Vertical dynamics in subterranean ecology: thinning and water-nitrogen additions drive multilayered responses in soil-fine root systems of Populus tomentosa plantations

Vertical dynamics in subterranean ecology: thinning and water-nitrogen additions drive multilayered responses in soil-fine root systems of Populus tomentosa plantations | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Vertical dynamics in subterranean ecology: thinning and water-nitrogen additions drive multilayered responses in soil-fine root systems of Populus tomentosa plantations Yafei Wang, Xiaofei Ding, Kai Wang, Yu Zou, Dongnan Wang, Liming Jia, and 1 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-7734837/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract Purpose Fine roots and soil properties show distinct vertical patterns, reflecting their coupled responses to thinning and water-fertilizer management. This study aimed to elucidate soil-root interactions and provide insights for the sustainable management of plantations. Methods A split-plot design was established with three thinning intensities (no thinning, moderate, heavy) and three water-nitrogen treatments (control, irrigation, irrigation + nitrogen). Soil profiles (0–6 m) and fine roots were sampled to assess changes in soil moisture, nutrient dynamics, and fine root traits. Multivariate analyses were used to identify key regulatory drivers. Results Soil water content (SWC) peaked at 300–400 cm and was sensitive to management in the 20–500 cm layer. Thinning and irrigation increased SWC, whereas water-nitrogen input reduced it in mid-depth layers. Thinning enhanced nitrogen accumulation, while water-nitrogen input offset nitrogen loss but increased nitrate leaching risk. Fine root biomass density was highest in the 0–20 cm layer, with deeper layers remaining stable. Water-nitrogen addition increased specific root area, with SWC as the main determinant after thinning, and both phosphorus and SWC driving responses under fertilization. Conclusion Thinning improved water availability but constrained nutrients, while water-nitrogen input shifted fine roots toward an acquisitive strategy, highlighting management-specific soil-root interactions. Thinning effect water and nitrogen addition effect soil nutrient characteristics fine root traits Populus Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 1. Introduction Fine roots, as the primary organs responsible for water and nutrient acquisition, play a crucial role in tree growth and ecosystem productivity through their spatial distribution and morphological plasticity (Zou et al., 2022 ; Tan et al., 2023 ). Serving as the key interface between plants and soil, and as the primary conduits for soil resource uptake, fine roots are indispensable for resource acquisition within forest ecosystems (Xi et al. 2013 ; Kramer-Walter and Laughlin 2017 ). Their critical ecological functions have sustained long-standing research interest in belowground processes (Jackson et al. 1997 ; Gill and Jackson 2000 ; Reich et al. 2014 ; Bergmann et al. 2020 ; Kou et al. 2025 ). In arid and semi-arid regions, the exploitation of deep soil resources (> 1 m) is particularly crucial for tree survival (Laclau et al. 2001 ; Di et al. 2018 ; Germon et al. 2020 ; Henriksson et al. 2021 ; Zhu et al. 2023 ). Nevertheless, studies examining deep soil characteristics and the vertical distribution of fine roots in plantations remain limited. The spatial distribution and physiological functions of fine roots are strongly influenced by plantation management practices, such as thinning, irrigation, and nitrogen addition, which regulate belowground resource acquisition strategies by modifying stand structure and soil conditions (He et al. 2022 ; Pang et al. 2022 ; Yang et al. 2025b ). Thinning reduces stand density and alleviates belowground competition, thereby improving the availability of light, water, and nutrients, which promotes the proliferation of fine roots in the surface soil (< 1 m) (Qin et al., 2024 ; Zhao et al., 2024 ). However, most existing studies have focused on soil layers within 1 m, leaving a knowledge gap regarding the spatial heterogeneity and functional adaptability of fine roots in deeper layers (Pang et al. 2022 ; Zhao et al. 2024 ). Similarly, water and nitrogen addition disrupt the original carbon-water-nitrogen balance, inducing phenotypic plasticity in fine roots (Pregitzer et al. 1993 ; Wang et al. 2021a ; He et al. 2022 ). Therefore, elucidating the dynamics of fine root distribution down to 6 m under water and nitrogen addition is theoretically significant for understanding deep resource acquisition strategies and provides a scientific basis for optimizing water and fertilizer management in plantations. Fine root plasticity represents a core adaptive strategy to environmental heterogeneity, manifested through coordinated adjustments in morphology, biomass, and physiological function (Kong et al. 2017 ; Zhang et al. 2024 ). This plasticity enables plants to optimize the cost-benefit balance of root system construction, thereby enhancing survival competitiveness (Lima et al. 2010 ), and can be conceptualized as an "investment-benefit" decision guided by ecological stoichiometry (Peng et al. 2017 ). Within plantation management contexts, thinning and water-nitrogen addition influence fine root plasticity via distinct pathways: thinning promotes the expression of fine root potential by reducing competition (Pang et al. 2022 ; Zhao et al. 2024 ), whereas water-nitrogen addition directly induces adaptation by modifying resource availability (Coleman 2007 ; Dou et al. 2022 ; Zhang et al. 2022 ). Consequently, fine roots adopt a gradient strategy along the soil profile, exhibiting a "foraging type" in topsoil (high specific root length, high root tip density) and a "conservative type" in deeper soil layers (larger diameter, reduced branching, extended lifespan) (Wang et al. 2016 ; Zhou et al. 2022 ). Although soil water and nutrient conditions are key drivers of fine root dynamics (Ellsworth and Sternberg 2019 ; Liu et al. 2024 ), management interventions can facilitate root penetration into deeper layers by improving subsurface conditions (Pang et al. 2022 ; He et al. 2022 ). However, a significant knowledge gap persists regarding the interactive network linking "management practices-soil environment-fine root plasticity," particularly in deep soil layers. The North China Plain, one of the largest regions globally with concentrated artificial forest plantations, hosts over 3 million hectares of poplar plantations. Nevertheless, this region faces severe ecological challenges, including declining groundwater levels leading to the formation of dry soil layers (Liu et al. 2022 ), and progressive depletion of organic matter in surface soils, which reduces fertility (Zhang et al. 2018 ; He et al. 2021 ; Yang et al. 2025a ). Investigating the effects of management practices on deep soil-root interactions is therefore of dual significance: theoretically, it advances understanding of deep resource acquisition strategies in arid-region trees; practically, it provides a foundation for sustainable management models emphasizing water conservation, soil nourishment, and efficiency enhancement. In particular, elucidating the response patterns of the soil-root system in deep layers under short-term management interventions can directly inform precision water and fertilizer strategies and promote efficient use of belowground ecological space in Populus plantations on the North China Plain. Populus tomentosa , a major native tree species widely cultivated in plantations on the North China Plain, delivers substantial ecological, economic, and social benefits (Zhu et al. 2024a ; Wang et al. 2025 ). This study assessed Populus tomentosa plantations to evaluate the short-term impacts of different management practices on soil properties and fine root plasticity through varying thinning intensities and water-nitrogen addition treatments. The objectives were: (1) to assess the short-term effects of thinning and water-nitrogen addition on soil moisture and nutrient properties across the 0–6 m soil profile; (2) to elucidate the responses of fine root vertical distribution and morphological plasticity to management interventions; and (3) to investigate the interrelationships between soil properties and fine root traits. We hypothesized that: (1) thinning and water-nitrogen addition would significantly improve soil fertility, with effects exhibiting distinct vertical gradients (He et al. 2021 ; Liu et al. 2022 ); and (2) water-nitrogen addition would induce "foraging-type" plasticity in fine roots, whereas thinning would more likely trigger "conservative-type" plasticity, with this differential response positively associated with the degree of soil resource heterogeneity (Wang et al. 2016 ; Zhou et al. 2022 ; Pang et al. 2022 ; Zhao et al. 2024 ). 2. Materials and methods 2.1. Study site and experimental plantations The research was conducted in 2023 at a state-owned forest farm located in Gaotang County, Liaocheng City, Shandong Province, China (36° 48′N, 116° 05′E; altitude of approximately 30 m). The area is part of the warm temperate semi-arid monsoon region of the northern hemisphere, characterized by four distinct seasons and abundant sunshine. Rainfall is concentrated in July and August, with an average annual precipitation of 553 mm and an average temperature of 13.9°C. The topography of the study site consists primarily of plains, with the water table located 6 to 9 meters below the surface (Liu et al. 2022 ). The soil is classified as sandy loam. Before afforestation, the soil contained 3.34 g·kg⁻¹ organic matter, 0.23 g·kg⁻¹ total nitrogen, and 4.92 mg·kg⁻¹ available phosphorus (He et al. 2021 ). The experimental plantation was established in the spring of 2016 using 2-year-old hybrid poplar saplings ( P. tomentosa × P. bolleana ) × ( P. alba × P. glandulosa ). The saplings had a breast height diameter of 2.12–3.51 cm (mean 2.68 cm) and a height of 2.60–3.62 m (mean 3.30 m). Trees were planted at 2 m× 3 m spacing, corresponding to a density of 1,666 trees ha⁻¹. Each tree received 70 g of basal fertilizer containing ≥ 31% N + P₂O₅ + K₂O (14-12-5) and ≥ 15% organic matter (OM). 2.2 Experimental design A split-plot design was employed for the experiment. In April 2022, thinning was performed in the experimental forest with two intensities: alternate row thinning (50% thinning, spacing adjusted to 2 m × 6 m, T50) and alternate row and tree thinning (75% thinning, spacing adjusted to 4 m × 6 m, T75), along with an unthinned control plot. Within each thinning intensity, three water-nitrogen treatments were applied: irrigation with nitrogen (WN), irrigation only (W), and no irrigation or nitrogen addition (CK). Following thinning, drip irrigation systems (Netafim, Israel) were installed with drippers spaced 50 cm apart at a flow rate of 1.6 L h⁻¹, positioned 30 cm from both sides of the tree row. Irrigation scheduling was based on the quantitative relationship between Populus tomentosa growth and soil water availability (Xi et al. 2016 ), triggered when soil water potential at 20 cm below the dripper reached − 20 kPa (79% field capacity). Irrigation commenced in early April and ended before the rainy season in late June. Nitrogen was applied as urea (46% N) at 156 g tree⁻¹ year⁻¹ in six applications at 20-day intervals, corresponding to the species’ growth rhythm (Wang et al. 2015 ). Routine weeding and pest control were conducted throughout the experimental period. To reduce the effects of light and soil heterogeneity, NT, T50, and T75 areas were divided into blocks along the north-south axis. Within each block, the three water-nitrogen treatments were randomly assigned, with plot sizes ranging from 16 to 64 trees, and protective rows established on the east and west sides. 2.3 Acquisition and measurement of soil and root samples Fine roots were sampled using soil cores. In October 2023, five standard trees ( n = 5) were selected per plot under five treatments: NT, T50, T75, T75W, and T75WN. Soil cores (0.0008836 m³) were collected 30 cm east of each tree to a depth of 6 m, divided into 20 cm increments from 0-100 cm and 100 cm increments from 100–600 cm, yielding 10 layers. Each soil layer was weighed in the field to obtain fresh weight (W). Three root-free subsamples were taken: one dried at 105°C to determine moisture content (W1 and W2), while the remaining two were air-dried for nutrient analysis and stored at − 4°C for inorganic nitrogen measurements. Soil water content (SWC, %) was calculated as: $$\:SWC=\frac{{W}_{1}-{W}_{2}}{{W}_{2}}\times\:100\%$$ 1 All roots were washed via wet sieving to remove soil and organic material. Live roots were separated from dead roots based on color, flexibility, and breakage resistance, and only roots < 2 mm in diameter were considered fine roots (Zou et al. 2022 ; Tan et al. 2023 ). Roots were scanned (Epson Twain Pro, 400 dpi) without overlap, and diameter, surface area, volume, and length were measured using WinRHIZO (Regent Instruments Inc., Quebec, Canada). Fine roots were then dried at 75°C to constant weight to determine biomass (FRB). Morphological traits included mean root diameter (MRD), fine root biomass density (FRBD), fine root length density (FRLD), fine root tissue density (FRTD), specific root surface area (SRA), and specific root length (SRL), calculated as: $$\:\text{F}\text{R}\text{B}\text{D}=FRB/SV$$ 2 $$\:\text{F}\text{R}\text{L}\text{D}=FRL/SV$$ 3 $$\:\text{F}\text{R}\text{T}\text{D}=FRB/FRV$$ 4 $$\:\text{S}\text{R}\text{A}=FRA/FRB$$ 5 $$\:\text{S}\text{R}\text{L}=FRL/FRB$$ 6 Where SV represents soil volume, FRL is root length, FRV is root volume, and FRA is root surface area. Soil samples, after root and stone removal, were air-dried and sieved (100-mesh, 0.15 mm; 20-mesh, 0.85 mm) for nutrient analysis. Samples stored at − 4°C were used for NO₃⁻-N and NH₄⁺-N determination. Organic carbon was measured by the potassium dichromate external heating method, available phosphorus (AP) via the molybdenum–antimony anti-colorimetric method, and total nitrogen (TN) and total phosphorus (TP) using concentrated H₂SO₄–HClO₄ digestion in a fully automated analyzer (SMARTCHEM 450, AMS Alliance, France). SOM was calculated as organic carbon × 1.724 (Zhang et al. 2022 ). NO₃⁻-N and NH₄⁺-N were extracted with 1 M KCl and measured via UV spectrophotometry and indophenol blue colorimetry, respectively. 2.4 Data analysis Data were organized in Excel 2016. Homogeneity of variance and normality were assessed using Levene’s test and the Kolmogorov-Smirnov test, respectively. Non-normal data were transformed to meet ANOVA assumptions. Two-way ANOVA evaluated the effects of thinning (T), water and nitrogen addition (W), soil depth (D), and their interactions (T×D, W×D) on soil properties and fine root traits. One-way ANOVA tested differences among treatments within the same soil layer, with post hoc Tukey HSD comparisons (α = 0.05). Pearson correlations were used to assess relationships among fine root traits (SPSS 25, IBM SPSS Statistics, IBM Corporation, Armonk, NY, USA). Redundancy analysis (RDA) was conducted in CANOCO 5 (Microcomputer Power, Ithaca, NY, USA) to evaluate the relative importance of soil properties on fine root variation. Based on the plant economics spectrum theory (Reich et al. 2014 ; Kong et al. 2017 ; Weigelt et al. 2023 ), MRD, FRBD, and FRTD were classified as "conservative" strategy indicators, reflecting substantial carbon allocation to robust tissues, whereas FRLD, SRA, and SRL represented the "acquisitive" strategy, reflecting minimal carbon investment for rapid resource uptake. Standardized data were used to construct and analyze structural equation models (SEM) in AMOS 20.0 (IBM SPSS Amos, IBM Corporation, Armonk, NY, USA). Figures were generated in Origin 2023 (Origin Lab Corporation, Northampton, MA, USA). 3 Result 3.1 Soil moisture content Soil water content (SWC) exhibited significant depth variability ( P < 0.05) (Table S1 ), with peak SWC observed at depths of 300–400 cm across all treatment groups (Fig. 1 ). The vertical distribution of SWC showed sensitivity to silvicultural measures in the 20–500 cm soil layer, while the 0–20 cm and 500–600 cm layers remained relatively stable (Fig. 1 ). Two-way ANOVA revealed significant effects of thinning intensity and water-nitrogen addition on SWC ( P < 0.05) (Table S1 ). Both thinning and water input significantly increased soil water content, with a particularly pronounced effect observed in the 80–400 cm soil layer (Fig. 1 ). However, the combined input of water and nitrogen resulted in a decrease in SWC, with this effect most pronounced in the 100–200 cm soil layer (Fig. 1 ). 3.2 Soil nutrients Soil organic matter (SOM) was significantly influenced by thinning intensity, water-nitrogen addition, and soil depth ( P < 0.05) (Table S1 ). Across the soil profile, SOM exhibited a nonlinear response to thinning intensity, with the highest values observed under moderate thinning (T50) followed by the unthinned control (NT), and the lowest in high-intensity thinning (T75) (Fig. 2 ). In low-density stands (T75), the application of water and nitrogen significantly enhanced SOM content, with values following the order T75WN > T75W > T75 (Fig. 2 ). Soil total nitrogen (TN) content was significantly affected by thinning and water-nitrogen addition ( P < 0.05) (Table S1 ). Thinning promoted TN accumulation, whereas water addition reduced TN content; the combined input of water and nitrogen mitigated this reduction. However, this mitigation was significant only in the 0–20 cm soil layer ( P < 0.05) (Fig. 3 ). Ammonium nitrogen (NH₄⁺-N) and nitrate nitrogen (NO₃⁻-N) contents were significantly affected by thinning intensity, water-nitrogen addition, and soil depth ( P < 0.05). Among these, the interaction between water-nitrogen addition and soil depth significantly influenced inorganic nitrogen ( P < 0.05), whereas the interaction between thinning intensity and soil depth was significant only for nitrate nitrogen ( P < 0.05) (Table S1 ). Thinning significantly increased ammonium nitrogen contents in the 0–40 cm, 200–300 cm, and 500–600 cm soil layers (Fig. S1 ), and markedly increased nitrate nitrogen contents at 100–200 cm, 300–400 cm, and 500–600 cm (Fig. S2). Water addition promoted the leaching and downward movement of ammonium nitrogen, whereas combined water and nitrogen addition mitigated nitrogen loss and enhanced accumulation in deeper soil layers (300–600 cm) (Fig. S1 ). Nitrate nitrogen exhibited greater stability in the 0–60 cm surface layer, with no significant differences detected among the T75, T75W, and T75WN treatments. The combined addition of water and fertilizer (T75WN) promoted nitrate nitrogen accumulation in the 60–100 cm and 300–500 cm soil layers, thereby increasing the potential risk of leaching (Fig. S2). Soil total phosphorus (TP) was significantly affected by thinning intensity, soil depth, and their interaction ( P < 0.05), and was also significantly regulated by water-nitrogen addition ( P < 0.05) (Table S1 ). In the 0–20 cm, 60–80 cm, and 100–200 cm soil layers, TP contents under the T75W and T75WN treatments were significantly higher than under T75, with the most pronounced increase occurring in the 100–200 cm layer, where TP increased by 196.82% and 59.23%, respectively, compared to T75 (Fig. 4 ). Soil available phosphorus (AP) contents were also significantly affected by thinning intensity, soil depth, and their interaction ( P NT > T75 with increasing thinning intensity (Fig. 5 ). 