Decrease of soil total and organic phosphorus with ECM tree dominance in a subtropical mountainous forest

Decrease of soil total and organic phosphorus with ECM tree dominance in a subtropical mountainous forest | 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 Decrease of soil total and organic phosphorus with ECM tree dominance in a subtropical mountainous forest Mi Yang, Mengzhen Lu, Long Chen, Qiuxiang Tian, Xiaorong Wang, and 3 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-9088640/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 Aims Arbuscular mycorrhizal (AM) and ectomycorrhizal (ECM) plants differ in phosphorus uptake strategies and litter quality, but their effects on soil total P (TP) and P fractions along natural dominance gradients remain unclear, especially in P-limited subtropical forests. This study aimed to clarify how tree mycorrhizal associations regulate soil P dynamics. Methods We established 35 plots across an AM and ECM tree dominance gradient. Soil P fractions were determined using the modified Hedley method. Plant traits, litter quality, soil properties, and microbial communities were analyzed to identify driving mechanisms. Results Soil TP, organic P (Po), and primary mineral P (HCl-Pi) decreased significantly with increasing ECM tree dominance. TP content was negatively related to the basal area of trees and the thickness of forest floor, and positively correlated with community-weighted-mean litter P content. Reduced TP was mainly driven by stronger aboveground P translocation and lower litter P input. Declines in Po and HCl-Pi were attributed to accelerated mineralization and dissolution mediated by soil enzymes, phosphorus-solubilizing microorganisms (PSM), and low pH. Conclusions Increasing ECM dominance accelerates soil P depletion and may exacerbate P limitation, but ECM plants can sustain P availability and forest productivity via enhanced organic P mineralization. This study highlights the key role of mycorrhizal fungi in regulating soil P cycling and alleviating P limitation in subtropical forests. Mycorrhizal type Hedley P fractionation acid phosphatase phosphorus-solubilizing microorganism subtropical forest Badagongshan Mountain Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Introduction Phosphorus (P) is a crucial nutrient element in plant physiology and metabolism, playing a key role in plant growth and forest productivity (Du et al. 2020 ; Lambers et al. 2015 ). As a nutrient primarily derived from soil parent material (Porder et al. 2007 ; Song et al. 2024 ) and not replenished through atmospheric inputs (Hedley et al. 1982 ), soil P content and availability generally decline with the increase in soil age and weathering degree(Yang and Post, 2011 ). Consequently, P frequently limits forest productivity in highly weathered tropical and subtropical forests (Manu et al. 2024 ). In soils, P occurs in multiple forms, including (1) inorganic P (Pi) adsorbed on the surface of minerals or organic matter, (2) organic P (Po) present as nucleic acids, phospholipids, and phytate, (3) primary mineral P retained within parent materials, and (4) residual P occluded within mineral lattices (Turner et al. 2007 ; He et al. 2023 ; Yan et al. 2023 ). Most of these P fractions exhibit low solubility and must be transformed into soluble inorganic forms before being taken up by plants. Characterizing soil total P and its various fractions is crucial for elucidating the pathway and capacity of soil P supply, providing key insights into nutrient availability and forest productivity. Most terrestrial plants can form symbiotic relationships with either arbuscular mycorrhizal fungi (AM) or ectomycorrhizal fungi (ECM) to cope with nutrient limitations (Liu et al. 2018 ). Previous studies have shown that AM-associated trees are predominantly distributed in tropical and subtropical forests characterized by low P availability, whereas ECM-associated trees are more common in cold regions with lower nitrogen availability and relatively higher P availability. Consequently, at global and regional scales, AM-dominated forests generally exhibit higher soil P content than ECM-dominated forests, largely due to differences in soil genesis processes (Phillips et al. 2013 ). In subtropical montane forests, however, large areas are characterized by the coexistence of AM and ECM tree species (Lin et al. 2017 ), and previous studies have reported that soil P availability differs between AM and ECM-dominated stands (Su et al. 2024 ). For example, Lian et al. ( 2024 ) reported higher soil total P (TP) and available P contents under ECM stands compared with AM stands, whereas significantly lower soil TP and available P contents in ECM-dominated stands were also observed (Qi et al. 2022 ). These inconsistent findings indicate that how AM and ECM-associated trees regulate soil P content and overall P cycling in subtropical forests remains unclear. Given the potential shifts in the distribution of AM and ECM plants under future climate change (Bennett and Classen, 2020; Jo et al. 2019), it is necessary to investigate how mycorrhizal types of tree