3.3 Fine root plasticity Thinning treatments were not found to significantly influence the short-term plasticity of most fine root traits. However, soil depth was observed to play a critical role in regulating fine root trait variation across experimental forests subjected to three thinning intensities. Analysis of variance indicated that soil depth significantly influenced several key metrics, including mean root diameter (MRD), fine root biomass density (FRBD), and fine root length density (FRLD) ( P < 0.05). Peak values of FRBD and FRLD were recorded in the top 0–20 cm soil layer for all treatments, with mean values of 456.83 g·m⁻³ and 2,655,151.30 cm·m⁻³, respectively. In contrast, these indices remained relatively stable in the deeper 200–600 cm soil layer (Fig. S3). MRD initially increased with soil depth before declining, with peak depths for the T50 and T75 treatments occurring in shallower soil layers compared to the control (NT). Under the NT treatment, specific root area (SRA) and specific root length (SRL) attained their maximum values in the 60–80 cm soil layer, whereas the T50 and T75 treatments peaked in the 80–100 cm soil layer (Fig. 6 ). These results indicate that fine root distribution and morphology are strongly stratified along the soil profile and are influenced primarily by soil depth rather than short-term thinning intensity. The water and nitrogen addition experiment conducted under T75 high-intensity thinning exhibited limited short-term effects on most fine root functional traits. A significant increase in specific root area (SRA) was the only notable change observed ( P < 0.05). However, the interaction between water-nitrogen addition and soil depth was found to significantly affect the vertical distribution patterns of SRA and SRL ( P < 0.05) (Fig. 6 ). Fine root morphological traits displayed distinct strategic differentiation under different management interventions. In the T75WN treatment, SRA and SRL remained stable within the 0-400 cm soil layer but increased sharply at 400–500 cm depth. In contrast, the T75W and T75 treatments primarily promoted increases in SRA and SRL at depths above 200 cm. Notably, the T75W treatment exhibited particularly significant increases in the 20–40 cm, 60–80 cm, and 80–100 cm soil layers (Fig. 6 ). These patterns suggest that water-nitrogen addition interacts with soil depth to modify fine root plasticity, with deeper soil layers responding more prominently under combined treatment, whereas single-factor interventions primarily affect upper soil horizons. Pearson correlation analysis (Fig. S4) further supported these observations, revealing highly significant positive correlations ( P < 0.001) between FRBD and FRLD across different experimental treatments and soil depths (0-600 cm, 0-100 cm, and 100–600 cm), indicating a strong consistency in the response of these traits to environmental variation. However, negative correlations were detected between these traits and both SRA and SRL. Notably, correlations between MRD and other fine root traits displayed distinct vertical differentiation across soil layers. Within the 0-100 cm surface layer, MRD was negatively correlated with FRBD, FRLD, and fine root tissue density (FRTD), whereas in the 100–600 cm deeper layer, MRD was positively correlated with these traits. These results suggest that MRD may fulfill different ecological functions in distinct soil layers, reflecting the adaptive differentiation of fine root traits in response to soil environmental heterogeneity. 3.4 The relationship between fine root plasticity and soil properties Following thinning, specific root area (SRA) and specific root length (SRL) were positively correlated with soil water content (SWC), total nitrogen (TN), and nitrate nitrogen (NO₃⁻-N), whereas fine root biomass density (FRBD) and fine root length density (FRLD) were predominantly negatively correlated. Soil water content (SWC) was identified as the primary factor influencing variation in fine root traits after thinning, accounting for 32.9% of the observed variance (Fig. 7 a). Soil depth exerted a significant influence on the interactions between fine root traits and soil factors. In the surface soil layer (0-100 cm), available phosphorus (AP) and SWC were identified as the key regulatory factors, with their correlation patterns closely aligning with those observed across the entire 0-600 cm soil profile (Fig. 7 c). In the deeper soil layers (100–600 cm), AP emerged as the primary factor regulating fine root development, explaining more than 38% of the observed variance (Fig. 7 e). The water-nitrogen addition experiment, conducted after thinning, further altered the interactions between soil properties and fine root traits. Fine root biomass density (FRBD) and fine root length density (FRLD) were positively correlated with total nitrogen (TN) and nitrate nitrogen (NO₃⁻-N), but negatively correlated with ammonium nitrogen (NH₄⁺-N), soil organic matter (SOM), available phosphorus (AP), and SWC. AP and SWC were identified as the primary regulatory factors under the water-nitrogen addition treatments ( P < 0.05) (Fig. 7 b). In the shallow soil layer (0-100 cm), nitrate nitrogen (NO₃⁻-N) stimulated the development of FRBD and FRLD, whereas SWC predominantly regulated SRA and SRL, accounting for 48.9% of the observed variation (Fig. 7 d). In the deeper soil layers, a distinctly different regulatory pattern was observed, with phosphorus (TP, AP) and soil moisture positively influencing FRBD and FRLD, whereas nitrogen (NO₃⁻-N, NH₄⁺-N) and SOM served as the primary drivers of variations in SRA and SRL (Fig. 7 f). 3.5 SEM under thinning and water-nitrogen addition experiments Both the thinning and water-nitrogen addition models showed a good goodness-of-fit. The χ² value for the thinning model was 187.680 ( d f = 69), and for the water-nitrogen addition model, it was 216.398 ( d f = 82). The chi-square to degrees of freedom ratios (χ²/ d f ) for the two models were 2.720 and 2.639, respectively, both below the threshold of 3.000. Furthermore, the RMSEA values for both models were 0.051 and 0.057, respectively, both below the critical value of 0.100, suggesting a good fit to the data (Fig. 8 ). Path analysis revealed that thinning and water-nitrogen addition had opposing regulatory effects on soil properties (Fig. 8 ). Thinning significantly increased soil moisture but inhibited soil nutrient availability (Fig. 8 a). In contrast, water and nitrogen addition treatments significantly enhanced soil nutrient content ( P < 0.001) (Fig. 8 b). Thinning has a limited short-term impact on the investment and development of fine roots, without significant direct regulatory pathways. However, thinning promotes a conservative development strategy that negatively impacts soil nutrient availability, thus inhibiting fine root development (Fig. 8 a). In contrast, water and nitrogen addition promote the acquisitive development of fine roots in Populus tomentosa and suppress the conservative development strategy (Fig. 8 b). This shift is primarily mediated through changes in soil nutrient availability. 4. Discussions 4.1 Vertical stratification of soil water and nutrients under management practices Soil moisture displayed a clear vertical stratification, with a pronounced peak at 300–400 cm across all treatments (Fig. 1 ). Deeper layers (> 200 cm) generally retained more water than the surface (0-100 cm), consistent with earlier studies on the hydrological characteristics of deep soil in the North China Plain (Liu et al. 2022 ; Zhu et al. 2023 ). This pattern was largely driven by the vertical distribution of poplar roots, where high root density in the topsoil enhanced water consumption, while reduced root activity at depth allowed moisture accumulation (Zou et al. 2022 ; Tan et al. 2023 ; Zhu et al. 2024b ). Thinning significantly enhanced soil moisture in the 80–400 cm layer (Fig. 1 ), partially supporting Hypothesis 1. This effect can be attributed to reduced canopy interception, enhanced precipitation infiltration, decreased transpiration after tree removal, and diminished root competition (Molina et al. 2021 ; Liu et al. 2025 ). Water addition also promoted soil moisture replenishment, but a decline in water content was observed in the 100–200 cm layer under combined water-nitrogen input, likely due to a fertilization-transpiration feedback in which nitrogen addition stimulated plant growth and water consumption (Samuelson et al. 2008 ). Furthermore, the upward shift of root distribution under fertilization (Fig. S3) may have intensified water competition in the middle soil layers, underscoring the need for precise fertilization strategies in water-limited environments. Soil nutrients exhibited complex vertical responses to management interventions. Under moderate thinning (T50), soil organic matter (SOM) reached its highest level (T50 > NT > T75) (Fig. 2 ), consistent with the Intermediate Disturbance Hypothesis (Fox 2013 ), suggesting that moderate thinning optimizes the balance between litter inputs and decomposition. Both irrigation and irrigation-fertilization treatments markedly increased total phosphorus (TP) in the 100–200 cm layer (up to 196.82%) (Fig. 4 ), largely through three mechanisms: transformation of Ca-bound P into more plant-available Fe/Al-bound P (Wolf et al. 2013 ); stimulation of phosphatase activity by fertilization, accelerating organic P mineralization (Gao et al. 2020 ; Wang et al. 2021b ); and secretion of root-derived organic acids (e.g., citric and oxalic acids), which chelated metal ions and enhanced P availability (Jiang et al. 2025 ). These processes acted synergistically, increasing phosphorus bioavailability following water and nitrogen inputs. Water input alone reduced total nitrogen (TN), whereas the combined water-nitrogen treatment offset this decline (Fig. 3 ), reflecting a balance between water-driven nitrogen leaching and fertilization-driven replenishment. Water addition also facilitated nitrate (NO₃⁻–N) migration into deeper layers (200–600 cm) (Fig. S2). Under drip fertigation, rapid urea hydrolysis, nitrification, and transport led to nitrate accumulation at varying depths, with the peak shifting downward under greater irrigation (Xu et al. 2020 ; He et al. 2021 ). In contrast, ammonium (NH₄⁺–N), being strongly adsorbed to soil colloids, showed low mobility (Dai et al. 2015 ). Although nitrate accumulation in deeper layers (300–600 cm) increased nutrient reserves, it also raised the risk of leaching, requiring careful nutrient management in ecologically sensitive areas. In summary, management practices that explicitly account for the vertical stratification of soil water and nutrients are essential to enhance both productivity and sustainability of poplar plantations in water-limited regions. 4.2 Fine root adaptations to vertical resource heterogeneity Our results demonstrate that fine root distribution across the 0-600 cm soil profile follows a clear resource-gradient adaptation strategy, consistent with recent advances in fine root ecology (Lu et al. 2025 ). The highest FRBD and FRLD values occurred in the 0–20 cm topsoil layer, supporting the optimal resource allocation hypothesis, as plants preferentially invest in absorptive roots within resource-rich layers (Coleman 2007 ; Giehl and Von Wiren 2014 ; Coleman and Aubrey 2018 ). Forest management practices significantly modified fine root traits. Thinning exhibited relatively limited short-term effects on SRL and SRA but altered MRD, with peaks shifting to shallower layers under high-intensity thinning. This indicates that adaptive trait adjustment requires longer response times, consistent with previous observations on the temporal dynamics of root plasticity (Shen et al. 2017 ). Water addition (T75W) strongly influenced fine root morphology, increasing SRA by up to 655% in the 20–100 cm layer, highlighting soil water availability as a major driver of morphological plasticity (Vanguelova et al. 2005 ; Li et al. 2022 ; Tan et al. 2023 ). In contrast, the combined water-nitrogen treatment (T75WN) enhanced SRA and SRL in the 400–600 cm layer, suggesting that water regulates root morphology directly via turgor-driven cell division (Spollen and Sharp 1991 ; Alrajhi et al. 2024 ), whereas nitrogen indirectly modulates root construction by altering carbon allocation and hormonal signaling (Jing and Strader 2019 ; Taleski et al. 2024 ). The synergistic effect of water and nitrogen may further involve the activation of specific gene expression pathways in poplar (Shen et al. 2024 ). A clear structure-function trade-off was also observed, with conservative traits (MRD, FRBD, FRTD) negatively correlated with acquisitive traits (SRA, SRL) (Fig. S4). This finding not only confirms the applicability of the root economic spectrum theory (Reich 2014 ; Reich et al. 2014 ) to deep soil systems but also extends its relevance. The trade-off reflects an adaptive balance between resource acquisition efficiency and construction costs: conservative strategies invest in thicker, denser roots with longer lifespans, whereas acquisitive strategies favor higher SRL and SRA, enhancing uptake efficiency but reducing stress tolerance (Kong et al. 2017 ; Weigelt et al. 2023 ). Management practices were shown to regulate these trade-offs. Thinning promoted conservative trait expression by reducing nutrient availability, while water-fertilizer addition favored acquisitive strategies, enhancing carbon input and nutrient uptake under resource-limited conditions. These results suggest that plantation management can actively shift fine root strategies along the conservative-acquisitive spectrum, with direct implications for belowground carbon allocation and nutrient cycling. 4.3Trade-offs in fine root strategies under management regimes After thinning, soil water content (SWC) and available phosphorus (AP) in the 0-100 cm surface layer jointly governed the variation in fine root traits (Fig. 7 ). This pattern highlights the strong regulation of shallow root growth by immediate resource availability (Guilbeault-Mayers et al. 2024 ). As the most dynamic zone for water and nutrient fluctuations, the surface soil is subject to frequent wet-dry cycles that directly influence phosphorus uptake efficiency. Under optimal moisture conditions, phosphorus diffusion rates increase (Shapiro et al. 1960 ; Sharpley and Ahuja 1983 ), thereby enhancing root nutrient acquisition. In addition, high microbial activity and organic matter turnover in surface soils maintain a continuous supply of AP (Hawkins et al. 2022 ; Sica et al. 2025 ), driving fine roots to develop densely branched structures in resource-rich environments to maximize absorptive surface area (Goebel et al. 2011 ; Du and Wei 2018 ). By contrast, in deeper layers (100–600 cm), the relationship between AP and fine root structural traits reversed (Fig. 7 ), reflecting the resource constraints typical of these strata. Deep soils are characterized by relatively stable moisture but limited phosphorus availability (Mao et al. 2024 ). Given the extremely low mobility of phosphorus, deep roots adapt through morphological adjustments—such as elongating axes and reducing branching—rather than relying on opportunistic uptake as in surface soils (Wang and Lambers 2020 ; Lambers 2022 ; Barrow and Lambers 2022 ). This adaptive strategy enabled AP to explain up to 38% of trait variation in deep layers of thinned forests, suggesting that phosphorus is a key limiting factor regulating deep root development in Populus tomentosa after thinning. Irrigation and fertigation further increased surface soil water availability in thinned stands (Fig. 1 ), alleviating fine root water limitation during growth and development. Accordingly, SWC was the primary factor explaining root trait variation under different water and nitrogen treatments (Fig. 7 ). In deeper layers, however, water mobility is restricted and anaerobic conditions are common, reducing the explanatory power of SWC. Under such conditions, roots responded more strongly to leached nitrogen (Fig. 7 ), optimizing resource acquisition by enhancing inorganic nitrogen utilization. For instance, under the T75WN treatment, fine roots increased specific root area (SRA) and specific root length (SRL), thereby improving nitrogen uptake efficiency in deep soil (Fig. 6 ). This vertical differentiation reflects the precise adaptive strategies of root systems to resource environments at contrasting soil depths. The adaptive shift in fine root construction strategy exhibited significant treatment specificity, primarily driven by dynamic changes in the soil nutrient pool, in full support of Hypothesis 2. Thinning profoundly modified the root environment by improving soil moisture conditions (Fig. 8 ). Reduced stand density alleviated belowground competition, enabling plants to optimize spatial distribution patterns (Qin et al. 2024 ). Enhanced light penetration and ventilation accelerated litter decomposition (Henneron et al. 2018 ; Latterini et al. 2024 ), improved soil aggregate stability (Ma et al. 2022 ), and reduced transpiration losses, thereby increasing deep soil water availability (Molina et al. 2021 ). These synergistic improvements favored the allocation of resources toward persistent root structures, as indicated by the enhancement of conservative traits such as fine root biomass density (FRBD) and mean root diameter (MRD), whereas acquisitive traits (e.g., SRL) were comparatively reduced (Figs. 6 and S3). This conservative shift suggests an adaptive transition from short-term resource capture toward long-term survival under reduced competition (Giehl and Von Wiren 2014 ). In contrast, water and nitrogen addition treatments primarily regulated root development through chemical pathways (Coleman 2007 ; He et al. 2022 ). By increasing soil nutrient availability, they lowered the metabolic cost of root exploration (Fig. 8 ) while simultaneously influencing moisture distribution and altering plant carbon-nitrogen allocation. Under such conditions, plants exhibited typical “resource enrichment” responses, favoring acquisitive root architectures with high SRL and SRA (Fig. 6 ), while reducing investment in persistent structures. This pattern reflects a precision foraging strategy that facilitates rapid adaptation to short-term enrichment environments, consistent with Hypothesis 2. Overall, thinning demonstrated unique advantages in improving soil water conditions and promoting stable root system development, whereas water and nitrogen additions were more effective in enhancing short-term productivity. A rational integration of these management strategies is likely to achieve synergistic optimization of root structural and functional traits, thereby improving both resilience and productivity in Populus tomentosa plantations. Future research should focus on elucidating the physiological and ecological mechanisms underlying trade-offs in root development strategies under combined management regimes, which will provide critical foundations for precision forest management frameworks grounded in ecological processes. 5. Conclusions Thinning and water-nitrogen addition treatments exerted significant effects on soil moisture, nutrient distribution, and fine root traits, exhibiting pronounced vertical differentiation. Thinning increased soil water content in the 80–400 cm layer, whereas the synergistic application of water and nitrogen reduced moisture in the 100–200 cm layer but enhanced total phosphorus by 59.23%. Soil organic matter and total nitrogen were most responsive to management in the surface layers, while inorganic nitrogen accumulation was more prominent in deeper layers (300–600 cm). Irrigation primarily increased specific root length (SRL) in the middle soil layers, whereas combined water-nitrogen addition promoted specific root area (SRA) in the deep soil (400–500 cm). Thinning had limited short-term impacts on fine root strategy, whereas water-nitrogen addition shifted root development toward a more acquisitive strategy by improving nutrient availability. These results provide novel insights into plantation root system responses and offer a scientific basis for optimizing sustainable management practices in managed forests. Abbreviations AP available phosphorus FRBD fine root biomass density FRLD fine root length density FRTD fine root tissue density MRD mean root diameter NT no thinning NH₄⁺-N ammonium nitrogen NO₃⁻-N nitrate nitrogen T50 50% tree removal T75 75% tree removal TN total nitrogen TP total phosphorus SOM soil organic matter SRA specific root surface area SRL specific root length SWC soil water content W irrigation only WN irrigation with nitrogen application Declarations Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Author contributions YFW conceived the ideas; YFW, BYX, and LMJ designed the methodology; YFW, XFD, and YZ collected the data; YFW, KW, and DNW analyzed the data; YFW, BYX, and LMJ led the writing of the manuscript. All authors contributed critically to the drafts and gave final approval for publication. Acknowledgment This research was supported by the National Key Research and Development Program of China (2024YFD2201004, 2021YFD2201203). References Alrajhi A, Alharbi S, Beecham S, Alotaibi F (2024) Regulation of root growth and elongation in wheat. Front Plant Sci 15:1397337. https://doi.org/10.3389/fpls.2024.1397337 Barrow NJ, Lambers H (2022) Phosphate-solubilising microorganisms mainly increase plant phosphate uptake by the effects of pH on root physiology. Plant Soil 476:397–402. https://doi.org/10.1007/s11104-021-05240-0 Bergmann J, Weigelt A, Kuyper TW et al (2020) The fungal collaboration gradient dominates the root economics space in plants. SCIENCE ADVANCES Coleman M (2007) Spatial and temporal patterns of root distribution in developing stands of four woody crop species grown with drip irrigation and fertilization. Plant Soil 299:195–213. https://doi.org/10.1007/s11104-007-9375-5 Coleman MD, Aubrey DP (2018) Stand development and other intrinsic factors largely control fine-root dynamics with only subtle modifications from resource availability. Tree Physiol 38:1805–1819. https://doi.org/10.1093/treephys/tpy033 Dai T, Xi B, Yan X, Jia L (2015) Effects of fertilization method and nitrogen application rate on soil nitrogen vertical migration in a Populus × euramericana cv. ‘Guariento’plantation. Chin J Appl Ecol 26:1641–1648. https://doi.org/10.13287/j.1001-9332.2015.0047 Di N, Liu Y, Mead DJ et al (2018) Root-system characteristics of plantation-grown Populus tomentosa adapted to seasonal fluctuation in the groundwater table. Trees 32:137–149. https://doi.org/10.1007/s00468-017-1619-2 Dou X, Wang R, Li C et al (2022) Spatial distribution of soil water, plant roots, and water use pattern under different drip fertigation regimes in an apple-soybean intercropping system on the Loess Plateau, China. Agric Water Manage 269:107718. https://doi.org/10.1016/j.agwat.2022.107718 Du X, Wei X (2018) Definition of fine roots on the basis of the root anatomy, diameter, and branch orders of one-year old Fraxinus mandshurica seedlings. J Res 29:1321–1327. https://doi.org/10.1007/s11676-017-0561-x Ellsworth PZ, Sternberg LSL (2019) Linking soil nutrient availability, fine root production and turnover, and species composition in a seasonally dry plant community. Plant Soil 442:49–63. https://doi.org/10.1007/s11104-019-04153-3 Fox JW (2013) The intermediate disturbance hypothesis should be abandoned. Trends Ecol Evol 28:86–92. https://doi.org/10.1016/j.tree.2012.08.014 Gao D, Bai E, Li M et al (2020) Responses of soil nitrogen and phosphorus cycling to drying and rewetting cycles: A meta-analysis. Soil Biol Biochem 148:107896. https://doi.org/10.1016/j.soilbio.2020.107896 Germon A, Laclau J-P, Robin A, Jourdan C (2020) Tamm Review: Deep fine roots in forest ecosystems: Why dig deeper? For Ecol Manag 466:118135. https://doi.org/10.1016/j.foreco.2020.118135 Giehl RFH, Von Wiren N (2014) Root Nutrient Foraging. Plant Physiol 166:509–517. https://doi.org/10.1104/pp.114.245225 Gill RA, Jackson RB (2000) Global patterns of root turnover for terrestrial ecosystems. New Phytol 147:13–31. https://doi.org/10.1046/j.1469-8137.2000.00681.x Goebel M, Hobbie SE, Bulaj B et al (2011) Decomposition of the finest root branching orders: linking belowground dynamics to fine-root function and structure. Ecol Monogr 81:89–102. https://doi.org/10.1890/09-2390.1 Guilbeault-Mayers X, Lambers H, Laliberté E (2024) Coordination among leaf and fine-root traits along a strong natural soil fertility gradient. Plant Soil. https://doi.org/10.1007/s11104-024-06740-5 Hawkins JMB, Vermeiren C, Blackwell MSA et al (2022) The effect of soil organic matter on long-term availability of phosphorus in soil: Evaluation in a biological P mining experiment. Geoderma 423:115965. https://doi.org/10.1016/j.geoderma.2022.115965 He Y, Li G, Xi B et al (2022) Fine root plasticity of young Populus tomentosa plantations under drip irrigation and nitrogen fertigation in the North China Plain. Agric Water Manage 261:107341. https://doi.org/10.1016/j.agwat.2021.107341 He Y, Xi B, Li G et al (2021) Influence of drip irrigation, nitrogen fertigation, and precipitation on soil water and nitrogen distribution, tree seasonal growth and nitrogen uptake in young triploid poplar (Populus tomentosa) plantations. Agric Water Manage 243:106460. https://doi.org/10.1016/j.agwat.2020.106460 Henneron L, Chauvat M, Archaux F et al (2018) Plasticity in leaf litter traits partly mitigates the impact of thinning on forest floor carbon cycling. Funct Ecol 32:2777–2789. https://doi.org/10.1111/1365-2435.13208 Henriksson N, Lim H, Marshall J et al (2021) Tree water uptake enhances nitrogen acquisition in a fertilized boreal forest – but not under nitrogen-poor conditions. New Phytol 232:113–122. https://doi.org/10.1111/nph.17578 Jackson RB, Mooney HA, Schulze E-D (1997) A global budget for fine root biomass, surface area, and nutrient contents. Proc Natl Acad Sci USA 94:7362–7366. https://doi.org/10.1073/pnas.94.14.7362 Jiang Z, Wang C, Sun H et al (2025) Soil phosphorus availability modulates root exudation under long-term nitrogen fertilization. Soil Tillage Res 254:106699. https://doi.org/10.1016/j.still.2025.106699 Jing H, Strader LC (2019) Interplay of Auxin and Cytokinin in Lateral Root Development. IJMS 20:486. https://doi.org/10.3390/ijms20030486 Kong D, Wang J, Zeng H et al (2017) The nutrient absorption–transportation hypothesis: optimizing structural traits in absorptive roots. New Phytol 213:1569–1572. https://doi.org/10.1111/nph.14344 Kou L, Ma N, Freschet GT et al (2025) Iterative effects: a new paradigm for root dynamics. Trends Plant Sci S1360138525001633. https://doi.org/10.1016/j.tplants.2025.05.018 Kramer-Walter KR, Laughlin DC (2017) Root nutrient concentration and biomass allocation are more plastic than morphological traits in response to nutrient limitation. Plant Soil 416:539–550. https://doi.org/10.1007/s11104-017-3234-9 Laclau J-P, Arnaud M, Bouillet J-P, Ranger J (2001) Spatial distribution of Eucalyptus roots in a deep sandy soil in the Congo: relationships with the ability of the stand to take up water and nutrients. Tree Physiol 21:129–136. https://doi.org/10.1093/treephys/21.2-3.129 Lambers H (2022) Phosphorus Acquisition and Utilization in Plants. Annu Rev Plant Biol 73:17–42. https://doi.org/10.1146/annurev-arplant-102720-125738 Latterini F, Horodecki P, Dyderski MK et al (2024) Mediterranean beech forests: Thinning and ground-based skidding are found to alter microarthropod biodiversity with no effect on litter decomposition rate. For Ecol Manag 569:122160. https://doi.org/10.1016/j.foreco.2024.122160 Li T, Ren J, He W et al (2022) Anatomical structure interpretation of the effect of soil environment on fine root function. Front Plant Sci 13:993127. https://doi.org/10.3389/fpls.2022.993127 Lima TTS, Miranda IS, Vasconcelos SS (2010) Effects of water and nutrient availability on fine root growth in eastern Amazonian forest regrowth, Brazil. New Phytol 187:622–630. https://doi.org/10.1111/j.1469-8137.2010.03299.x Liu J, Li D, Fernández J-E et al (2022) Variations in water-balance components and carbon stocks in poplar plantations with differing water inputs over a whole rotation: implications for sustainable forest management under climate change. Agric For Meteorol 320:108958. https://doi.org/10.1016/j.agrformet.2022.108958 Liu J, Zhou L, Wang D et al (2024) The Effect of Balanced N, P and K Fertilization on Fine Root Traits and Soil Properties in Sapindus mukorossi. Forests 15:94. https://doi.org/10.3390/f15010094 Liu Y, Li Y, Gao G et al (2025) Effects of thinning on soil water content and water use characteristics of artificial forest in the Loess Plateau of China. CATENA 254:108997. https://doi.org/10.1016/j.catena.2025.108997 Lu M, Wang S, Malhotra A et al (2025) A continental scale analysis reveals widespread root bimodality. Nat Commun 16:5281. https://doi.org/10.1038/s41467-025-60055-2 Ma Y, Cheng X, Kang F, Han H (2022) Effects of thinning on soil aggregation, organic carbon and labile carbon component distribution in Larix principis-rupprechtii plantations in North China. Ecol Ind 139:108873. https://doi.org/10.1016/j.ecolind.2022.108873 Mao L, Miao Y, Ge Y et al (2024) Effects of different afforestation years on soil moisture and nutrient content in Maxian Mountain of the Loess Plateau. Sci Rep 14:16194. https://doi.org/10.1038/s41598-024-66408-z Molina AJ, González-Sanchis M, Biel C, Del Campo AD (2021) Ecohydrological turnover in overstocked Aleppo pine plantations: Does the effect of thinning, in relation to water, persist at the mid-term? For Ecol Manag 483:118781. https://doi.org/10.1016/j.foreco.2020.118781 Pang Y, Tian J, Yang H et al (2022) Responses of Fine Roots at Different Soil Depths to Different Thinning Intensities in a Secondary Forest in the Qinling Mountains, China. Biology 11:351. https://doi.org/10.3390/biology11030351 Peng Y, Guo D, Yang Y (2017) Global patterns of root dynamics under nitrogen enrichment. Global Ecol Biogeogr 26:102–114. https://doi.org/10.1111/geb.12508 Pregitzer KS, Hendrick RL, Fogel R (1993) The demography of fine roots in response to patches of water and nitrogen. New Phytol 125:575–580. https://doi.org/10.1111/j.1469-8137.1993.tb03905.x Qin J, Lu J, Peng Y et al (2024) Thinning-induced decrease in fine root biomass, but not other fine root traits in global forests. J Environ Manage 370:122938. https://doi.org/10.1016/j.jenvman.2024.122938 Reich PB (2014) The world-wide ‘fast–slow’ plant economics spectrum: a traits manifesto. J Ecol 102:275–301. https://doi.org/10.1111/1365-2745.12211 Reich PB, Luo Y, Bradford JB et al (2014) Temperature drives global patterns in forest biomass distribution in leaves, stems, and roots. Proc Natl Acad Sci USA 111:13721–13726. https://doi.org/10.1073/pnas.1216053111 Samuelson LJ, Farris MG, Stokes TA, Coleman MD (2008) Fertilization but not irrigation influences hydraulic traits in plantation-grown loblolly pine. For Ecol Manag 255:3331–3339. https://doi.org/10.1016/j.foreco.2008.02.014 Shapiro RE, Armiger WH, Fried M (1960) The Effect of Soil Water Movement vs. Phosphate Diffusion On Growth and Phosphorus Content of Corn and Soybeans. Soil Sci Soc Amer J 24:161–164. https://doi.org/10.2136/sssaj1960.03615995002400030012x Sharpley AN, Ahuja LR (1983) A diffusion interpretation of soil phosphorus desorption. Soil Sci 135:322–326. https://doi.org/10.1097/00010694-198305000-00008 Shen J, Li X, Jiang L et al (2024) Molecular Mechanisms of Poplar Adaptation to Water–Fertilizer Coupling: Insights from Transcriptomic and Metabolomic Analyses. Forests 15:1967. https://doi.org/10.3390/f15111967 Shen Y, Wang N, Cheng R et al (2017) Short-Term Effects of Low Intensity Thinning on the Fine Root Dynamics of Pinus massoniana Plantations in the Three Gorges Reservoir Area, China. Forests 8:428. https://doi.org/10.3390/f8110428 Sica P, Sitzmann TJ, Müller-Stöver D, Magid J (2025) Strategic placement of mineral and biobased fertilizers for optimizing phosphorus use efficiency: A comprehensive review. Soil Use Manag 41:e70039. https://doi.org/10.1111/sum.70039 Spollen WG, Sharp RE (1991) Spatial Distribution of Turgor and Root Growth at Low Water Potentials. Plant Physiol 96:438–443. https://doi.org/10.1104/pp.96.2.438 Taleski M, Jin M, Chapman K et al (2024) CEP hormones at the nexus of nutrient acquisition and allocation, root development, and plant–microbe interactions. J Exp Bot 75:538–552. https://doi.org/10.1093/jxb/erad444 Tan J, Yu W, Liu Y et al (2023) Response of Fine-Root Traits of Populus tomentosa to Drought in Shallow and Deep Soil. Forests 14:951. https://doi.org/10.3390/f14050951 Vanguelova EI, Nortcliff S, Moffat AJ, Kennedy F (2005) Morphology, biomass and nutrient status of fine roots of Scots pine (Pinus sylvestris) as influenced by seasonal fluctuations in soil moisture and soil solution chemistry. Plant Soil 270:233–247. https://doi.org/10.1007/s11104-004-1523-6 Wang C, Brunner I, Guo W et al (2021a) Effects of long-term water reduction and nitrogen addition on fine roots and fungal hyphae in a mixed mature Pinus koraiensis forest. Plant Soil 467:451–463. https://doi.org/10.1007/s11104-021-05092-8 Wang P, Diao F, Yin L, Huo C (2016) Absorptive roots trait plasticity explains the variation of root foraging strategies in Cunninghamia lanceolata. Environ Exp Bot 129:127–135. https://doi.org/10.1016/j.envexpbot.2016.01.001 Wang Y, Bauke SL, Von Sperber C et al (2021b) Soil phosphorus cycling is modified by carbon and nitrogen fertilization in a long-term field experiment. J Plant Nutr Soil Sci 184:282–293. https://doi.org/10.1002/jpln.202000261 Wang Y, Lambers H (2020) Root-released organic anions in response to low phosphorus availability: recent progress, challenges and future perspectives. Plant Soil 447:135–156. https://doi.org/10.1007/s11104-019-03972-8 Wang Y, Liu J, He Y et al (2025) Growth and Biomass Distribution Responses of Populus tomentosa to Long-Term Water–Nitrogen Coupling in the North China Plain. Plants 14:1833. https://doi.org/10.3390/plants14121833 Wang Y, Xi B, Bloomberg M et al (2015) Response of diameter growth, biomass allocation and N uptake to N fertigation in a triploid Populus tomentosa plantation in the North China Plain: Ontogenetic shift does not exclude plasticity. Eur J For Res 134:889–898. https://doi.org/10.1007/s10342-015-0897-8 Weigelt A, Mommer L, Andraczek K et al (2023) The importance of trait selection in ecology. Nature 618:E29–E30. https://doi.org/10.1038/s41586-023-06148-8 Wolf KL, Noe GB, Ahn C (2013) Hydrologic Connectivity to Streams Increases Nitrogen and Phosphorus Inputs and Cycling in Soils of Created and Natural Floodplain Wetlands. J Environ Qual 42:1245–1255. https://doi.org/10.2134/jeq2012.0466 Xi B, Bloomberg M, Watt MS et al (2016) Modeling growth response to soil water availability simulated by HYDRUS for a mature triploid Populus tomentosa plantation located on the North China Plain. Agric Water Manage 176:243–254. https://doi.org/10.1016/j.agwat.2016.06.017 Xi B, Wang Y, Jia L et al (2013) Characteristics of fine root system and water uptake in a triploid Populus tomentosa plantation in the North China Plain: Implications for irrigation water management. Agric Water Manage 117:83–92. https://doi.org/10.1016/j.agwat.2012.11.006 Xu J, Wang X, Ding Y et al (2020) Effects of irrigation and nitrogen fertilization management on crop yields and long-term dynamic characteristics of water and nitrogen transport at deep soil depths. Soil Tillage Res 198:104536. https://doi.org/10.1016/j.still.2019.104536 Yang J, Zhang S, Zhang J et al (2025a) Incorporating crop rotation into the winter wheat-summer maize system to enhance soil multifunctionality and sustainable grain production in the North China Plain. Field Crops Res 325:109834. https://doi.org/10.1016/j.fcr.2025.109834 Yang Y, Ballauff J, Polle A (2025b) Drought, forest management, and soil chemistry affect fine root biomass and nutrient contents in 150 temperate forests plots. For Ecol Manag 593:122836. https://doi.org/10.1016/j.foreco.2025.122836 Zhang R, Nie L, Huang M et al (2022) Effects of Irrigation and Nitrogen Application on Soil Nutrients in Triploid Populus tomentosa Stands. Forests 13:1046. https://doi.org/10.3390/f13071046 Zhang X, Zhu A, Xin X et al (2018) Tillage and residue management for long-term wheat-maize cropping in the North China Plain: I. Crop yield and integrated soil fertility index. Field Crops Res 221:157–165. https://doi.org/10.1016/j.fcr.2018.02.025 Zhang Y, Cao J, Lu M et al (2024) The origin of bi-dimensionality in plant root traits. Trends Ecol Evol 39:78–88. https://doi.org/10.1016/j.tree.2023.09.002 Zhao J, Sun X, Wang D et al (2024) Responses of Fine Root Morphological and Chemical Traits among Branch Orders to Forest Thinning in Pinus massoniana Plantations. Forests 15:495. https://doi.org/10.3390/f15030495 Zhou Z, Wang Y, An Z et al (2022) Deep root information hidden in the dark: A case study on the 21-m soil profile of Robinia pseudoacacia in the critical zone of the Chinese loess Plateau. CATENA 213:106121. https://doi.org/10.1016/j.catena.2022.106121 Zhu W, Wang Y, Qu G et al (2024a) Cultivar and tree size, but not climate, are principal factors affecting stem quality of Populus tomentosa plantations in the North China Plain. For Ecol Manag 573:122348. https://doi.org/10.1016/j.foreco.2024.122348 Zhu W, Zhao D, Di N et al (2024b) Matching root water uptake patterns to fine root and soil water distributions. Plant Soil 495:499–516. https://doi.org/10.1007/s11104-023-06349-0 Zhu W, Zhou O, Sun Y et al (2023) Effects of stand age and structure on root distribution and root water uptake in fast-growing poplar plantations. J Hydrol 616:128831. https://doi.org/10.1016/j.jhydrol.2022.128831 Zou S, Li D, Di N et al (2022) Stand development modifies effects of soil water availability on poplar fine-root traits: evidence from a six-year experiment. 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17:49:39","extension":"xml","order_by":24,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":180957,"visible":true,"origin":"","legend":"","description":"","filename":"PLSOD25037080structuring.xml","url":"https://assets-eu.researchsquare.com/files/rs-7734837/v1/23b619ef35c4044ca3d8a2f2.xml"},{"id":93963503,"identity":"e80a81e6-4c59-449d-8e38-04e02c881a09","added_by":"auto","created_at":"2025-10-20 17:49:39","extension":"html","order_by":25,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":193139,"visible":true,"origin":"","legend":"","description":"","filename":"earlyproof.html","url":"https://assets-eu.researchsquare.com/files/rs-7734837/v1/4326edd1c4c5f0ee116f93c5.html"},{"id":93963496,"identity":"831a00db-1ae7-4c62-9e62-40f876842483","added_by":"auto","created_at":"2025-10-20 17:49:39","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":22629,"visible":true,"origin":"","legend":"\u003cp\u003eVariation patterns of soil moisture content (SWC) at 0-6 m depth under five treatments (\u003cem\u003en\u003c/em\u003e = 5). NT: No thinning; T50: 50% tree removal; T75: 75% tree removal; T75W: 75% tree removal with water addition; T75WN: 75% tree removal with water and nitrogen addition. Note: Different lowercase letters indicate significant differences among the five treatments (\u003cem\u003eP\u003c/em\u003e\u0026lt;0.05), and ns indicates no significant differences among the five treatments.\u003c/p\u003e","description":"","filename":"Onlinefloatimage1.png","url":"https://assets-eu.researchsquare.com/files/rs-7734837/v1/8933d44c470bbbcd8512723e.png"},{"id":93963367,"identity":"c2fc29a4-2c07-48a3-99d0-2759d87373b3","added_by":"auto","created_at":"2025-10-20 17:41:39","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":23127,"visible":true,"origin":"","legend":"\u003cp\u003eVariation patterns of soil organic matter (SOM) at 0-6 m depth under five treatments (\u003cem\u003en\u003c/em\u003e = 5). NT: No thinning; T50: 50% tree removal; T75: 75% tree removal; T75W: 75% tree removal with water addition; T75WN: 75% tree removal with water and nitrogen addition. Note: Different lowercase letters indicate significant differences among the five treatments (\u003cem\u003eP\u003c/em\u003e\u0026lt;0.05), and ns indicates no significant differences among the five treatments.