species shape soil TP content and P fractions to better predict and explain the maintenance of forest productivity. Apart from pedogenic processes, soil TP content is mainly controlled by biological processes, including aboveground P translocation and the replenishment of soil P via litterfall (Turner et al. 2007 ; Yan et al. 2023 ). First, the widely observed negative relationship between aboveground biomass and soil TP suggests that a substantial proportion of P source can be sequestrated in living biomass, and the translocation of P into aboveground vegetation can lead to a decline in soil TP pools (An and Li, 2013 ). Meanwhile, long-term litter manipulation experiments have shown that litter addition increases soil TP content, while litter removal results in significant loss of multiple soil P fractions (Turner et al.2024). These findings indicate that litter, through continuous nutrient return, plays an important role in replenishing the soil P pool. Compared to AM trees, ECM trees typically exhibit a more conservative growth and nutrient strategy. Consequently, ECM trees tend to have longer lifespans and greater biomass accumulation (Averill et al. 2014 ; Shao et al. 2023). In addition, ECM trees tend to produce lower-quality litter, leading to slower decomposition rates and lower nutrient return efficiency (Phillips et al. 2013 ). Collectively, these traits can potentially influence soil TP content and its spatial variability. Fine roots and their associated mycorrhizal fungi can mobilize soil P either directly or indirectly through the recruitment of phosphorus-solubilizing microorganisms (PSM). The mechanisms include secreting organic acids to activate adsorbed Pi, by producing phosphatases to mineralize Po, and by releasing protons to hydrolyze mineral P (Hinsinger, 2001 ; Richardson and Simpson, 2011 ). All of these processes have been reported in both types of mycorrhizal trees (Smith and Read, 2010 ; Phillips et al. 2013 ), thereby potentially influencing soil P fractions. Previous studies have reported that AM-associated trees preferentially rely on Pi sources (Smith et al. 2011 ; Hinsinger et al. 2011 ), whereas ECM-associated trees tend to utilize Po (Liu et al. 2018 ). Thus, ECM-dominated forests generally exhibit higher phosphatase activity than AM-dominated forests and stronger Po mineralization (Cao et al. 2023 ; Wang et al. 2023 ), whereas AM-dominated forests show greater depletion of inorganic Pi fractions (Phillips et al. 2013 ; Smith et al. 2011 ). However, whether these P-use preferences can modulate the relative contributions and the intensity of the P mobilization process, and consequently soil P fractions, remains largely unexplored. In addition to the P- mobilization processes described above, soil properties such as soil organic carbon (SOC), soil microbial biomass (MBC), and soil pH have also been shown to influence soil P fractions. For example, higher SOC provides more adsorption sites, facilitating the retention of P on organic matter (Gérard, 2016 ; Richardson et al. 2009 ). MBC reflects the size and activity of the soil microbial community and plays a key role in regulating P transformation through microbial uptake, turnover, and enzyme-mediated mineralization processes (Richardson et al. 2009 ). Lower soil pH can accelerate the dissolution of primary mineral P (Phillips et al. 2013 ). ECM-dominated forests have been reported to exhibit higher SOC content, lower MBC content, and lower pH (Yang et al. 2025 ), which may further influence soil P fractions. To explore the influence of mycorrhizal types of trees on soil TP content and P fractions, we established 35 plots with a natural gradient of ECM tree dominance (quantified as the percentage of total basal area contributed by ECM trees) in a subtropical montane forest. Soil TP and different P fractions were determined based on the modified Hedley-P method (Tiessen 1993 ). Plant traits, litter quality, soil physico-chemical properties, and microbial communities were determined to assess their influence on soil P fractions. Due to the stronger above-ground P translocation and lower nutrient return via litterfall in ECM-dominated plots, we hypothesize that (1) soil TP content will decrease with higher ECM tree dominance. Since AM plants may preferentially utilize Pi and ECM plants preferentially utilize Po, we hypothesize (2) soil Pi content will increase, and Po content will decrease with the higher ECM tree dominance. We also hypothesize (3) HCl-Pi content will decrease with increasing ECM tree dominance due to the lower soil pH. Materials and Methods Site description This study was conducted in Badagongshan National Nature Reserve (29.66°–29.83° N, 109.70°–110.16° E) in Hunan Province (Fig. S1 ). This region features a humid subtropical climate, with an average annual precipitation of 2100 mm and a mean annual temperature of 10.7°C. The soil type is mountainous yellow-brown soil with a loamy texture. The vegetation type is evergreen-deciduous broadleaf mixed forest. The dominant species include Cyclobalanopsis multinervis , CaCarpinus turczaninovii , Cornus kousa , Fagus lucida , Castanea seguinii , and Sorbus folgneri . In May 2023, 35 plots (20 m × 20 m) were established. Each plot was located at similar elevations with identical parent materials. Within each plot, woody plants with a diameter at breast height (DBH) greater than 1 cm were recorded. Mycorrhizal types were assigned for each species based on existing literature and fine root staining analysis (Soudzilovskaia et al. 2020 ). The ECM tree dominance varied from 2 to 89% across the plots, and reflected a natural mycorrhizal gradient. Sample collection and processing Within each plot, four random points were selected to collect soil samples. Before collecting the mineral soil, we use a ruler to measure the thickness of forest floor. After removing the forest floor, we collected the mineral soil samples from the 0–10 cm layer using a 5.72 cm diameter soil auger. Samples from the four points within the same plot were combined to form a composite soil sample. Fresh soil samples were sieved through a 2 mm mesh to remove visible plant debris and stones. A portion of the sieved samples was stored at -80°C for microbial sequencing, and another portion was kept at 4°C for the determination of biological parameters, and the remaining samples were air-dried for the analysis of soil physico-chemical properties and P fractions. Data of SOC, total nitrogen (TN), dissolved organic carbon (DOC), dissolved nitrogen (DN), MBC, soil acid phosphatase (ACP) activity, soil pH, and soil texture were obtained from Lu et al. ( 2025 ). In autumn 2023, fresh leaf litter was collected from the 28 dominant tree species (accounting for 78% of the total basal area) in the study area. The litter samples were dried in a 65°C to constant weight in the laboratory. After pulverization, litter carbon, nitrogen, P, and manganese (Mn) contents were determined. In this study, community weighted-mean litter carbon (CWM-LC) , nitrogen (CWM-LN) , and P (CWM-LP) contents were calculated to indicate litter quality at community level (Chen et al. 2022 ). Weighted-mean litter Mn ( CWM-LMn ) content was calculated as an indicator of rhizosphere carboxylate concentration and used as a proxy of the mobilization of soil adsorbed Pi (Yan et al. 2025 ). Determination of soil P fractions Soil P fractions were extracted using a modified sequential extraction procedure developed by Hedley et al. ( 1982 ) and modified by Tiessen ( 1993 ). In brief, 1 g of soil was weighed into a 50-mL centrifuge tube, and sequentially extracted with solutions of increasing strength, including 0.5 mol L − 1 NaHCO 3 , 0.1 mol L − 1 NaOH, and 1 mol L − 1 HCl. Each extraction was performed at 25°C with shaking for 16 h. After extraction, the supernatant was collected by centrifugation and filtered through a 0.45-µm membrane. For NaHCO 3 and NaOH extracts, Pi and total P (Pt) were determined, respectively. Pi was measured by the molybdenum-antimony colorimetric method at 880 nm, and Pt was quantified after digestion with sulfate-sulfuric acid and analyzed using the same colorimetric procedure. Po content was calculated as the difference between Pt and Pi. The HCl extract was directly analyzed for Pi (HCl-Pi) due to the negligible Po content. The extraction residue was washed with deionized water, dried and ground, and then 0.2g of the residue was digested with concentrated H 2 SO 4 -HClO 4 to measure residual P. In this study, NaHCO 3 -Pi represents the most labile Pi fractions. NaOH-Pi represents moderate Pi fractions and is primarily adsorbed on soil minerals or co-precipitated with organic matter. Po represents the sum of NaHCO 3 -Po and NaOH-Po. HCl-Pi corresponds to primary mineral P. Soil TP content was also determined following H 2 SO 4 -HClO 4 digestion using a microwave digestion system (ETHOS ONE, Milestone, Milan, Italy). Analysis of soil microbial properties Soil bacterial and fungal community composition was analyzed using high-throughput sequencing techniques, and DNA was extracted from the frozen soil samples using the MoBio PowerSoil DNA Isolation Extraction Kit (MoBio Laboratories, USA). For bacterial community, the bacterial 16S rRNA gene was amplified using the primer pair: 799F (5' AACMGGATTAGATACCCKG-3') and 1193R (5'-ACGTCATCCCCACCTTCC-3'). For fungal community, the fungal internal transcribed spacer 1(ITS1) region was amplified using the following primer pair: ITS1F (5'-CTTGGTCATTTAGAGGAAGTAA-3') and ITS2-2043R (5'-GCTGCGTTCTTCATCGATGC-3'). The PCR products were purified and sequenced on the same Illumina MiSeq platform (PE-250). Raw sequencing data were processed using QIIME2 (version 2023.2) for quality control and denoising, and sequences were clustered into operational taxonomic units (OTUs) at 97% similarity. OTUs were then annotated taxonomically using the SILVA (for bacteria) and UNITE (for fungi) databases. Microbial groups known to possess functions related to Po mineralization (e.g., phosphatase production) or Pi solubilization (e.g., organic acid secretion) were identified according to (Richardson et al. 2009 ) and hereafter referred to as OPSM (organic phosphorus-solubilizing microorganism) and IPSM (inorganic phosphorus-solubilizing