\u003c/p\u003e","description":"","filename":"Onlinefloatimage2.png","url":"https://assets-eu.researchsquare.com/files/rs-7734837/v1/b6c1b98adda9942c51583774.png"},{"id":93963497,"identity":"58bae10d-3872-415f-b9cb-cae603d36056","added_by":"auto","created_at":"2025-10-20 17:49:39","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":24290,"visible":true,"origin":"","legend":"\u003cp\u003eVariation patterns of soil total nitrogen (TN) at 0-6 m depth under five treatments (\u003cem\u003en\u003c/em\u003e = 5). NT: No thinning; T50: 50% tree removal; T75: 75% tree removal; T75W: 75% tree removal with water addition; T75WN: 75% tree removal with water and nitrogen addition. Note: Different lowercase letters indicate significant differences among the five treatments (\u003cem\u003eP\u003c/em\u003e\u0026lt;0.05), and ns indicates no significant differences among the five treatments.\u003c/p\u003e","description":"","filename":"Onlinefloatimage3.png","url":"https://assets-eu.researchsquare.com/files/rs-7734837/v1/62b35691392d09e81f01624d.png"},{"id":93963369,"identity":"d667ebfc-e579-44d5-9703-1f997b4e8e5f","added_by":"auto","created_at":"2025-10-20 17:41:39","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":23289,"visible":true,"origin":"","legend":"\u003cp\u003eVariation patterns of soil total phosphorus (TP) at 0-6 m depth under five treatments (\u003cem\u003en\u003c/em\u003e = 5). NT: No thinning; T50: 50% tree removal; T75: 75% tree removal; T75W: 75% tree removal with water addition; T75WN: 75% tree removal with water and nitrogen addition. Note: Different lowercase letters indicate significant differences among the five treatments (\u003cem\u003eP\u003c/em\u003e\u0026lt;0.05), and ns indicates no significant differences among the five treatments.\u003c/p\u003e","description":"","filename":"Onlinefloatimage4.png","url":"https://assets-eu.researchsquare.com/files/rs-7734837/v1/253b2b6bd45ac0297938e975.png"},{"id":93963371,"identity":"00274113-3cdc-4dc2-a6ae-05c94d8decd3","added_by":"auto","created_at":"2025-10-20 17:41:39","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":22010,"visible":true,"origin":"","legend":"\u003cp\u003eVariation patterns of soil available phosphorus (AP) at 0-6 m depth under five treatments (\u003cem\u003en\u003c/em\u003e = 5). NT: No thinning; T50: 50% tree removal; T75: 75% tree removal; T75W: 75% tree removal with water addition; T75WN: 75% tree removal with water and nitrogen addition. Note: Different lowercase letters indicate significant differences among the five treatments (\u003cem\u003eP\u003c/em\u003e\u0026lt;0.05), and ns indicates no significant differences among the five treatments.\u003c/p\u003e","description":"","filename":"Onlinefloatimage5.png","url":"https://assets-eu.researchsquare.com/files/rs-7734837/v1/d8a4ab2a6539f0e63b466998.png"},{"id":93963499,"identity":"a190dfef-e9cd-4b94-b081-629c8a9e346a","added_by":"auto","created_at":"2025-10-20 17:49:39","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":360458,"visible":true,"origin":"","legend":"\u003cp\u003eThe plastic responses of mean fine root diameter (MRD, a, e), fine root tissue density (FRTD, b, f), specific root surface area (SRA, c, g), and specific root length (SRL, d, h) to different treatments within the 0-6 m soil depth (\u003cem\u003en\u003c/em\u003e = 5). NT: No thinning; T50: 50% tree removal; T75: 75% tree removal; T75W: 75% tree removal with water addition; T75WN: 75% tree removal with water and nitrogen addition. Note: T or W represents the main effect of thinning or water-nitrogen addition in the two-way ANOVA, D represents the main effect of soil depth, and T×D and W×D represent the interaction effect of the two. The analysis parameters displayed include the \u003cem\u003eF\u003c/em\u003e-statistic (\u003cem\u003eF\u003c/em\u003e) and \u003cem\u003eP\u003c/em\u003e-values (\u003cem\u003eP\u003c/em\u003e).\u003c/p\u003e","description":"","filename":"Onlinefloatimage6.png","url":"https://assets-eu.researchsquare.com/files/rs-7734837/v1/8804b89f10c270f286b12813.png"},{"id":93963374,"identity":"ab3c0ea7-4799-49de-896a-e9a9808f8d9f","added_by":"auto","created_at":"2025-10-20 17:41:39","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":139514,"visible":true,"origin":"","legend":"\u003cp\u003eRedundancy analysis (RDA) of fine root plasticity and soil properties at different depths under thinning and water-nitrogen treatments. (a) Thinning at 0-600 cm; (b) Water-nitrogen addition at 0-600 cm; (c) Thinning at 0-100 cm; (d) Water-nitrogen addition at 0-100 cm; (e) Thinning at 100-600 cm; (f) Water-nitrogen addition at 100-600 cm. Note: SWC for soil water content, SOM for soil organic matter, TN for total nitrogen, TP for total phosphorus, AP for available phosphorus, NH4+-N for ammonium nitrogen, and NO3--N for nitrate nitrogen, MRD stands for mean fine root diameter, FRBD for fine root biomass density, FRLD for fine root length density, FRTD for fine root tissue density, SRA for specific root surface area, and SRL for specific root length.\u003c/p\u003e","description":"","filename":"Onlinefloatimage7.png","url":"https://assets-eu.researchsquare.com/files/rs-7734837/v1/8316f72dec9c763d47f307bf.png"},{"id":93963375,"identity":"527697ca-1bf3-46ae-9260-e429b6eb2e13","added_by":"auto","created_at":"2025-10-20 17:41:39","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":82956,"visible":true,"origin":"","legend":"\u003cp\u003eStructural equation models (SEM) of soil moisture and nutrient properties and fine root development strategies under thinning (a) and water-nitrogen addition (b). Boxes represent latent variables. Blue indicates positive effects, red indicates negative effects, solid lines indicate significant effects (\u003cem\u003eP\u003c/em\u003e\u0026lt;0.05), dashed lines indicate non-significant effects (\u003cem\u003eP\u003c/em\u003e\u0026gt;0.05), and numbers represent correlation coefficients. The goodness of fit of the SEM models was evaluated using the χ2 test and the root mean square error of approximation (RMSEA). Note:\u003csup\u003e ***\u003c/sup\u003e: \u003cem\u003eP\u003c/em\u003e\u0026lt;0.001; \u003csup\u003e**\u003c/sup\u003e: \u003cem\u003eP\u003c/em\u003e\u0026lt;0.01; \u003csup\u003e*\u003c/sup\u003e: \u003cem\u003eP\u003c/em\u003e\u0026lt;0.05.\u003c/p\u003e","description":"","filename":"Onlinefloatimage8.png","url":"https://assets-eu.researchsquare.com/files/rs-7734837/v1/d208237c203463367568ccc4.png"},{"id":95655461,"identity":"34518c28-7357-44bd-a795-b3d738b67d40","added_by":"auto","created_at":"2025-11-11 16:16:14","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1739794,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-7734837/v1/1cfa6d19-1cd0-41a2-aa9c-68f74644300b.pdf"},{"id":93963389,"identity":"ad8c3047-6b62-4225-bd31-6a96332b5a08","added_by":"auto","created_at":"2025-10-20 17:41:40","extension":"docx","order_by":6,"title":"","display":"","copyAsset":false,"role":"supplement","size":125239279,"visible":true,"origin":"","legend":"","description":"","filename":"3.Supplementalmaterials.docx","url":"https://assets-eu.researchsquare.com/files/rs-7734837/v1/0b98f621ad91506cddaa3264.docx"}],"financialInterests":"","formattedTitle":"Vertical dynamics in subterranean ecology: thinning and water-nitrogen additions drive multilayered responses in soil-fine root systems of Populus tomentosa plantations","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eFine roots, as the primary organs responsible for water and nutrient acquisition, play a crucial role in tree growth and ecosystem productivity through their spatial distribution and morphological plasticity (Zou et al., \u003cspan citationid=\"CR78\" class=\"CitationRef\"\u003e2022\u003c/span\u003e; Tan et al., \u003cspan citationid=\"CR55\" class=\"CitationRef\"\u003e2023\u003c/span\u003e). Serving as the key interface between plants and soil, and as the primary conduits for soil resource uptake, fine roots are indispensable for resource acquisition within forest ecosystems (Xi et al. \u003cspan citationid=\"CR66\" class=\"CitationRef\"\u003e2013\u003c/span\u003e; Kramer-Walter and Laughlin \u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e2017\u003c/span\u003e). Their critical ecological functions have sustained long-standing research interest in belowground processes (Jackson et al. \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e1997\u003c/span\u003e; Gill and Jackson \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e2000\u003c/span\u003e; Reich et al. \u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e2014\u003c/span\u003e; Bergmann et al. \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e2020\u003c/span\u003e; Kou et al. \u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e2025\u003c/span\u003e). In arid and semi-arid regions, the exploitation of deep soil resources (\u0026gt;\u0026thinsp;1 m) is particularly crucial for tree survival (Laclau et al. \u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e2001\u003c/span\u003e; Di et al. \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e2018\u003c/span\u003e; Germon et al. \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e2020\u003c/span\u003e; Henriksson et al. \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e2021\u003c/span\u003e; Zhu et al. \u003cspan citationid=\"CR77\" class=\"CitationRef\"\u003e2023\u003c/span\u003e). Nevertheless, studies examining deep soil characteristics and the vertical distribution of fine roots in plantations remain limited.\u003c/p\u003e\u003cp\u003eThe spatial distribution and physiological functions of fine roots are strongly influenced by plantation management practices, such as thinning, irrigation, and nitrogen addition, which regulate belowground resource acquisition strategies by modifying stand structure and soil conditions (He et al. \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e2022\u003c/span\u003e; Pang et al. \u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e2022\u003c/span\u003e; Yang et al. \u003cspan citationid=\"CR69\" class=\"CitationRef\"\u003e2025b\u003c/span\u003e). Thinning reduces stand density and alleviates belowground competition, thereby improving the availability of light, water, and nutrients, which promotes the proliferation of fine roots in the surface soil (\u0026lt;\u0026thinsp;1 m) (Qin et al., \u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e2024\u003c/span\u003e; Zhao et al., \u003cspan citationid=\"CR73\" class=\"CitationRef\"\u003e2024\u003c/span\u003e). However, most existing studies have focused on soil layers within 1 m, leaving a knowledge gap regarding the spatial heterogeneity and functional adaptability of fine roots in deeper layers (Pang et al. \u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e2022\u003c/span\u003e; Zhao et al. \u003cspan citationid=\"CR73\" class=\"CitationRef\"\u003e2024\u003c/span\u003e). Similarly, water and nitrogen addition disrupt the original carbon-water-nitrogen balance, inducing phenotypic plasticity in fine roots (Pregitzer et al. \u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e1993\u003c/span\u003e; Wang et al. \u003cspan citationid=\"CR57\" class=\"CitationRef\"\u003e2021a\u003c/span\u003e; He et al. \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). Therefore, elucidating the dynamics of fine root distribution down to 6 m under water and nitrogen addition is theoretically significant for understanding deep resource acquisition strategies and provides a scientific basis for optimizing water and fertilizer management in plantations.\u003c/p\u003e\u003cp\u003eFine root plasticity represents a core adaptive strategy to environmental heterogeneity, manifested through coordinated adjustments in morphology, biomass, and physiological function (Kong et al. \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e2017\u003c/span\u003e; Zhang et al. \u003cspan citationid=\"CR72\" class=\"CitationRef\"\u003e2024\u003c/span\u003e). This plasticity enables plants to optimize the cost-benefit balance of root system construction, thereby enhancing survival competitiveness (Lima et al. \u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e2010\u003c/span\u003e), and can be conceptualized as an \"investment-benefit\" decision guided by ecological stoichiometry (Peng et al. \u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e2017\u003c/span\u003e). Within plantation management contexts, thinning and water-nitrogen addition influence fine root plasticity via distinct pathways: thinning promotes the expression of fine root potential by reducing competition (Pang et al. \u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e2022\u003c/span\u003e; Zhao et al. \u003cspan citationid=\"CR73\" class=\"CitationRef\"\u003e2024\u003c/span\u003e), whereas water-nitrogen addition directly induces adaptation by modifying resource availability (Coleman \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e2007\u003c/span\u003e; Dou et al. \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e2022\u003c/span\u003e; Zhang et al. \u003cspan citationid=\"CR70\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). Consequently, fine roots adopt a gradient strategy along the soil profile, exhibiting a \"foraging type\" in topsoil (high specific root length, high root tip density) and a \"conservative type\" in deeper soil layers (larger diameter, reduced branching, extended lifespan) (Wang et al. \u003cspan citationid=\"CR58\" class=\"CitationRef\"\u003e2016\u003c/span\u003e; Zhou et al. \u003cspan citationid=\"CR74\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). Although soil water and nutrient conditions are key drivers of fine root dynamics (Ellsworth and Sternberg \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e2019\u003c/span\u003e; Liu et al. \u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e2024\u003c/span\u003e), management interventions can facilitate root penetration into deeper layers by improving subsurface conditions (Pang et al. \u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e2022\u003c/span\u003e; He et al. \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). However, a significant knowledge gap persists regarding the interactive network linking \"management practices-soil environment-fine root plasticity,\" particularly in deep soil layers.\u003c/p\u003e\u003cp\u003eThe North China Plain, one of the largest regions globally with concentrated artificial forest plantations, hosts over 3\u0026nbsp;million hectares of poplar plantations. Nevertheless, this region faces severe ecological challenges, including declining groundwater levels leading to the formation of dry soil layers (Liu et al. \u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e2022\u003c/span\u003e), and progressive depletion of organic matter in surface soils, which reduces fertility (Zhang et al. \u003cspan citationid=\"CR71\" class=\"CitationRef\"\u003e2018\u003c/span\u003e; He et al. \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e2021\u003c/span\u003e; Yang et al. \u003cspan citationid=\"CR68\" class=\"CitationRef\"\u003e2025a\u003c/span\u003e). Investigating the effects of management practices on deep soil-root interactions is therefore of dual significance: theoretically, it advances understanding of deep resource acquisition strategies in arid-region trees; practically, it provides a foundation for sustainable management models emphasizing water conservation, soil nourishment, and efficiency enhancement. In particular, elucidating the response patterns of the soil-root system in deep layers under short-term management interventions can directly inform precision water and fertilizer strategies and promote efficient use of belowground ecological space in \u003cem\u003ePopulus plantations\u003c/em\u003e on the North China Plain.\u003c/p\u003e\u003cp\u003e\u003cem\u003ePopulus tomentosa\u003c/em\u003e, a major native tree species widely cultivated in plantations on the North China Plain, delivers substantial ecological, economic, and social benefits (Zhu et al. \u003cspan citationid=\"CR75\" class=\"CitationRef\"\u003e2024a\u003c/span\u003e; Wang et al. \u003cspan citationid=\"CR61\" class=\"CitationRef\"\u003e2025\u003c/span\u003e). This study assessed \u003cem\u003ePopulus tomentosa\u003c/em\u003e plantations to evaluate the short-term impacts of different management practices on soil properties and fine root plasticity through varying thinning intensities and water-nitrogen addition treatments. The objectives were: (1) to assess the short-term effects of thinning and water-nitrogen addition on soil moisture and nutrient properties across the 0\u0026ndash;6 m soil profile; (2) to elucidate the responses of fine root vertical distribution and morphological plasticity to management interventions; and (3) to investigate the interrelationships between soil properties and fine root traits. We hypothesized that: (1) thinning and water-nitrogen addition would significantly improve soil fertility, with effects exhibiting distinct vertical gradients (He et al. \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e2021\u003c/span\u003e; Liu et al. \u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e2022\u003c/span\u003e); and (2) water-nitrogen addition would induce \"foraging-type\" plasticity in fine roots, whereas thinning would more likely trigger \"conservative-type\" plasticity, with this differential response positively associated with the degree of soil resource heterogeneity (Wang et al. \u003cspan citationid=\"CR58\" class=\"CitationRef\"\u003e2016\u003c/span\u003e; Zhou et al. \u003cspan citationid=\"CR74\" class=\"CitationRef\"\u003e2022\u003c/span\u003e; Pang et al. \u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e2022\u003c/span\u003e; Zhao et al. \u003cspan citationid=\"CR73\" class=\"CitationRef\"\u003e2024\u003c/span\u003e).\u003c/p\u003e"},{"header":"2. Materials and methods","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e\u003ch2\u003e2.1. Study site and experimental plantations\u003c/h2\u003e\u003cp\u003eThe research was conducted in 2023 at a state-owned forest farm located in Gaotang County, Liaocheng City, Shandong Province, China (36\u0026deg; 48\u0026prime;N, 116\u0026deg; 05\u0026prime;E; altitude of approximately 30 m). The area is part of the warm temperate semi-arid monsoon region of the northern hemisphere, characterized by four distinct seasons and abundant sunshine. Rainfall is concentrated in July and August, with an average annual precipitation of 553 mm and an average temperature of 13.9\u0026deg;C. The topography of the study site consists primarily of plains, with the water table located 6 to 9 meters below the surface (Liu et al. \u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). The soil is classified as sandy loam. Before afforestation, the soil contained 3.34 g\u0026middot;kg⁻\u0026sup1; organic matter, 0.23 g\u0026middot;kg⁻\u0026sup1; total nitrogen, and 4.92 mg\u0026middot;kg⁻\u0026sup1; available phosphorus (He et al. \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e2021\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eThe experimental plantation was established in the spring of 2016 using 2-year-old hybrid poplar saplings (\u003cem\u003eP. tomentosa \u0026times; P. bolleana\u003c/em\u003e) \u0026times; (\u003cem\u003eP. alba \u0026times; P. glandulosa\u003c/em\u003e). The saplings had a breast height diameter of 2.12\u0026ndash;3.51 cm (mean 2.68 cm) and a height of 2.60\u0026ndash;3.62 m (mean 3.30 m). Trees were planted at 2 m\u0026times; 3 m spacing, corresponding to a density of 1,666 trees ha⁻\u0026sup1;. Each tree received 70 g of basal fertilizer containing\u0026thinsp;\u0026ge;\u0026thinsp;31% N\u0026thinsp;+\u0026thinsp;P₂O₅ + K₂O (14-12-5) and \u0026ge;\u0026thinsp;15% organic matter (OM).