microorganism), respectively. The main genera of IPSM and OPSM in bacterial and fungal communities are listed in Table S1 . The relative abundance of IPSM and OPSM was calculated separately for bacterial and fungal communities. Statistical analyses Linear regression analysis was employed to examine the effects of ECM tree dominance on the contents of soil TP and P fractions, as well as plant, litter, soil physico-chemical, and microbial properties. Pearson correlation analysis was performed to examine relationships between soil P fractions and all these environmental variables. Additionally, partial least squares path modelling (PLS-PM) was conducted to assess the direct and indirect drivers of soil P fractions. The strengths and directions of these relationships were assessed based on the standardized path coefficients. The predictive power of the model was then evaluated using the coefficient of determination (R 2 ) for endogenous latent variables, and the goodness-of-fit of the model was assessed using the Goodness of Fit (GoF) index (Sanchez, 2013). All data processing in this study was performed using R 4.5.2 software. Result Plant, litter, soil, and microbial properties across the gradient of ECM tree dominance ECM tree dominance was positively correlated with basal area of trees, thickness of forest floor, and APH activity, and negatively correlated with tree diversity, the contents of litter CWM-LC, CWM-LN, CWM-LP, soil pH, and bacterial and fungi diversities. Moreover, ECM tree dominance had no significant correlations with the contents of litter CWM-LMn, SOC, and MBC (Fig. S2). The relative abundance of IPSM and OPSM in bacterial communities averaged 10.4% and 4.2%, respectively, and both increased significantly with increasing ECM tree dominance (Fig. 1 ). IPSM in bacterial community were primarily represented by Candidatus and Bradyrhizobium , and OPSM were dominated by Burkholderia (Figure S3). In contrast, the relative abundance of IPSM and OPSM in fungal community averaged 1.0% and 0.2%, respectively, and both showed no significant relationship with ECM tree dominance (Fig. 1 ). IPSM in fungal community were dominated by Piloderma , and OPSM was dominated by Trichoderma (Figure. S3). Soil TP and P fractions across the gradient of ECM tree dominance In this subtropical forest, soil TP ranged from 226.4 to 747.0 µg g − 1 , with NaHCO 3 -Pi, NaOH-Pi, Po, HCl-Pi, and residual P contributing averagely 0.48%, 14.4%, 52.3%, 1.2% and 31.4% of TP, respectively (Figure S5). With the increasing of ECM tree dominance, the contents of NaHCO 3 -Pi, NaOH-Pi, and residual P showed no significant change, while the contents of Po, HCl-Pi, and TP decreased significantly (Fig. 2 ). Controlling factors of soil TP and P fractions Soil TP was negatively correlated with basal area of trees and the thickness of forest floor, and showed a slightly positive correlation with litter CWM-LP content (Fig. 3 ). NaOH-Pi content was positively correlated with MBC, SOC, and the relative abundances of IPSM and OPSM in fungi community, but negatively correlated with the thickness of forest floor and the relative abundances of IPSM and OPSM in bacterial community (Fig. 4 and Fig. S4). The contents of Po, HCl-Pi and residual P were negatively correlated with basal area of trees and the relative abundance of IPSM and OPSM in bacterial community, but positively correlated with MBC, fungal richness and bacterial richness (Fig. 4 and Fig. S4). Additionally, Po content was positively correlated with litter CWM-LP and negatively correlated with the thickness of forest floor and APH activity. NaHCO 3 -Pi content was positively correlated with MBC, SOC and the relative abundances of IPSM and OPSM in fungi community, and negatively correlated with soil pH. Additionally, NaHCO 3 -Pi content was positively correlated with the contents of NaOH-Pi, HCl-Pi, and residual P (Fig. S6). Since Po and HCl-Pi contents were also correlated with ECM tree dominance, we further use PLS-PM analysis to examine direct and indirect drivers of Po and HCl-Pi. Litter quality, APH, and microbial diversity could significantly regulate Po concentrations directly, whereas ECM tree dominance could decrease the Po concentrations indirectly by reducing litter quality and by increasing the relative abundance of bacterial OPSM, which then enhances the APH activity. The relative abundance of bacterial IPSM could regulate HCl-Pi content directly, whereas ECM tree dominance could indirectly decrease HCl-Pi content by increasing the relative abundance of bacterial IPSM. The PLS-PM models explained 56.97% and 49.04% of the variance in the Po and HCl-Pi, respectively (Fig. 5 ). Discussion Effect of ECM tree dominance on soil TP content Consistent with our first hypothesis, soil TP content decreased significantly with the increase in ECM tree dominance (Fig. 2 ). Since all soils were collected under similar climatic conditions and parent material, the soil formation processes are expected to be broadly comparable among sites. Therefore, the pronounced variation in TP content highlights the strong influence of biogeochemical processes. Our findings indeed revealed soil TP content significant correlated with basal area of trees, forest floor thickness, and litter CWM-LP, confirming the critical role of biogeochemical processes in shaping soil P dynamics. Firstly, the negative relationship between TP content and basal area of trees was aligned with previous studies (Qi et al. 2022 ). Phosphorus, as a key element of plant cell structure, nucleic acids, and energy metabolism-related substances, is largely sequestered within plant biomass (Marschner, 1995 ). As biomass increases, large amounts of P are stored in aboveground tissues, resulting in a significant reduction in soil TP content. In this study, plots with higher ECM tree dominance exhibited significantly greater aboveground biomass, indicated by the basal area of trees (Fig. S2). The higher biomass is mainly due to the conservative growth strategy and increased lifespan of ECM tree species, resulting in increased biomass storage (Averill et al. 2014 ). Therefore, the increase in ECM tree dominance enhanced above-ground P translocation, ultimately leading to a significant decline in soil TP (Fig. 3 ). Secondly, soil TP content exhibited a weakly positive correlation with litter P content (Fig. 3 ). This was primarily because litter decomposition serves as a crucial replenishment for soil P pools. Litter with higher P content can release greater amounts of P source during decomposition, thereby strengthening soil P replenishment (Turner et al. 2007 ). In this study, ECM-dominated plots had lower litter CWM-LP content than AM-dominated plots. This finding was consistent with previous studies showing that litter of ECM-associated trees was characterized by significantly lower P content and higher C:P ratios compared with AM-associated trees (Phillips et al. 2013 ; Averill et al. 2019 ), resulting in reduced P return via litterfall. Such reductions in litter P inputs can constrain P replenishment to surface soils, thereby accelerating soil TP depletion with increasing ECM dominance. Additionally, the forest floor serves as a crucial P “reservoir” in ecosystems (Peng et al. 2020 ). Larger forest floor biomass implies that more P is sequestered in undecomposed or partially decomposed litter, delaying its return into the soil P pool. Consequently, forest floor biomass is also considered an important factor regulating soil TP content (Spohn, 2020 ). Litter from ECM tree species is typically low in nutrients and contains a higher proportion of recalcitrant components (Cheeke et al. 2017 ), resulting in a slower decomposition rate. Moreover, ECM fungi have a nutrient competition relationship with soil saprophytic microorganisms (Fernandez et al. 2020 ), further inhibiting litter decomposition. As a result, ECM tree-dominant plots accumulated higher forest floor biomass (Fig. S2), reducing the amount of P returned into the soil and ultimately contributing to lower soil TP content. Based on the variations of soil P fractions, the decline in soil TP content was mainly driven by reductions in soil Po and HCl-Pi, while NaOH-Pi, and residual P remained largely unchanged across the ECM gradient (Fig. 2 ). These results are consistent with our second and third hypotheses, except that the trend in NaOH-Pi was opposite to our expectation. Effect of ECM tree dominance on soil Po content Our results indicated that Po was the predominant P fraction in this subtropical forest (Fig. S5). Since soil Po is largely derived from plant residue as well as microbial biomass and its necromass (Wang et al. 2023 ), variations in litter P content and microbial processes are expected to influence soil Po dynamics. Previous studies have shown that higher litter P content can promote soil Po accumulation (Chi et al. 2022 ). Consistent with this, soil Po content in our study was positively correlated with litter CWM-LP content (Fig. 4 ), suggesting that plant-derived P inputs play an important role in regulating soil Po accumulation (Turner et al. 2007 ; Spohn, 2020 ). As ECM tree dominance increased, litter CWM-LP content declined (Fig. S2), which might partly explain the low soil Po in the ECM-dominated plots. Soil microorganisms represent a major pool of Po in soils, and increases in MBC are therefore commonly accompanied by higher soil Po content due to enhanced microbial assimilation and immobilization of P sources (Joergensen et al. 2008). Consistent with previous studies, our results also demonstrated a significant positive relationship between MBC and soil Po content (Fig. 4 ). Moreover, we observed a significant positive relationship between soil microbial diversity and Po content (Fig. S4). Higher microbial diversity may accelerate microbial turnover and necromass production, thereby enhancing the formation and retention of microbially derived Po in soils and ultimately contributing to greater soil Po accumulation (Liang et al. 2017 ; Delgado-Baquerizo et al. 2016 ). The mineralization of Po is a key pathway for soil P mobilization (Wang et al. 2023 ), and can potentially reduce soil Po accumulation by accelerating its conversion to inorganic forms. Po