\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec4\" class=\"Section2\"\u003e\u003ch2\u003e2.2 Experimental design\u003c/h2\u003e\u003cp\u003eA split-plot design was employed for the experiment. In April 2022, thinning was performed in the experimental forest with two intensities: alternate row thinning (50% thinning, spacing adjusted to 2 m \u0026times; 6 m, T50) and alternate row and tree thinning (75% thinning, spacing adjusted to 4 m \u0026times; 6 m, T75), along with an unthinned control plot. Within each thinning intensity, three water-nitrogen treatments were applied: irrigation with nitrogen (WN), irrigation only (W), and no irrigation or nitrogen addition (CK). Following thinning, drip irrigation systems (Netafim, Israel) were installed with drippers spaced 50 cm apart at a flow rate of 1.6 L h⁻\u0026sup1;, positioned 30 cm from both sides of the tree row. Irrigation scheduling was based on the quantitative relationship between \u003cem\u003ePopulus tomentosa\u003c/em\u003e growth and soil water availability (Xi et al. \u003cspan citationid=\"CR65\" class=\"CitationRef\"\u003e2016\u003c/span\u003e), triggered when soil water potential at 20 cm below the dripper reached \u0026minus;\u0026thinsp;20 kPa (79% field capacity). Irrigation commenced in early April and ended before the rainy season in late June. Nitrogen was applied as urea (46% N) at 156 g tree⁻\u0026sup1; year⁻\u0026sup1; in six applications at 20-day intervals, corresponding to the species\u0026rsquo; growth rhythm (Wang et al. \u003cspan citationid=\"CR62\" class=\"CitationRef\"\u003e2015\u003c/span\u003e). Routine weeding and pest control were conducted throughout the experimental period. To reduce the effects of light and soil heterogeneity, NT, T50, and T75 areas were divided into blocks along the north-south axis. Within each block, the three water-nitrogen treatments were randomly assigned, with plot sizes ranging from 16 to 64 trees, and protective rows established on the east and west sides.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec5\" class=\"Section2\"\u003e\u003ch2\u003e2.3 Acquisition and measurement of soil and root samples\u003c/h2\u003e\u003cp\u003eFine roots were sampled using soil cores. In October 2023, five standard trees (\u003cem\u003en\u003c/em\u003e\u0026thinsp;=\u0026thinsp;5) were selected per plot under five treatments: NT, T50, T75, T75W, and T75WN. Soil cores (0.0008836 m\u0026sup3;) were collected 30 cm east of each tree to a depth of 6 m, divided into 20 cm increments from 0-100 cm and 100 cm increments from 100\u0026ndash;600 cm, yielding 10 layers. Each soil layer was weighed in the field to obtain fresh weight (W). Three root-free subsamples were taken: one dried at 105\u0026deg;C to determine moisture content (W1 and W2), while the remaining two were air-dried for nutrient analysis and stored at \u0026minus;\u0026thinsp;4\u0026deg;C for inorganic nitrogen measurements. Soil water content (SWC, %) was calculated as:\u003cdiv id=\"Equ1\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ1\" name=\"EquationSource\"\u003e\n$$\\:SWC=\\frac{{W}_{1}-{W}_{2}}{{W}_{2}}\\times\\:100\\%$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e1\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e\u003cp\u003eAll roots were washed via wet sieving to remove soil and organic material. Live roots were separated from dead roots based on color, flexibility, and breakage resistance, and only roots\u0026thinsp;\u0026lt;\u0026thinsp;2 mm in diameter were considered fine roots (Zou et al. \u003cspan citationid=\"CR78\" class=\"CitationRef\"\u003e2022\u003c/span\u003e; Tan et al. \u003cspan citationid=\"CR55\" class=\"CitationRef\"\u003e2023\u003c/span\u003e). Roots were scanned (Epson Twain Pro, 400 dpi) without overlap, and diameter, surface area, volume, and length were measured using WinRHIZO (Regent Instruments Inc., Quebec, Canada). Fine roots were then dried at 75\u0026deg;C to constant weight to determine biomass (FRB). Morphological traits included mean root diameter (MRD), fine root biomass density (FRBD), fine root length density (FRLD), fine root tissue density (FRTD), specific root surface area (SRA), and specific root length (SRL), calculated as:\u003cdiv id=\"Equ2\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ2\" name=\"EquationSource\"\u003e\n$$\\:\\text{F}\\text{R}\\text{B}\\text{D}=FRB/SV$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e2\u003c/div\u003e\u003c/div\u003e\u003cdiv id=\"Equ3\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ3\" name=\"EquationSource\"\u003e\n$$\\:\\text{F}\\text{R}\\text{L}\\text{D}=FRL/SV$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e3\u003c/div\u003e\u003c/div\u003e\u003cdiv id=\"Equ4\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ4\" name=\"EquationSource\"\u003e\n$$\\:\\text{F}\\text{R}\\text{T}\\text{D}=FRB/FRV$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e4\u003c/div\u003e\u003c/div\u003e\u003cdiv id=\"Equ5\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ5\" name=\"EquationSource\"\u003e\n$$\\:\\text{S}\\text{R}\\text{A}=FRA/FRB$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e5\u003c/div\u003e\u003c/div\u003e\u003cdiv id=\"Equ6\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ6\" name=\"EquationSource\"\u003e\n$$\\:\\text{S}\\text{R}\\text{L}=FRL/FRB$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e6\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e\u003cp\u003eWhere SV represents soil volume, FRL is root length, FRV is root volume, and FRA is root surface area.\u003c/p\u003e\u003cp\u003eSoil samples, after root and stone removal, were air-dried and sieved (100-mesh, 0.15 mm; 20-mesh, 0.85 mm) for nutrient analysis. Samples stored at \u0026minus;\u0026thinsp;4\u0026deg;C were used for NO₃⁻-N and NH₄⁺-N determination. Organic carbon was measured by the potassium dichromate external heating method, available phosphorus (AP) via the molybdenum\u0026ndash;antimony anti-colorimetric method, and total nitrogen (TN) and total phosphorus (TP) using concentrated H₂SO₄\u0026ndash;HClO₄ digestion in a fully automated analyzer (SMARTCHEM 450, AMS Alliance, France). SOM was calculated as organic carbon \u0026times; 1.724 (Zhang et al. \u003cspan citationid=\"CR70\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). NO₃⁻-N and NH₄⁺-N were extracted with 1 M KCl and measured via UV spectrophotometry and indophenol blue colorimetry, respectively.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec6\" class=\"Section2\"\u003e\u003ch2\u003e2.4 Data analysis\u003c/h2\u003e\u003cp\u003eData were organized in Excel 2016. Homogeneity of variance and normality were assessed using Levene\u0026rsquo;s test and the Kolmogorov-Smirnov test, respectively. Non-normal data were transformed to meet ANOVA assumptions. Two-way ANOVA evaluated the effects of thinning (T), water and nitrogen addition (W), soil depth (D), and their interactions (T\u0026times;D, W\u0026times;D) on soil properties and fine root traits. One-way ANOVA tested differences among treatments within the same soil layer, with post hoc Tukey HSD comparisons (α\u0026thinsp;=\u0026thinsp;0.05).\u003c/p\u003e\u003cp\u003ePearson correlations were used to assess relationships among fine root traits (SPSS 25, IBM SPSS Statistics, IBM Corporation, Armonk, NY, USA). Redundancy analysis (RDA) was conducted in CANOCO 5 (Microcomputer Power, Ithaca, NY, USA) to evaluate the relative importance of soil properties on fine root variation. Based on the plant economics spectrum theory (Reich et al. \u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e2014\u003c/span\u003e; Kong et al. \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e2017\u003c/span\u003e; Weigelt et al. \u003cspan citationid=\"CR63\" class=\"CitationRef\"\u003e2023\u003c/span\u003e), MRD, FRBD, and FRTD were classified as \"conservative\" strategy indicators, reflecting substantial carbon allocation to robust tissues, whereas FRLD, SRA, and SRL represented the \"acquisitive\" strategy, reflecting minimal carbon investment for rapid resource uptake. Standardized data were used to construct and analyze structural equation models (SEM) in AMOS 20.0 (IBM SPSS Amos, IBM Corporation, Armonk, NY, USA). Figures were generated in Origin 2023 (Origin Lab Corporation, Northampton, MA, USA).\u003c/p\u003e\u003c/div\u003e"},{"header":"3 Result","content":"\u003cdiv id=\"Sec8\" class=\"Section2\"\u003e\u003ch2\u003e\u003cb\u003e3.1 Soil moisture content\u003c/b\u003e\u003c/h2\u003e\u003cp\u003eSoil water content (SWC) exhibited significant depth variability (\u003cem\u003eP\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.05) (Table \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e), with peak SWC observed at depths of 300\u0026ndash;400 cm across all treatment groups (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e1\u003c/span\u003e). The vertical distribution of SWC showed sensitivity to silvicultural measures in the 20\u0026ndash;500 cm soil layer, while the 0\u0026ndash;20 cm and 500\u0026ndash;600 cm layers remained relatively stable (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e1\u003c/span\u003e). Two-way ANOVA revealed significant effects of thinning intensity and water-nitrogen addition on SWC (\u003cem\u003eP\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.05) (Table \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e). Both thinning and water input significantly increased soil water content, with a particularly pronounced effect observed in the 80\u0026ndash;400 cm soil layer (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e1\u003c/span\u003e). However, the combined input of water and nitrogen resulted in a decrease in SWC, with this effect most pronounced in the 100\u0026ndash;200 cm soil layer (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e1\u003c/span\u003e).\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec9\" class=\"Section2\"\u003e\u003ch2\u003e3.2 Soil nutrients\u003c/h2\u003e\u003cp\u003eSoil organic matter (SOM) was significantly influenced by thinning intensity, water-nitrogen addition, and soil depth (\u003cem\u003eP\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.05) (Table \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e). Across the soil profile, SOM exhibited a nonlinear response to thinning intensity, with the highest values observed under moderate thinning (T50) followed by the unthinned control (NT), and the lowest in high-intensity thinning (T75) (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e2\u003c/span\u003e). In low-density stands (T75), the application of water and nitrogen significantly enhanced SOM content, with values following the order T75WN\u0026thinsp;\u0026gt;\u0026thinsp;T75W\u0026thinsp;\u0026gt;\u0026thinsp;T75 (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e2\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eSoil total nitrogen (TN) content was significantly affected by thinning and water-nitrogen addition (\u003cem\u003eP\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.05) (Table \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e). Thinning promoted TN accumulation, whereas water addition reduced TN content; the combined input of water and nitrogen mitigated this reduction. However, this mitigation was significant only in the 0\u0026ndash;20 cm soil layer (\u003cem\u003eP\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.05) (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e3\u003c/span\u003e). Ammonium nitrogen (NH₄⁺-N) and nitrate nitrogen (NO₃⁻-N) contents were significantly affected by thinning intensity, water-nitrogen addition, and soil depth (\u003cem\u003eP\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.05). Among these, the interaction between water-nitrogen addition and soil depth significantly influenced inorganic nitrogen (\u003cem\u003eP\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.05), whereas the interaction between thinning intensity and soil depth was significant only for nitrate nitrogen (\u003cem\u003eP\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.05) (Table \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e). Thinning significantly increased ammonium nitrogen contents in the 0\u0026ndash;40 cm, 200\u0026ndash;300 cm, and 500\u0026ndash;600 cm soil layers (Fig. \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e), and markedly increased nitrate nitrogen contents at 100\u0026ndash;200 cm, 300\u0026ndash;400 cm, and 500\u0026ndash;600 cm (Fig. S2). Water addition promoted the leaching and downward movement of ammonium nitrogen, whereas combined water and nitrogen addition mitigated nitrogen loss and enhanced accumulation in deeper soil layers (300\u0026ndash;600 cm) (Fig. \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e). Nitrate nitrogen exhibited greater stability in the 0\u0026ndash;60 cm surface layer, with no significant differences detected among the T75, T75W, and T75WN treatments. The combined addition of water and fertilizer (T75WN) promoted nitrate nitrogen accumulation in the 60\u0026ndash;100 cm and 300\u0026ndash;500 cm soil layers, thereby increasing the potential risk of leaching (Fig. S2).\u003c/p\u003e\u003cp\u003eSoil total phosphorus (TP) was significantly affected by thinning intensity, soil depth, and their interaction (\u003cem\u003eP\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.05), and was also significantly regulated by water-nitrogen addition (\u003cem\u003eP\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.05) (Table \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e). In the 0\u0026ndash;20 cm, 60\u0026ndash;80 cm, and 100\u0026ndash;200 cm soil layers, TP contents under the T75W and T75WN treatments were significantly higher than under T75, with the most pronounced increase occurring in the 100\u0026ndash;200 cm layer, where TP increased by 196.82% and 59.23%, respectively, compared to T75 (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e4\u003c/span\u003e). Soil available phosphorus (AP) contents were also significantly affected by thinning intensity, soil depth, and their interaction (\u003cem\u003eP\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.05) (Table \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e). High-intensity thinning (T75) significantly reduced AP contents in the 0-400 cm soil layer, whereas in the 200\u0026ndash;400 cm layer, AP contents followed the trend T50\u0026thinsp;\u0026gt;\u0026thinsp;NT\u0026thinsp;\u0026gt;\u0026thinsp;T75 with increasing thinning intensity (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e5\u003c/span\u003e).\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec10\" class=\"Section2\"\u003e\u003ch2\u003e3.3 Fine root plasticity\u003c/h2\u003e\u003cp\u003eThinning treatments were not found to significantly influence the short-term plasticity of most fine root traits. However, soil depth was observed to play a critical role in regulating fine root trait variation across experimental forests subjected to three thinning intensities. Analysis of variance indicated that soil depth significantly influenced several key metrics, including mean root diameter (MRD), fine root biomass density (FRBD), and fine root length density (FRLD) (\u003cem\u003eP\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.05). Peak values of FRBD and FRLD were recorded in the top 0\u0026ndash;20 cm soil layer for all treatments, with mean values of 456.83 g\u0026middot;m⁻\u0026sup3; and 2,655,151.30 cm\u0026middot;m⁻\u0026sup3;, respectively. In contrast, these indices remained relatively stable in the deeper 200\u0026ndash;600 cm soil layer (Fig. S3). MRD initially increased with soil depth before declining, with peak depths for the T50 and T75 treatments occurring in shallower soil layers compared to the control (NT). Under the NT treatment, specific root area (SRA) and specific root length (SRL) attained their maximum values in the 60\u0026ndash;80 cm soil layer, whereas the T50 and T75 treatments peaked in the 80\u0026ndash;100 cm soil layer (Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e6\u003c/span\u003e). These results indicate that fine root distribution and morphology are strongly stratified along the soil profile and are influenced primarily by soil depth rather than short-term thinning intensity.\u003c/p\u003e\u003cp\u003eThe water and nitrogen addition experiment conducted under T75 high-intensity thinning exhibited limited short-term effects on most fine root functional traits. A significant increase in specific root area (SRA) was the only notable change observed (\u003cem\u003eP\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.05). However, the interaction between water-nitrogen addition and soil depth was found to significantly affect the vertical distribution patterns of SRA and SRL (\u003cem\u003eP\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.05) (Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e6\u003c/span\u003e). Fine root morphological traits displayed distinct strategic differentiation under different management interventions. In the T75WN treatment, SRA and SRL remained stable within the 0-400 cm soil layer but increased sharply at 400\u0026ndash;500 cm depth. In contrast, the T75W and T75 treatments primarily promoted increases in SRA and SRL at depths above 200 cm. Notably, the T75W treatment exhibited particularly significant increases in the 20\u0026ndash;40 cm, 60\u0026ndash;80 cm, and 80\u0026ndash;100 cm soil layers (Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e6\u003c/span\u003e). These patterns suggest that water-nitrogen addition interacts with soil depth to modify fine root plasticity, with deeper soil layers responding more prominently under combined treatment, whereas single-factor interventions primarily affect upper soil horizons.\u003c/p\u003e\u003cp\u003ePearson correlation analysis (Fig. S4) further supported these observations, revealing highly significant positive correlations (\u003cem\u003eP\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.001) between FRBD and FRLD across different experimental treatments and soil depths (0-600 cm, 0-100 cm, and 100\u0026ndash;600 cm), indicating a strong consistency in the response of these traits to environmental variation. However, negative correlations were detected between these traits and both SRA and SRL. Notably, correlations between MRD and other fine root traits displayed distinct vertical differentiation across soil layers. Within the 0-100 cm surface layer, MRD was negatively correlated with FRBD, FRLD, and fine root tissue density (FRTD), whereas in the 100\u0026ndash;600 cm deeper layer, MRD was positively correlated with these traits. These results suggest that MRD may fulfill different ecological functions in distinct soil layers, reflecting the adaptive differentiation of fine root traits in response to soil environmental heterogeneity.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec11\" class=\"Section2\"\u003e\u003ch2\u003e\u003cb\u003e3.4 The relationship between fine root plasticity and soil properties\u003c/b\u003e\u003c/h2\u003e\u003cp\u003eFollowing thinning, specific root area (SRA) and specific root length (SRL) were positively correlated with soil water content (SWC), total nitrogen (TN), and nitrate nitrogen (NO₃⁻-N), whereas fine root biomass density (FRBD) and fine root length density (FRLD) were predominantly negatively correlated. Soil water content (SWC) was identified as the primary factor influencing variation in fine root traits after thinning, accounting for 32.9% of the observed variance (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e7\u003c/span\u003ea). Soil depth exerted a significant influence on the interactions between fine root traits and soil factors. In the surface soil layer (0-100 cm), available phosphorus (AP) and SWC were identified as the key regulatory factors, with their correlation patterns closely aligning with those observed across the entire 0-600 cm soil profile (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e7\u003c/span\u003ec). In the deeper soil layers (100\u0026ndash;600 cm), AP emerged as the primary factor regulating fine root development, explaining more than 38% of the observed variance (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e7\u003c/span\u003ee).