mineralization relies on phosphatase activity, which originates mainly from plant roots and PSM (Ragot et al. 2015 ). In this study, APH activity was significantly negatively correlated with soil Po content, indicating the enhanced phosphatase activity accelerated Po mineralization. Moreover, APH activity increased with the increasing ECM tree dominance (Fig. S2) and was mainly accompanied by a higher relative abundance of bacterial OPSM (Fig. 1 ), which likely further contributed to the low soil Po observed in the ECM-dominated plots. Previous studies have confirmed that ECM tree species can reduce soil Po pool by regulating microbial activity and enzyme activity associated with Po mineralization (Khan et al. 2024 ; Rosling et al. 2016 ). Notably, compared to PSMs in fungal community, bacterial PSMs exhibited greater relative abundance and functional contribution (Fig. 1 ), suggesting that bacteria may play a more important role in soil Po mineralization. This finding aligns with previous studies indicating that bacteria are the primary source of phosphatases in forest soils (Fraser et al. 2015 ; Ragot et al. 2015 ). Furthermore, we observed a higher abundance of PSM in ECM-dominant plots (Fig. 1 ), suggesting that ECM tree species can recruit PSM more efficiently than AM trees. However, the underlying mechanisms driving differences in PSM abundance between AM and ECM-dominated plots remain unclear and warrant further investigation. Effect of ECM tree dominance of soil on NaOH-Pi Soil NaOH-Pi is another important P fraction. Both SOC and MBC contents exerted a strong positive influence on NaOH-Pi. First, organic matter contains abundant functional groups, such as carboxyl, phenolic, and hydroxyl groups, which can directly bind phosphate through ligand exchange or indirectly enhance Pi retention by forming organo–mineral complexes, thereby enhancing the stability of NaOH-Pi and reducing its mobility and leaching risk (Gérard, 2016 ; Guppy et al. 2005 ; Kaiser and Guggenberger, 2000 ). Previous studies in temperate and tropical forest soils have shown that higher SOC is associated with great concentrations of NaOH-Pi due to enhanced organo–mineral interactions and stronger P sorption capacity (Hou et al. 2014 ). Second, soil microorganisms produce a wide range of extracellular phosphatases that hydrolyze Po compounds, and the released Pi can be rapidly adsorbed onto soil mineral surface or co-precipitated with organic matter, potentially enhancing soil NaOH-Pi pool (Bünemann et al. 2012 ). In this study, MBC was significantly positively correlated with NaOH-Pi (Fig. 4 ), suggesting that microbial-mediated Po mineralization likely contributed to the formation and maintenance of NaOH-Pi pool. Similar relationships have been reported in forest and grassland soils, where higher MBC and activity are associated with greater Pi sorption capacity and more inorganic P fractions (Richardson and Simpson, 2011 ). Previous studies have shown that soil acidity plays a dominant role in regulating NaOH-Pi content by modifying the abundance, surface charge, and reactivity of iron and aluminum (Fe/Al) oxides (Zarif et al. 2020 ). Under acidic conditions, increased protonation of mineral surfaces enhances phosphate sorption through ligand exchange, thereby increasing the proportion of NaOH-Pi (Hinsinger, 2001 ; Gérard, 2016 ; Walker and Syers, 1976 ; Barrow, 2017 ). However, in the present study, soil pH showed no significant relationship with NaOH-Pi. This discrepancy was likely attributable to the uniformly strong acidity of soils (mean pH 4.15) across the study area. Under this condition, minerals form stable iron-aluminum-phosphate complexes via coordination exchange mechanisms (Sanyal and De Datta, 1991 ), which chemically protect the NaOH-Pi pool and make it less susceptible to soil pH (Cross and Schlesinger, 1995 ). Since ECM tree dominance showed no significant effects on SOC and MBC contents (Fig. S3), NaOH-Pi content had no significant correlation with ECM tree dominance. Some previous studies have suggested that AM trees preferentially utilize Pi sources (Lian et al. 2024 ), which could potentially reduce soil Pi pool. However, in our study, litter CWM-LMn, an indicator of rhizosphere carboxylate concentration and a proxy of the mobilization of soil adsorbed Pi, showed no significant correlation with ECM tree dominance (Fig. S2). Additionally, the relative abundance of IPSM, an indicator of microorganisms capable of mobilizing inorganic P, was positively, rather than negatively, correlated with ECM tree dominance (Fig. S2). Thus, in our study, we did not observe preferential Pi utilization by AM plants. Overall, ECM tree dominance did not influence soil Pi dynamics. Effect of ECM tree dominance on soil primary mineral P content Primary mineral P accounted for only 1.2% of TP in this subtropical forest, indicating an advanced degree of soil weathering. We observed a significant negative relationship between basal area of trees and HCl-Pi content (Fig. 4 ), consistent with previous studies (Tie et al. 2024 ). Greater basal area of trees indicates intensified root growth and mycorrhizal fungal activities, which can accelerate the weathering of primary phosphate minerals through the release of organic acids and protons (Kohler et al. 2015 ; van Hees et al. 2006 ), thereby reducing the soil primary mineral P pool. The relative abundance of IPSM also showed a negative role on soil primary mineral P content. This pattern likely reflects the ability of IPSM to produce low-molecular-weight organic acids (Tian et al. 2021 ). These acids can chelate metal cations (e.g., Ca²⁺, Fe³⁺, and Al³⁺) and destabilize mineral P-bearing phases, thereby facilitating the dissolution and depletion of mineral P. The dissolution of primary mineral P can further be reinforced under acidic soil conditions, as lower pH enhances mineral solubility and weakens the structural stability of primary P minerals (Hinsinger, 2001 ; Gérard, 2016 ). In our study, the basal area of trees and the relative abundance of IPSM increased, and soil pH decreased markedly with increasing ECM tree dominance (Fig. S2). Together, these coordinated changes resulted in a significant decline in mineral P along the ECM tree dominance gradient (Fig. 2 ). Overall, we propose a conceptual framework illustrating how ECM tree dominance reshapes soil P pools through coupled changes in P translocation, replenishment, and mobilization (Fig. 6 ). Soil TP declined in ECM-dominated plots primarily due to enhanced aboveground P translocation and reduced P replenishment via litterfall. Specifically, soil Po decreased with increasing ECM tree dominance, driven by both lower Po inputs from litterfall and elevated APH activity, which accelerates Po mineralization. In addition, mineral P was progressively depleted as ECM tree dominance increased, likely due to promoted mineral P dissolution via organic acid release and enhanced soil acidification. In contrast, NaOH-Pi remained relatively stable across the ECM tree dominance gradient. Generally, mobilized P from NaOH-Pi, Po, and HCl-Pi contributes to the labile P pool, known as NaHCO 3 -Pi. We further observed that NaHCO 3 -Pi content was positively correlated with all P fractions except Po (Fig. S6), further highlighting the influence of different P fractions on soil P availability. Moreover, although Po content was low in ECM-dominated plots, enhanced phosphatase activity can maintain Po mineralization and thus sustain soil P supply for plants. Conclusion Our study demonstrated that increasing dominance of ECM tree species leads to a significant decline in soil TP content in the subtropical mountainous forest. The reduction in TP was mainly caused by the enhanced aboveground P translocation and reduced P replenishment via litterfall. The decline in TP was mainly reflected in reductions of Po and primary mineral P, whereas NaOH-Pi and residual P remained relatively stable. The decline in Po was primarily driven by higher Po mineralization and lower Po replenishment from litterfall, while the decline in primary mineral P was mainly due to acid-driven dissolution. Our findings highlight that shifts in tree mycorrhizal dominance can substantially alter soil P fractions by regulating both plant-derived P inputs and microbially mediated P mobilization. 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. No author has any financial or non-financial interests that are directly or indirectly related to the subject matter of this research. Funding This work was financially supported by the National Natural Science Foundation of China (Grant No. 32371736; Grant No. 32171599). Acknowledgments We acknowledge with gratitude the assistance and support provided by the Administration of Badagongshan National Nature Reserve. Special thanks are extended to Wei Guorong and Qin Biwu from Tianpingshan Station of Badagongshan for their valuable help in sample plot selection and the sampling process. Data availability statement The raw and processed data supporting the findings of this study are available from the corresponding author (Q. Tian, [email protected] ) upon reasonable request. References An H, Li GQ (2013) Effects of grazing on plant biomass and soil nutrient in desert steppe. Plant Nutr Fertil Sci 19:705–712 Averill C, Bhatnagar JM, Dietze MC, Pearse WD, Kivlin SN (2019) Global imprint of mycorrhizal fungi on whole-plant nutrient economics. Proc. Natl. Acad. Sci. U.S.A. 116: 23163–23168. https://doi.org/10.1073/pnas.1906655116 Averill C, Turner BL, Finzi AC (2014) Mycorrhiza-mediated competition between plants and decomposers drives soil carbon storage. Nature. https://doi.org/10.1038/nature12901 . 505:543+ Barrow NJ (2017) The effects of pH on phosphate uptake from the soil. 