\u003c/p\u003e\u003cp\u003eThe water-nitrogen addition experiment, conducted after thinning, further altered the interactions between soil properties and fine root traits. Fine root biomass density (FRBD) and fine root length density (FRLD) were positively correlated with total nitrogen (TN) and nitrate nitrogen (NO₃⁻-N), but negatively correlated with ammonium nitrogen (NH₄⁺-N), soil organic matter (SOM), available phosphorus (AP), and SWC. AP and SWC were identified as the primary regulatory factors under the water-nitrogen addition treatments (\u003cem\u003eP\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.05) (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e7\u003c/span\u003eb). In the shallow soil layer (0-100 cm), nitrate nitrogen (NO₃⁻-N) stimulated the development of FRBD and FRLD, whereas SWC predominantly regulated SRA and SRL, accounting for 48.9% of the observed variation (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e7\u003c/span\u003ed). In the deeper soil layers, a distinctly different regulatory pattern was observed, with phosphorus (TP, AP) and soil moisture positively influencing FRBD and FRLD, whereas nitrogen (NO₃⁻-N, NH₄⁺-N) and SOM served as the primary drivers of variations in SRA and SRL (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e7\u003c/span\u003ef).\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec12\" class=\"Section2\"\u003e\u003ch2\u003e3.5 SEM under thinning and water-nitrogen addition experiments\u003c/h2\u003e\u003cp\u003eBoth the thinning and water-nitrogen addition models showed a good goodness-of-fit. The χ\u0026sup2; value for the thinning model was 187.680 (\u003cem\u003ed\u003c/em\u003e\u003csub\u003e\u003cem\u003ef\u003c/em\u003e\u003c/sub\u003e = 69), and for the water-nitrogen addition model, it was 216.398 (\u003cem\u003ed\u003c/em\u003e\u003csub\u003e\u003cem\u003ef\u003c/em\u003e\u003c/sub\u003e = 82). The chi-square to degrees of freedom ratios (χ\u0026sup2;/\u003cem\u003ed\u003c/em\u003e\u003csub\u003e\u003cem\u003ef\u003c/em\u003e\u003c/sub\u003e) for the two models were 2.720 and 2.639, respectively, both below the threshold of 3.000. Furthermore, the RMSEA values for both models were 0.051 and 0.057, respectively, both below the critical value of 0.100, suggesting a good fit to the data (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e8\u003c/span\u003e).\u003c/p\u003e\u003cp\u003ePath analysis revealed that thinning and water-nitrogen addition had opposing regulatory effects on soil properties (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e8\u003c/span\u003e). Thinning significantly increased soil moisture but inhibited soil nutrient availability (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e8\u003c/span\u003ea). In contrast, water and nitrogen addition treatments significantly enhanced soil nutrient content (\u003cem\u003eP\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.001) (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e8\u003c/span\u003eb). Thinning has a limited short-term impact on the investment and development of fine roots, without significant direct regulatory pathways. However, thinning promotes a conservative development strategy that negatively impacts soil nutrient availability, thus inhibiting fine root development (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e8\u003c/span\u003ea). In contrast, water and nitrogen addition promote the acquisitive development of fine roots in \u003cem\u003ePopulus tomentosa\u003c/em\u003e and suppress the conservative development strategy (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e8\u003c/span\u003eb). This shift is primarily mediated through changes in soil nutrient availability.\u003c/p\u003e\u003c/div\u003e"},{"header":"4. Discussions","content":"\u003cdiv id=\"Sec14\" class=\"Section2\"\u003e\u003ch2\u003e4.1 Vertical stratification of soil water and nutrients under management practices\u003c/h2\u003e\u003cp\u003eSoil moisture displayed a clear vertical stratification, with a pronounced peak at 300\u0026ndash;400 cm across all treatments (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e1\u003c/span\u003e). Deeper layers (\u0026gt;\u0026thinsp;200 cm) generally retained more water than the surface (0-100 cm), consistent with earlier studies on the hydrological characteristics of deep soil in the North China Plain (Liu et al. \u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e2022\u003c/span\u003e; Zhu et al. \u003cspan citationid=\"CR77\" class=\"CitationRef\"\u003e2023\u003c/span\u003e). This pattern was largely driven by the vertical distribution of poplar roots, where high root density in the topsoil enhanced water consumption, while reduced root activity at depth allowed moisture accumulation (Zou et al. \u003cspan citationid=\"CR78\" class=\"CitationRef\"\u003e2022\u003c/span\u003e; Tan et al. \u003cspan citationid=\"CR55\" class=\"CitationRef\"\u003e2023\u003c/span\u003e; Zhu et al. \u003cspan citationid=\"CR76\" class=\"CitationRef\"\u003e2024b\u003c/span\u003e). Thinning significantly enhanced soil moisture in the 80\u0026ndash;400 cm layer (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e1\u003c/span\u003e), partially supporting Hypothesis 1. This effect can be attributed to reduced canopy interception, enhanced precipitation infiltration, decreased transpiration after tree removal, and diminished root competition (Molina et al. \u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e2021\u003c/span\u003e; Liu et al. \u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e2025\u003c/span\u003e). Water addition also promoted soil moisture replenishment, but a decline in water content was observed in the 100\u0026ndash;200 cm layer under combined water-nitrogen input, likely due to a fertilization-transpiration feedback in which nitrogen addition stimulated plant growth and water consumption (Samuelson et al. \u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e2008\u003c/span\u003e). Furthermore, the upward shift of root distribution under fertilization (Fig. S3) may have intensified water competition in the middle soil layers, underscoring the need for precise fertilization strategies in water-limited environments.\u003c/p\u003e\u003cp\u003eSoil nutrients exhibited complex vertical responses to management interventions. Under moderate thinning (T50), soil organic matter (SOM) reached its highest level (T50\u0026thinsp;\u0026gt;\u0026thinsp;NT\u0026thinsp;\u0026gt;\u0026thinsp;T75) (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e2\u003c/span\u003e), consistent with the Intermediate Disturbance Hypothesis (Fox \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e2013\u003c/span\u003e), suggesting that moderate thinning optimizes the balance between litter inputs and decomposition. Both irrigation and irrigation-fertilization treatments markedly increased total phosphorus (TP) in the 100\u0026ndash;200 cm layer (up to 196.82%) (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e4\u003c/span\u003e), largely through three mechanisms: transformation of Ca-bound P into more plant-available Fe/Al-bound P (Wolf et al. \u003cspan citationid=\"CR64\" class=\"CitationRef\"\u003e2013\u003c/span\u003e); stimulation of phosphatase activity by fertilization, accelerating organic P mineralization (Gao et al. \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e2020\u003c/span\u003e; Wang et al. \u003cspan citationid=\"CR59\" class=\"CitationRef\"\u003e2021b\u003c/span\u003e); and secretion of root-derived organic acids (e.g., citric and oxalic acids), which chelated metal ions and enhanced P availability (Jiang et al. \u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e2025\u003c/span\u003e). These processes acted synergistically, increasing phosphorus bioavailability following water and nitrogen inputs.\u003c/p\u003e\u003cp\u003eWater input alone reduced total nitrogen (TN), whereas the combined water-nitrogen treatment offset this decline (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e3\u003c/span\u003e), reflecting a balance between water-driven nitrogen leaching and fertilization-driven replenishment. Water addition also facilitated nitrate (NO₃⁻\u0026ndash;N) migration into deeper layers (200\u0026ndash;600 cm) (Fig. S2). Under drip fertigation, rapid urea hydrolysis, nitrification, and transport led to nitrate accumulation at varying depths, with the peak shifting downward under greater irrigation (Xu et al. \u003cspan citationid=\"CR67\" class=\"CitationRef\"\u003e2020\u003c/span\u003e; He et al. \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e2021\u003c/span\u003e). In contrast, ammonium (NH₄⁺\u0026ndash;N), being strongly adsorbed to soil colloids, showed low mobility (Dai et al. \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e2015\u003c/span\u003e). Although nitrate accumulation in deeper layers (300\u0026ndash;600 cm) increased nutrient reserves, it also raised the risk of leaching, requiring careful nutrient management in ecologically sensitive areas.\u003c/p\u003e\u003cp\u003eIn summary, management practices that explicitly account for the vertical stratification of soil water and nutrients are essential to enhance both productivity and sustainability of poplar plantations in water-limited regions.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec15\" class=\"Section2\"\u003e\u003ch2\u003e4.2 Fine root adaptations to vertical resource heterogeneity\u003c/h2\u003e\u003cp\u003eOur results demonstrate that fine root distribution across the 0-600 cm soil profile follows a clear resource-gradient adaptation strategy, consistent with recent advances in fine root ecology (Lu et al. \u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e2025\u003c/span\u003e). The highest FRBD and FRLD values occurred in the 0\u0026ndash;20 cm topsoil layer, supporting the optimal resource allocation hypothesis, as plants preferentially invest in absorptive roots within resource-rich layers (Coleman \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e2007\u003c/span\u003e; Giehl and Von Wiren \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e2014\u003c/span\u003e; Coleman and Aubrey \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e2018\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eForest management practices significantly modified fine root traits. Thinning exhibited relatively limited short-term effects on SRL and SRA but altered MRD, with peaks shifting to shallower layers under high-intensity thinning. This indicates that adaptive trait adjustment requires longer response times, consistent with previous observations on the temporal dynamics of root plasticity (Shen et al. \u003cspan citationid=\"CR51\" class=\"CitationRef\"\u003e2017\u003c/span\u003e). Water addition (T75W) strongly influenced fine root morphology, increasing SRA by up to 655% in the 20\u0026ndash;100 cm layer, highlighting soil water availability as a major driver of morphological plasticity (Vanguelova et al. \u003cspan citationid=\"CR56\" class=\"CitationRef\"\u003e2005\u003c/span\u003e; Li et al. \u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e2022\u003c/span\u003e; Tan et al. \u003cspan citationid=\"CR55\" class=\"CitationRef\"\u003e2023\u003c/span\u003e). In contrast, the combined water-nitrogen treatment (T75WN) enhanced SRA and SRL in the 400\u0026ndash;600 cm layer, suggesting that water regulates root morphology directly via turgor-driven cell division (Spollen and Sharp \u003cspan citationid=\"CR53\" class=\"CitationRef\"\u003e1991\u003c/span\u003e; Alrajhi et al. \u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e2024\u003c/span\u003e), whereas nitrogen indirectly modulates root construction by altering carbon allocation and hormonal signaling (Jing and Strader \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e2019\u003c/span\u003e; Taleski et al. \u003cspan citationid=\"CR54\" class=\"CitationRef\"\u003e2024\u003c/span\u003e). The synergistic effect of water and nitrogen may further involve the activation of specific gene expression pathways in poplar (Shen et al. \u003cspan citationid=\"CR50\" class=\"CitationRef\"\u003e2024\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eA clear structure-function trade-off was also observed, with conservative traits (MRD, FRBD, FRTD) negatively correlated with acquisitive traits (SRA, SRL) (Fig. S4). This finding not only confirms the applicability of the root economic spectrum theory (Reich \u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e2014\u003c/span\u003e; Reich et al. \u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e2014\u003c/span\u003e) to deep soil systems but also extends its relevance. The trade-off reflects an adaptive balance between resource acquisition efficiency and construction costs: conservative strategies invest in thicker, denser roots with longer lifespans, whereas acquisitive strategies favor higher SRL and SRA, enhancing uptake efficiency but reducing stress tolerance (Kong et al. \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e2017\u003c/span\u003e; Weigelt et al. \u003cspan citationid=\"CR63\" class=\"CitationRef\"\u003e2023\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eManagement practices were shown to regulate these trade-offs. Thinning promoted conservative trait expression by reducing nutrient availability, while water-fertilizer addition favored acquisitive strategies, enhancing carbon input and nutrient uptake under resource-limited conditions. These results suggest that plantation management can actively shift fine root strategies along the conservative-acquisitive spectrum, with direct implications for belowground carbon allocation and nutrient cycling.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec16\" class=\"Section2\"\u003e\u003ch2\u003e4.3Trade-offs in fine root strategies under management regimes\u003c/h2\u003e\u003cp\u003eAfter thinning, soil water content (SWC) and available phosphorus (AP) in the 0-100 cm surface layer jointly governed the variation in fine root traits (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e7\u003c/span\u003e). This pattern highlights the strong regulation of shallow root growth by immediate resource availability (Guilbeault-Mayers et al. \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e2024\u003c/span\u003e). As the most dynamic zone for water and nutrient fluctuations, the surface soil is subject to frequent wet-dry cycles that directly influence phosphorus uptake efficiency. Under optimal moisture conditions, phosphorus diffusion rates increase (Shapiro et al. \u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e1960\u003c/span\u003e; Sharpley and Ahuja \u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e1983\u003c/span\u003e), thereby enhancing root nutrient acquisition. In addition, high microbial activity and organic matter turnover in surface soils maintain a continuous supply of AP (Hawkins et al. \u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e2022\u003c/span\u003e; Sica et al. \u003cspan citationid=\"CR52\" class=\"CitationRef\"\u003e2025\u003c/span\u003e), driving fine roots to develop densely branched structures in resource-rich environments to maximize absorptive surface area (Goebel et al. \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e2011\u003c/span\u003e; Du and Wei \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e2018\u003c/span\u003e). By contrast, in deeper layers (100\u0026ndash;600 cm), the relationship between AP and fine root structural traits reversed (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e7\u003c/span\u003e), reflecting the resource constraints typical of these strata. Deep soils are characterized by relatively stable moisture but limited phosphorus availability (Mao et al. \u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e2024\u003c/span\u003e). Given the extremely low mobility of phosphorus, deep roots adapt through morphological adjustments\u0026mdash;such as elongating axes and reducing branching\u0026mdash;rather than relying on opportunistic uptake as in surface soils (Wang and Lambers \u003cspan citationid=\"CR60\" class=\"CitationRef\"\u003e2020\u003c/span\u003e; Lambers \u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e2022\u003c/span\u003e; Barrow and Lambers \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). This adaptive strategy enabled AP to explain up to 38% of trait variation in deep layers of thinned forests, suggesting that phosphorus is a key limiting factor regulating deep root development in \u003cem\u003ePopulus tomentosa\u003c/em\u003e after thinning.\u003c/p\u003e\u003cp\u003eIrrigation and fertigation further increased surface soil water availability in thinned stands (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e1\u003c/span\u003e), alleviating fine root water limitation during growth and development. Accordingly, SWC was the primary factor explaining root trait variation under different water and nitrogen treatments (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e7\u003c/span\u003e). In deeper layers, however, water mobility is restricted and anaerobic conditions are common, reducing the explanatory power of SWC. Under such conditions, roots responded more strongly to leached nitrogen (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e7\u003c/span\u003e), optimizing resource acquisition by enhancing inorganic nitrogen utilization. For instance, under the T75WN treatment, fine roots increased specific root area (SRA) and specific root length (SRL), thereby improving nitrogen uptake efficiency in deep soil (Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e6\u003c/span\u003e). This vertical differentiation reflects the precise adaptive strategies of root systems to resource environments at contrasting soil depths.\u003c/p\u003e\u003cp\u003eThe adaptive shift in fine root construction strategy exhibited significant treatment specificity, primarily driven by dynamic changes in the soil nutrient pool, in full support of Hypothesis 2. Thinning profoundly modified the root environment by improving soil moisture conditions (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e8\u003c/span\u003e). Reduced stand density alleviated belowground competition, enabling plants to optimize spatial distribution patterns (Qin et al. \u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e2024\u003c/span\u003e). Enhanced light penetration and ventilation accelerated litter decomposition (Henneron et al. \u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e2018\u003c/span\u003e; Latterini et al. \u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e2024\u003c/span\u003e), improved soil aggregate stability (Ma et al. \u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e2022\u003c/span\u003e), and reduced transpiration losses, thereby increasing deep soil water availability (Molina et al. \u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e2021\u003c/span\u003e). These synergistic improvements favored the allocation of resources toward persistent root structures, as indicated by the enhancement of conservative traits such as fine root biomass density (FRBD) and mean root diameter (MRD), whereas acquisitive traits (e.g., SRL) were comparatively reduced (Figs.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e6\u003c/span\u003e and S3). This conservative shift suggests an adaptive transition from short-term resource capture toward long-term survival under reduced competition (Giehl and Von Wiren \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e2014\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eIn contrast, water and nitrogen addition treatments primarily regulated root development through chemical pathways (Coleman \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e2007\u003c/span\u003e; He et al. \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). By increasing soil nutrient availability, they lowered the metabolic cost of root exploration (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e8\u003c/span\u003e) while simultaneously influencing moisture distribution and altering plant carbon-nitrogen allocation. Under such conditions, plants exhibited typical \u0026ldquo;resource enrichment\u0026rdquo; responses, favoring acquisitive root architectures with high SRL and SRA (Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e6\u003c/span\u003e), while reducing investment in persistent structures. This pattern reflects a precision foraging strategy that facilitates rapid adaptation to short-term enrichment environments, consistent with Hypothesis 2.\u003c/p\u003e\u003cp\u003eOverall, thinning demonstrated unique advantages in improving soil water conditions and promoting stable root system development, whereas water and nitrogen additions were more effective in enhancing short-term productivity. A rational integration of these management strategies is likely to achieve synergistic optimization of root structural and functional traits, thereby improving both resilience and productivity in \u003cem\u003ePopulus tomentosa\u003c/em\u003e plantations. Future research should focus on elucidating the physiological and ecological mechanisms underlying trade-offs in root development strategies under combined management regimes, which will provide critical foundations for precision forest management frameworks grounded in ecological processes.\u003c/p\u003e\u003c/div\u003e"},{"header":"5. Conclusions","content":"\u003cp\u003eThinning and water-nitrogen addition treatments exerted significant effects on soil moisture, nutrient distribution, and fine root traits, exhibiting pronounced vertical differentiation. Thinning increased soil water content in the 80\u0026ndash;400 cm layer, whereas the synergistic application of water and nitrogen reduced moisture in the 100\u0026ndash;200 cm layer but enhanced total phosphorus by 59.23%. Soil organic matter and total nitrogen were most responsive to management in the surface layers, while inorganic nitrogen accumulation was more prominent in deeper layers (300\u0026ndash;600 cm). Irrigation primarily increased specific root length (SRL) in the middle soil layers, whereas combined water-nitrogen addition promoted specific root area (SRA) in the deep soil (400\u0026ndash;500 cm). Thinning had limited short-term impacts on fine root strategy, whereas water-nitrogen addition shifted root development toward a more acquisitive strategy by improving nutrient availability. These results provide novel insights into plantation root system responses and offer a scientific basis for optimizing sustainable management practices in managed forests.\u003c/p\u003e"},{"header":"Abbreviations","content":"\u003cdiv class=\"DefinitionList\"\u003e\u003cdiv class=\"DefinitionListEntry\"\u003e\u003cdiv class=\"Term\"\u003eAP\u003c/div\u003e\u003cdiv class=\"Description\"\u003e\u003cp\u003eavailable phosphorus\u003c/p\u003e\u003c/div\u003e\u003c/div\u003e\u003cdiv class=\"DefinitionListEntry\"\u003e\u003cdiv class=\"Term\"\u003eFRBD\u003c/div\u003e\u003cdiv class=\"Description\"\u003e\u003cp\u003efine root biomass density\u003c/p\u003e\u003c/div\u003e\u003c/div\u003e\u003cdiv class=\"DefinitionListEntry\"\u003e\u003cdiv class=\"Term\"\u003eFRLD\u003c/div\u003e\u003cdiv class=\"Description\"\u003e\u003cp\u003efine root length density\u003c/p\u003e\u003c/div\u003e\u003c/div\u003e\u003cdiv class=\"DefinitionListEntry\"\u003e\u003cdiv class=\"Term\"\u003eFRTD\u003c/div\u003e\u003cdiv class=\"Description\"\u003e\u003cp\u003efine root tissue density\u003c/p\u003e\u003c/div\u003e\u003c/div\u003e\u003cdiv class=\"DefinitionListEntry\"\u003e\u003cdiv class=\"Term\"\u003eMRD\u003c/div\u003e\u003cdiv class=\"Description\"\u003e\u003cp\u003emean root diameter\u003c/p\u003e\u003c/div\u003e\u003c/div\u003e\u003cdiv class=\"DefinitionListEntry\"\u003e\u003cdiv class=\"Term\"\u003eNT\u003c/div\u003e\u003cdiv class=\"Description\"\u003e\u003cp\u003eno thinning\u003c/p\u003e\u003c/div\u003e\u003c/div\u003e\u003cdiv class=\"DefinitionListEntry\"\u003e\u003cdiv class=\"Term\"\u003eNH₄⁺-N\u003c/div\u003e\u003cdiv class=\"Description\"\u003e\u003cp\u003eammonium nitrogen\u003c/p\u003e\u003c/div\u003e\u003c/div\u003e\u003cdiv class=\"DefinitionListEntry\"\u003e\u003cdiv class=\"Term\"\u003eNO₃⁻-N\u003c/div\u003e\u003cdiv class=\"Description\"\u003e\u003cp\u003enitrate nitrogen\u003c/p\u003e\u003c/div\u003e\u003c/div\u003e\u003cdiv class=\"DefinitionListEntry\"\u003e\u003cdiv class=\"Term\"\u003eT50\u003c/div\u003e\u003cdiv class=\"Description\"\u003e\u003cp\u003e50% tree removal\u003c/p\u003e\u003c/div\u003e\u003c/div\u003e\u003cdiv class=\"DefinitionListEntry\"\u003e\u003cdiv class=\"Term\"\u003eT75\u003c/div\u003e\u003cdiv class=\"Description\"\u003e\u003cp\u003e75% tree removal\u003c/p\u003e\u003c/div\u003e\u003c/div\u003e\u003cdiv class=\"DefinitionListEntry\"\u003e\u003cdiv class=\"Term\"\u003eTN\u003c/div\u003e\u003cdiv class=\"Description\"\u003e\u003cp\u003etotal nitrogen\u003c/p\u003e\u003c/div\u003e\u003c/div\u003e\u003cdiv class=\"DefinitionListEntry\"\u003e\u003cdiv class=\"Term\"\u003eTP\u003c/div\u003e\u003cdiv class=\"Description\"\u003e\u003cp\u003etotal phosphorus\u003c/p\u003e\u003c/div\u003e\u003c/div\u003e\u003cdiv class=\"DefinitionListEntry\"\u003e\u003cdiv class=\"Term\"\u003eSOM\u003c/div\u003e\u003cdiv class=\"Description\"\u003e\u003cp\u003esoil organic matter\u003c/p\u003e\u003c/div\u003e\u003c/div\u003e\u003cdiv class=\"DefinitionListEntry\"\u003e\u003cdiv class=\"Term\"\u003eSRA\u003c/div\u003e\u003cdiv class=\"Description\"\u003e\u003cp\u003especific root surface area\u003c/p\u003e\u003c/div\u003e\u003c/div\u003e\u003cdiv class=\"DefinitionListEntry\"\u003e\u003cdiv class=\"Term\"\u003eSRL\u003c/div\u003e\u003cdiv class=\"Description\"\u003e\u003cp\u003especific root length\u003c/p\u003e\u003c/div\u003e\u003c/div\u003e\u003cdiv class=\"DefinitionListEntry\"\u003e\u003cdiv class=\"Term\"\u003eSWC\u003c/div\u003e\u003cdiv class=\"Description\"\u003e\u003cp\u003esoil water content\u003c/p\u003e\u003c/div\u003e\u003c/div\u003e\u003cdiv class=\"DefinitionListEntry\"\u003e\u003cdiv class=\"Term\"\u003eW\u003c/div\u003e\u003cdiv class=\"Description\"\u003e\u003cp\u003eirrigation only\u003c/p\u003e\u003c/div\u003e\u003c/div\u003e\u003cdiv class=\"DefinitionListEntry\"\u003e\u003cdiv class=\"Term\"\u003eWN\u003c/div\u003e\u003cdiv class=\"Description\"\u003e\u003cp\u003eirrigation with nitrogen application\u003c/p\u003e\u003c/div\u003e\u003c/div\u003e\u003c/div\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003ch2\u003eDeclaration of competing interest\u003c/h2\u003e\u003cp\u003eThe authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.\u003c/p\u003e\u003c/p\u003e\u003ch2\u003eAuthor contributions\u003c/h2\u003e\u003cp\u003eYFW conceived the ideas; YFW, BYX, and LMJ designed the methodology; YFW, XFD, and YZ collected the data; YFW, KW, and DNW analyzed the data; YFW, BYX, and LMJ led the writing of the manuscript. All authors contributed critically to the drafts and gave final approval for publication.\u003c/p\u003e\u003ch2\u003eAcknowledgment\u003c/h2\u003e\u003cp\u003eThis research was supported by the National Key Research and Development Program of China (2024YFD2201004, 2021YFD2201203).\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eAlrajhi A, Alharbi S, Beecham S, Alotaibi F (2024) Regulation of root growth and elongation in wheat. Front Plant Sci 15:1397337. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.3389/fpls.2024.1397337\u003c/span\u003e\u003cspan address=\"10.3389/fpls.2024.1397337\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eBarrow NJ, Lambers H (2022) Phosphate-solubilising microorganisms mainly increase plant phosphate uptake by the effects of pH on root physiology. Plant Soil 476:397\u0026ndash;402. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1007/s11104-021-05240-0\u003c/span\u003e\u003cspan address=\"10.1007/s11104-021-05240-0\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eBergmann J, Weigelt A, Kuyper TW et al (2020) The fungal collaboration gradient dominates the root economics space in plants. SCIENCE ADVANCES\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eColeman M (2007) Spatial and temporal patterns of root distribution in developing stands of four woody crop species grown with drip irrigation and fertilization. Plant Soil 299:195\u0026ndash;213. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1007/s11104-007-9375-5\u003c/span\u003e\u003cspan address=\"10.1007/s11104-007-9375-5\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eColeman MD, Aubrey DP (2018) Stand development and other intrinsic factors largely control fine-root dynamics with only subtle modifications from resource availability. Tree Physiol 38:1805\u0026ndash;1819. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1093/treephys/tpy033\u003c/span\u003e\u003cspan address=\"10.1093/treephys/tpy033\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eDai T, Xi B, Yan X, Jia L (2015) Effects of fertilization method and nitrogen application rate on soil nitrogen vertical migration in a Populus \u0026times; euramericana cv. \u0026lsquo;Guariento\u0026rsquo;plantation. Chin J Appl Ecol 26:1641\u0026ndash;1648. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.13287/j.1001-9332.2015.0047\u003c/span\u003e\u003cspan address=\"10.13287/j.1001-9332.2015.0047\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eDi N, Liu Y, Mead DJ et al (2018) Root-system characteristics of plantation-grown Populus tomentosa adapted to seasonal fluctuation in the groundwater table. Trees 32:137\u0026ndash;149. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1007/s00468-017-1619-2\u003c/span\u003e\u003cspan address=\"10.1007/s00468-017-1619-2\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eDou X, Wang R, Li C et al (2022) Spatial distribution of soil water, plant roots, and water use pattern under different drip fertigation regimes in an apple-soybean intercropping system on the Loess Plateau, China. Agric Water Manage 269:107718. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.agwat.2022.107718\u003c/span\u003e\u003cspan address=\"10.1016/j.agwat.2022.107718\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eDu X, Wei X (2018) Definition of fine roots on the basis of the root anatomy, diameter, and branch orders of one-year old Fraxinus mandshurica seedlings. J Res 29:1321\u0026ndash;1327. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1007/s11676-017-0561-x\u003c/span\u003e\u003cspan address=\"10.1007/s11676-017-0561-x\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eEllsworth PZ, Sternberg LSL (2019) Linking soil nutrient availability, fine root production and turnover, and species composition in a seasonally dry plant community. Plant Soil 442:49\u0026ndash;63. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1007/s11104-019-04153-3\u003c/span\u003e\u003cspan address=\"10.1007/s11104-019-04153-3\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eFox JW (2013) The intermediate disturbance hypothesis should be abandoned. Trends Ecol Evol 28:86\u0026ndash;92. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.tree.2012.08.014\u003c/span\u003e\u003cspan address=\"10.1016/j.tree.2012.08.014\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eGao D, Bai E, Li M et al (2020) Responses of soil nitrogen and phosphorus cycling to drying and rewetting cycles: A meta-analysis. Soil Biol Biochem 148:107896. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.soilbio.2020.107896\u003c/span\u003e\u003cspan address=\"10.1016/j.soilbio.2020.107896\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eGermon A, Laclau J-P, Robin A, Jourdan C (2020) Tamm Review: Deep fine roots in forest ecosystems: Why dig deeper? For Ecol Manag 466:118135. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.foreco.2020.118135\u003c/span\u003e\u003cspan address=\"10.1016/j.foreco.2020.118135\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eGiehl RFH, Von Wiren N (2014) Root Nutrient Foraging. Plant Physiol 166:509\u0026ndash;517. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1104/pp.114.245225\u003c/span\u003e\u003cspan address=\"10.1104/pp.114.245225\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eGill RA, Jackson RB (2000) Global patterns of root turnover for terrestrial ecosystems. New Phytol 147:13\u0026ndash;31. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1046/j.1469-8137.2000.00681.x\u003c/span\u003e\u003cspan address=\"10.1046/j.1469-8137.2000.00681.x\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eGoebel M, Hobbie SE, Bulaj B et al (2011) Decomposition of the finest root branching orders: linking belowground dynamics to fine-root function and structure. Ecol Monogr 81:89\u0026ndash;102. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1890/09-2390.1\u003c/span\u003e\u003cspan address=\"10.1890/09-2390.1\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eGuilbeault-Mayers X, Lambers H, Lalibert\u0026eacute; E (2024) Coordination among leaf and fine-root traits along a strong natural soil fertility gradient. Plant Soil. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1007/s11104-024-06740-5\u003c/span\u003e\u003cspan address=\"10.1007/s11104-024-06740-5\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eHawkins JMB, Vermeiren C, Blackwell MSA et al (2022) The effect of soil organic matter on long-term availability of phosphorus in soil: Evaluation in a biological P mining experiment. Geoderma 423:115965. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.geoderma.2022.115965\u003c/span\u003e\u003cspan address=\"10.1016/j.geoderma.2022.115965\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eHe Y, Li G, Xi B et al (2022) Fine root plasticity of young Populus tomentosa plantations under drip irrigation and nitrogen fertigation in the North China Plain. Agric Water Manage 261:107341. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.agwat.2021.107341\u003c/span\u003e\u003cspan address=\"10.1016/j.agwat.2021.107341\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eHe Y, Xi B, Li G et al (2021) Influence of drip irrigation, nitrogen fertigation, and precipitation on soil water and nitrogen distribution, tree seasonal growth and nitrogen uptake in young triploid poplar (Populus tomentosa) plantations. Agric Water Manage 243:106460. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.agwat.2020.106460\u003c/span\u003e\u003cspan address=\"10.1016/j.agwat.2020.106460\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eHenneron L, Chauvat M, Archaux F et al (2018) Plasticity in leaf litter traits partly mitigates the impact of thinning on forest floor carbon cycling. Funct Ecol 32:2777\u0026ndash;2789. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1111/1365-2435.13208\u003c/span\u003e\u003cspan address=\"10.1111/1365-2435.13208\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eHenriksson N, Lim H, Marshall J et al (2021) Tree water uptake enhances nitrogen acquisition in a fertilized boreal forest \u0026ndash; but not under nitrogen-poor conditions. New Phytol 232:113\u0026ndash;122. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1111/nph.17578\u003c/span\u003e\u003cspan address=\"10.1111/nph.17578\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eJackson RB, Mooney HA, Schulze E-D (1997) A global budget for fine root biomass, surface area, and nutrient contents. Proc Natl Acad Sci USA 94:7362\u0026ndash;7366. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1073/pnas.94.14.7362\u003c/span\u003e\u003cspan address=\"10.1073/pnas.94.14.7362\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eJiang Z, Wang C, Sun H et al (2025) Soil phosphorus availability modulates root exudation under long-term nitrogen fertilization. Soil Tillage Res 254:106699. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.still.2025.106699\u003c/span\u003e\u003cspan address=\"10.1016/j.still.2025.106699\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eJing H, Strader LC (2019) Interplay of Auxin and Cytokinin in Lateral Root Development. IJMS 20:486. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.3390/ijms20030486\u003c/span\u003e\u003cspan address=\"10.3390/ijms20030486\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eKong D, Wang J, Zeng H et al (2017) The nutrient absorption\u0026ndash;transportation hypothesis: optimizing structural traits in absorptive roots. New Phytol 213:1569\u0026ndash;1572. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1111/nph.14344\u003c/span\u003e\u003cspan address=\"10.1111/nph.14344\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eKou L, Ma N, Freschet GT et al (2025) Iterative effects: a new paradigm for root dynamics. Trends Plant Sci S1360138525001633. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.tplants.2025.05.018\u003c/span\u003e\u003cspan address=\"10.1016/j.tplants.2025.05.018\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eKramer-Walter KR, Laughlin DC (2017) Root nutrient concentration and biomass allocation are more plastic than morphological traits in response to nutrient limitation. Plant Soil 416:539\u0026ndash;550. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1007/s11104-017-3234-9\u003c/span\u003e\u003cspan address=\"10.1007/s11104-017-3234-9\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eLaclau J-P, Arnaud M, Bouillet J-P, Ranger J (2001) Spatial distribution of Eucalyptus roots in a deep sandy soil in the Congo: relationships with the ability of the stand to take up water and nutrients. Tree Physiol 21:129\u0026ndash;136. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1093/treephys/21.2-3.129\u003c/span\u003e\u003cspan address=\"10.1093/treephys/21.2-3.129\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eLambers H (2022) Phosphorus Acquisition and Utilization in Plants. Annu Rev Plant Biol 73:17\u0026ndash;42. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1146/annurev-arplant-102720-125738\u003c/span\u003e\u003cspan address=\"10.1146/annurev-arplant-102720-125738\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eLatterini F, Horodecki P, Dyderski MK et al (2024) Mediterranean beech forests: Thinning and ground-based skidding are found to alter microarthropod biodiversity with no effect on litter decomposition rate. For Ecol Manag 569:122160. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.foreco.2024.122160\u003c/span\u003e\u003cspan address=\"10.1016/j.foreco.2024.122160\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eLi T, Ren J, He W et al (2022) Anatomical structure interpretation of the effect of soil environment on fine root function. Front Plant Sci 13:993127. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.3389/fpls.2022.993127\u003c/span\u003e\u003cspan address=\"10.3389/fpls.2022.993127\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eLima TTS, Miranda IS, Vasconcelos SS (2010) Effects of water and nutrient availability on fine root growth in eastern Amazonian forest regrowth, Brazil. New Phytol 187:622\u0026ndash;630. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1111/j.1469-8137.2010.03299.x\u003c/span\u003e\u003cspan address=\"10.1111/j.1469-8137.2010.03299.x\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eLiu J, Li D, Fern\u0026aacute;ndez J-E et al (2022) Variations in water-balance components and carbon stocks in poplar plantations with differing water inputs over a whole rotation: implications for sustainable forest management under climate change. Agric For Meteorol 320:108958. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.agrformet.2022.108958\u003c/span\u003e\u003cspan address=\"10.1016/j.agrformet.2022.108958\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eLiu J, Zhou L, Wang D et al (2024) The Effect of Balanced N, P and K Fertilization on Fine Root Traits and Soil Properties in Sapindus mukorossi. Forests 15:94. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.3390/f15010094\u003c/span\u003e\u003cspan address=\"10.3390/f15010094\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eLiu Y, Li Y, Gao G et al (2025) Effects of thinning on soil water content and water use characteristics of artificial forest in the Loess Plateau of China. CATENA 254:108997. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.catena.2025.108997\u003c/span\u003e\u003cspan address=\"10.1016/j.catena.2025.108997\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eLu M, Wang S, Malhotra A et al (2025) A continental scale analysis reveals widespread root bimodality. Nat Commun 16:5281. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1038/s41467-025-60055-2\u003c/span\u003e\u003cspan address=\"10.1038/s41467-025-60055-2\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eMa Y, Cheng X, Kang F, Han H (2022) Effects of thinning on soil aggregation, organic carbon and labile carbon component distribution in Larix principis-rupprechtii plantations in North China. Ecol Ind 139:108873. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.ecolind.2022.108873\u003c/span\u003e\u003cspan address=\"10.1016/j.ecolind.2022.108873\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eMao L, Miao Y, Ge Y et al (2024) Effects of different afforestation years on soil moisture and nutrient content in Maxian Mountain of the Loess Plateau. Sci Rep 14:16194. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1038/s41598-024-66408-z\u003c/span\u003e\u003cspan address=\"10.1038/s41598-024-66408-z\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eMolina AJ, Gonz\u0026aacute;lez-Sanchis M, Biel C, Del Campo AD (2021) Ecohydrological turnover in overstocked Aleppo pine plantations: Does the effect of thinning, in relation to water, persist at the mid-term? For Ecol Manag 483:118781. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.foreco.2020.118781\u003c/span\u003e\u003cspan address=\"10.1016/j.foreco.2020.118781\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003ePang Y, Tian J, Yang H et al (2022) Responses of Fine Roots at Different Soil Depths to Different Thinning Intensities in a Secondary Forest in the Qinling Mountains, China. Biology 11:351. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.3390/biology11030351\u003c/span\u003e\u003cspan address=\"10.3390/biology11030351\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003ePeng Y, Guo D, Yang Y (2017) Global patterns of root dynamics under nitrogen enrichment. Global Ecol Biogeogr 26:102\u0026ndash;114. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1111/geb.12508\u003c/span\u003e\u003cspan address=\"10.1111/geb.12508\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003ePregitzer KS, Hendrick RL, Fogel R (1993) The demography of fine roots in response to patches of water and nitrogen. New Phytol 125:575\u0026ndash;580. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1111/j.1469-8137.1993.tb03905.x\u003c/span\u003e\u003cspan address=\"10.1111/j.1469-8137.1993.tb03905.x\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eQin J, Lu J, Peng Y et al (2024) Thinning-induced decrease in fine root biomass, but not other fine root traits in global forests. J Environ Manage 370:122938. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.jenvman.2024.122938\u003c/span\u003e\u003cspan address=\"10.1016/j.jenvman.2024.122938\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eReich PB (2014) The world-wide \u0026lsquo;fast\u0026ndash;slow\u0026rsquo; plant economics spectrum: a traits manifesto. J Ecol 102:275\u0026ndash;301. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1111/1365-2745.12211\u003c/span\u003e\u003cspan address=\"10.1111/1365-2745.12211\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eReich PB, Luo Y, Bradford JB et al (2014) Temperature drives global patterns in forest biomass distribution in leaves, stems, and roots. Proc Natl Acad Sci USA 111:13721\u0026ndash;13726. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1073/pnas.1216053111\u003c/span\u003e\u003cspan address=\"10.1073/pnas.1216053111\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eSamuelson LJ, Farris MG, Stokes TA, Coleman MD (2008) Fertilization but not irrigation influences hydraulic traits in plantation-grown loblolly pine. For Ecol Manag 255:3331\u0026ndash;3339. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.foreco.2008.02.014\u003c/span\u003e\u003cspan address=\"10.1016/j.foreco.2008.02.014\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eShapiro RE, Armiger WH, Fried M (1960) The Effect of Soil Water Movement vs. Phosphate Diffusion On Growth and Phosphorus Content of Corn and Soybeans. Soil Sci Soc Amer J 24:161\u0026ndash;164. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.2136/sssaj1960.03615995002400030012x\u003c/span\u003e\u003cspan address=\"10.2136/sssaj1960.03615995002400030012x\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eSharpley AN, Ahuja LR (1983) A diffusion interpretation of soil phosphorus desorption. Soil Sci 135:322\u0026ndash;326. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1097/00010694-198305000-00008\u003c/span\u003e\u003cspan address=\"10.1097/00010694-198305000-00008\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eShen J, Li X, Jiang L et al (2024) Molecular Mechanisms of Poplar Adaptation to Water\u0026ndash;Fertilizer Coupling: Insights from Transcriptomic and Metabolomic Analyses. Forests 15:1967. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.3390/f15111967\u003c/span\u003e\u003cspan address=\"10.3390/f15111967\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eShen Y, Wang N, Cheng R et al (2017) Short-Term Effects of Low Intensity Thinning on the Fine Root Dynamics of Pinus massoniana Plantations in the Three Gorges Reservoir Area, China. Forests 8:428. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.3390/f8110428\u003c/span\u003e\u003cspan address=\"10.3390/f8110428\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eSica P, Sitzmann TJ, M\u0026uuml;ller-St\u0026ouml;ver D, Magid J (2025) Strategic placement of mineral and biobased fertilizers for optimizing phosphorus use efficiency: A comprehensive review. Soil Use Manag 41:e70039. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1111/sum.70039\u003c/span\u003e\u003cspan address=\"10.1111/sum.70039\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eSpollen WG, Sharp RE (1991) Spatial Distribution of Turgor and Root Growth at Low Water Potentials. Plant Physiol 96:438\u0026ndash;443. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1104/pp.96.2.438\u003c/span\u003e\u003cspan address=\"10.1104/pp.96.2.438\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eTaleski M, Jin M, Chapman K et al (2024) CEP hormones at the nexus of nutrient acquisition and allocation, root development, and plant\u0026ndash;microbe interactions. J Exp Bot 75:538\u0026ndash;552. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1093/jxb/erad444\u003c/span\u003e\u003cspan address=\"10.1093/jxb/erad444\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eTan J, Yu W, Liu Y et al (2023) Response of Fine-Root Traits of Populus tomentosa to Drought in Shallow and Deep Soil. Forests 14:951. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.3390/f14050951\u003c/span\u003e\u003cspan address=\"10.3390/f14050951\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eVanguelova EI, Nortcliff S, Moffat AJ, Kennedy F (2005) Morphology, biomass and nutrient status of fine roots of Scots pine (Pinus sylvestris) as influenced by seasonal fluctuations in soil moisture and soil solution chemistry. Plant Soil 270:233\u0026ndash;247. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1007/s11104-004-1523-6\u003c/span\u003e\u003cspan address=\"10.1007/s11104-004-1523-6\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eWang C, Brunner I, Guo W et al (2021a) Effects of long-term water reduction and nitrogen addition on fine roots and fungal hyphae in a mixed mature Pinus koraiensis forest. Plant Soil 467:451\u0026ndash;463. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1007/s11104-021-05092-8\u003c/span\u003e\u003cspan address=\"10.1007/s11104-021-05092-8\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eWang P, Diao F, Yin L, Huo C (2016) Absorptive roots trait plasticity explains the variation of root foraging strategies in Cunninghamia lanceolata. Environ Exp Bot 129:127\u0026ndash;135. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.envexpbot.2016.01.001\u003c/span\u003e\u003cspan address=\"10.1016/j.envexpbot.2016.01.001\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eWang Y, Bauke SL, Von Sperber C et al (2021b) Soil phosphorus cycling is modified by carbon and nitrogen fertilization in a long-term field experiment. J Plant Nutr Soil Sci 184:282\u0026ndash;293. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1002/jpln.202000261\u003c/span\u003e\u003cspan address=\"10.1002/jpln.202000261\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eWang Y, Lambers H (2020) Root-released organic anions in response to low phosphorus availability: recent progress, challenges and future perspectives. Plant Soil 447:135\u0026ndash;156. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1007/s11104-019-03972-8\u003c/span\u003e\u003cspan address=\"10.1007/s11104-019-03972-8\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eWang Y, Liu J, He Y et al (2025) Growth and Biomass Distribution Responses of Populus tomentosa to Long-Term Water\u0026ndash;Nitrogen Coupling in the North China Plain. Plants 14:1833. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.3390/plants14121833\u003c/span\u003e\u003cspan address=\"10.3390/plants14121833\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eWang Y, Xi B, Bloomberg M et al (2015) Response of diameter growth, biomass allocation and N uptake to N fertigation in a triploid Populus tomentosa plantation in the North China Plain: Ontogenetic shift does not exclude plasticity. Eur J For Res 134:889\u0026ndash;898. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1007/s10342-015-0897-8\u003c/span\u003e\u003cspan address=\"10.1007/s10342-015-0897-8\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eWeigelt A, Mommer L, Andraczek K et al (2023) The importance of trait selection in ecology. Nature 618:E29\u0026ndash;E30. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1038/s41586-023-06148-8\u003c/span\u003e\u003cspan address=\"10.1038/s41586-023-06148-8\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eWolf KL, Noe GB, Ahn C (2013) Hydrologic Connectivity to Streams Increases Nitrogen and Phosphorus Inputs and Cycling in Soils of Created and Natural Floodplain Wetlands. J Environ Qual 42:1245\u0026ndash;1255. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.2134/jeq2012.0466\u003c/span\u003e\u003cspan address=\"10.2134/jeq2012.0466\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eXi B, Bloomberg M, Watt MS et al (2016) Modeling growth response to soil water availability simulated by HYDRUS for a mature triploid Populus tomentosa plantation located on the North China Plain. Agric Water Manage 176:243\u0026ndash;254. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.agwat.2016.06.017\u003c/span\u003e\u003cspan address=\"10.1016/j.agwat.2016.06.017\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eXi B, Wang Y, Jia L et al (2013) Characteristics of fine root system and water uptake in a triploid Populus tomentosa plantation in the North China Plain: Implications for irrigation water management. Agric Water Manage 117:83\u0026ndash;92. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.agwat.2012.11.006\u003c/span\u003e\u003cspan address=\"10.1016/j.agwat.2012.11.006\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eXu J, Wang X, Ding Y et al (2020) Effects of irrigation and nitrogen fertilization management on crop yields and long-term dynamic characteristics of water and nitrogen transport at deep soil depths. Soil Tillage Res 198:104536. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.still.2019.104536\u003c/span\u003e\u003cspan address=\"10.1016/j.still.2019.104536\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eYang J, Zhang S, Zhang J et al (2025a) Incorporating crop rotation into the winter wheat-summer maize system to enhance soil multifunctionality and sustainable grain production in the North China Plain. Field Crops Res 325:109834. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.fcr.2025.109834\u003c/span\u003e\u003cspan address=\"10.1016/j.fcr.2025.109834\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eYang Y, Ballauff J, Polle A (2025b) Drought, forest management, and soil chemistry affect fine root biomass and nutrient contents in 150 temperate forests plots. For Ecol Manag 593:122836. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.foreco.2025.122836\u003c/span\u003e\u003cspan address=\"10.1016/j.foreco.2025.122836\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eZhang R, Nie L, Huang M et al (2022) Effects of Irrigation and Nitrogen Application on Soil Nutrients in Triploid Populus tomentosa Stands. Forests 13:1046. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.3390/f13071046\u003c/span\u003e\u003cspan address=\"10.3390/f13071046\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eZhang X, Zhu A, Xin X et al (2018) Tillage and residue management for long-term wheat-maize cropping in the North China Plain: I. Crop yield and integrated soil fertility index. Field Crops Res 221:157\u0026ndash;165. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.fcr.2018.02.025\u003c/span\u003e\u003cspan address=\"10.1016/j.fcr.2018.02.025\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eZhang Y, Cao J, Lu M et al (2024) The origin of bi-dimensionality in plant root traits. Trends Ecol Evol 39:78\u0026ndash;88. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.tree.2023.09.002\u003c/span\u003e\u003cspan address=\"10.1016/j.tree.2023.09.002\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eZhao J, Sun X, Wang D et al (2024) Responses of Fine Root Morphological and Chemical Traits among Branch Orders to Forest Thinning in Pinus massoniana Plantations. Forests 15:495. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.3390/f15030495\u003c/span\u003e\u003cspan address=\"10.3390/f15030495\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eZhou Z, Wang Y, An Z et al (2022) Deep root information hidden in the dark: A case study on the 21-m soil profile of Robinia pseudoacacia in the critical zone of the Chinese loess Plateau. CATENA 213:106121. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.catena.2022.106121\u003c/span\u003e\u003cspan address=\"10.1016/j.catena.2022.106121\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eZhu W, Wang Y, Qu G et al (2024a) Cultivar and tree size, but not climate, are principal factors affecting stem quality of \u003cem\u003ePopulus tomentosa\u003c/em\u003e plantations in the North China Plain. For Ecol Manag 573:122348. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.foreco.2024.122348\u003c/span\u003e\u003cspan address=\"10.1016/j.foreco.2024.122348\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eZhu W, Zhao D, Di N et al (2024b) Matching root water uptake patterns to fine root and soil water distributions. Plant Soil 495:499\u0026ndash;516. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1007/s11104-023-06349-0\u003c/span\u003e\u003cspan address=\"10.1007/s11104-023-06349-0\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eZhu W, Zhou O, Sun Y et al (2023) Effects of stand age and structure on root distribution and root water uptake in fast-growing poplar plantations. J Hydrol 616:128831. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.jhydrol.2022.128831\u003c/span\u003e\u003cspan address=\"10.1016/j.jhydrol.2022.128831\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eZou S, Li D, Di N et al (2022) Stand development modifies effects of soil water availability on poplar fine-root traits: evidence from a six-year experiment. Plant Soil 480:165\u0026ndash;184. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1007/s11104-022-05568-1\u003c/span\u003e\u003cspan address=\"10.1007/s11104-022-05568-1\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"Thinning effect, water and nitrogen addition effect, soil nutrient characteristics, fine root traits, Populus","lastPublishedDoi":"10.21203/rs.3.rs-7734837/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-7734837/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003ch2\u003ePurpose\u003c/h2\u003e\u003cp\u003eFine roots and soil properties show distinct vertical patterns, reflecting their coupled responses to thinning and water-fertilizer management. This study aimed to elucidate soil-root interactions and provide insights for the sustainable management of plantations.\u003c/p\u003e\u003ch2\u003eMethods\u003c/h2\u003e\u003cp\u003eA split-plot design was established with three thinning intensities (no thinning, moderate, heavy) and three water-nitrogen treatments (control, irrigation, irrigation\u0026thinsp;+\u0026thinsp;nitrogen). Soil profiles (0\u0026ndash;6 m) and fine roots were sampled to assess changes in soil moisture, nutrient dynamics, and fine root traits. Multivariate analyses were used to identify key regulatory drivers.\u003c/p\u003e\u003ch2\u003eResults\u003c/h2\u003e\u003cp\u003eSoil water content (SWC) peaked at 300\u0026ndash;400 cm and was sensitive to management in the 20\u0026ndash;500 cm layer. Thinning and irrigation increased SWC, whereas water-nitrogen input reduced it in mid-depth layers. Thinning enhanced nitrogen accumulation, while water-nitrogen input offset nitrogen loss but increased nitrate leaching risk. Fine root biomass density was highest in the 0\u0026ndash;20 cm layer, with deeper layers remaining stable. Water-nitrogen addition increased specific root area, with SWC as the main determinant after thinning, and both phosphorus and SWC driving responses under fertilization.\u003c/p\u003e\u003ch2\u003eConclusion\u003c/h2\u003e\u003cp\u003eThinning improved water availability but constrained nutrients, while water-nitrogen input shifted fine roots toward an acquisitive strategy, highlighting management-specific soil-root interactions.\u003c/p\u003e","manuscriptTitle":"Vertical dynamics in subterranean ecology: thinning and water-nitrogen additions drive multilayered responses in soil-fine root systems of Populus tomentosa plantations","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-10-20 17:41:34","doi":"10.21203/rs.3.rs-7734837/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"b277bc7b-1f34-4beb-9867-a3351df386c9","owner":[],"postedDate":"October 20th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[],"tags":[],"updatedAt":"2025-11-10T14:41:26+00:00","versionOfRecord":[],"versionCreatedAt":"2025-10-20 17:41:34","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-7734837","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-7734837","identity":"rs-7734837","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}