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Forests 11:1274. https://doi.org/10.3390/f11121274 Supplementary Files Attachment2026.docx Cite Share Download PDF Status: Posted Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-9088640","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":617168760,"identity":"c7e1f5a9-78cd-4b1c-b6d4-5e885614dc72","order_by":0,"name":"Mi Yang","email":"","orcid":"","institution":"","correspondingAuthor":false,"prefix":"","firstName":"Mi","middleName":"","lastName":"Yang","suffix":""},{"id":617168761,"identity":"ee24d358-b165-41ac-bbd5-e284746bef73","order_by":1,"name":"Mengzhen Lu","email":"","orcid":"","institution":"","correspondingAuthor":false,"prefix":"","firstName":"Mengzhen","middleName":"","lastName":"Lu","suffix":""},{"id":617168762,"identity":"9007404b-7813-4e22-a25f-58c604eef690","order_by":2,"name":"Long Chen","email":"","orcid":"","institution":"","correspondingAuthor":false,"prefix":"","firstName":"Long","middleName":"","lastName":"Chen","suffix":""},{"id":617168763,"identity":"8613de37-295f-4af7-90b6-febd3368060d","order_by":3,"name":"Qiuxiang Tian","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA9UlEQVRIiWNgGAWjYDACZhBhwMDABqI/MBwgrIMHWQvjDKK0oNjIQ4wWe3bmZw/eFBxO7GM/e/i1bdsdu/724w8Yfu7A5zA2c8M5BoeN2Xjy0qxz254lzziTY8DYewavX8ykeQwOy7Ex5JgZ5247nGzAkMPAzNiGTwv7N5AWHjb+N2bGliAt/M8fENDCA7VFIsf4MeO2w3YGEgkG+LUc5imTnGOQbswm8caMsfffswSJG28MDvbi0cLef3ybxJs/1onz+3OMP/w4c8eevz/94YOfeLRArIJQbBJAIrEBSBwgoAGuhfkDkLAnqHoUjIJRMApGHAAAADdL6YY4QFoAAAAASUVORK5CYII=","orcid":"https://orcid.org/0000-0002-4640-7741","institution":"Chinese Academy of Sciences Wuhan Botanical Garden","correspondingAuthor":true,"prefix":"","firstName":"Qiuxiang","middleName":"","lastName":"Tian","suffix":""},{"id":617168764,"identity":"9c128b28-93a7-479c-a6b2-ca8cd50819e6","order_by":4,"name":"Xiaorong Wang","email":"","orcid":"","institution":"","correspondingAuthor":false,"prefix":"","firstName":"Xiaorong","middleName":"","lastName":"Wang","suffix":""},{"id":617168765,"identity":"9c40dafc-5c38-492d-9a98-72dd29daec15","order_by":5,"name":"Zhiyang Feng","email":"","orcid":"","institution":"","correspondingAuthor":false,"prefix":"","firstName":"Zhiyang","middleName":"","lastName":"Feng","suffix":""},{"id":617168766,"identity":"a2d36abf-0827-45cb-b9b0-7ca733fc5ffa","order_by":6,"name":"Zhongfeng Sun","email":"","orcid":"","institution":"","correspondingAuthor":false,"prefix":"","firstName":"Zhongfeng","middleName":"","lastName":"Sun","suffix":""},{"id":617168767,"identity":"364c80ce-8d54-4e0f-83df-4622663496dc","order_by":7,"name":"Feng Liu","email":"","orcid":"","institution":"","correspondingAuthor":false,"prefix":"","firstName":"Feng","middleName":"","lastName":"Liu","suffix":""}],"badges":[],"createdAt":"2026-03-11 01:39:28","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-9088640/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-9088640/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":106508009,"identity":"42a6c94b-efb5-4c9d-a05d-4a3a34f6c34a","added_by":"auto","created_at":"2026-04-09 10:14:12","extension":"jpg","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":2929405,"visible":true,"origin":"","legend":"\u003cp\u003eRelationships between ECM tree dominance and the relative abundance of bacterial IPSM (a), bacterial OPSM (b), fungal OPSM (c), and fungal IPSM (d), respectively (\u003cem\u003en\u003c/em\u003e=35). IPSM\u003csub\u003eb\u003c/sub\u003e and OPSM\u003csub\u003eb\u003c/sub\u003e, bacteria involved in inorganic P solubilization and organic P mineralization, respectively; IPSM\u003csub\u003ef\u003c/sub\u003e, OPSM\u003csub\u003ef\u003c/sub\u003e, fungi involved in inorganic P solubilization and organic P mineralization, respectively. Solid lines indicate significant correlation (\u003cem\u003ep\u003c/em\u003e\u0026lt;0.05), and dashed lines indicate non-significant correlation\u003c/p\u003e","description":"","filename":"image1.jpg","url":"https://assets-eu.researchsquare.com/files/rs-9088640/v1/c929138cb51dc3c2cc585cef.jpg"},{"id":106507899,"identity":"31f0db94-0497-4a5a-84af-034ef2c4b031","added_by":"auto","created_at":"2026-04-09 10:13:45","extension":"jpg","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":3303133,"visible":true,"origin":"","legend":"\u003cp\u003eRelationships between ECM tree dominance and the content of NaHCO\u003csub\u003e3\u003c/sub\u003e-Pi, NaOH-Pi, Po, HCl-Pi, residual P, and TP (\u003cem\u003en\u003c/em\u003e=35). Po, the sum of NaHCO\u003csub\u003e3\u003c/sub\u003e-Po and NaOH-Po; TP, soil total P. Solid lines indicate significant correlation (\u003cem\u003ep\u003c/em\u003e\u0026lt;0.05), and dashed lines indicate non-significant correlation\u003c/p\u003e","description":"","filename":"image2.jpg","url":"https://assets-eu.researchsquare.com/files/rs-9088640/v1/dc7ed2ac5a7573f0c67d73b5.jpg"},{"id":106507960,"identity":"5dce86b0-0db4-4105-9a57-7d3d943bfe7a","added_by":"auto","created_at":"2026-04-09 10:13:52","extension":"jpg","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":1800426,"visible":true,"origin":"","legend":"\u003cp\u003eRelationships of Basal area of trees, thickness of forest floor, and litter CWM-LP content with soil TP content along the gradient of ECM tree dominance. Litter CWM-LP, community-weighted mean litter P. The solid lines denote the fitted regression lines where the relationship is statistically significant, while the dashed lines indicate non-significant relationships. Shaded areas show 95% confidence intervals\u003c/p\u003e","description":"","filename":"image3.jpg","url":"https://assets-eu.researchsquare.com/files/rs-9088640/v1/c08a7fe8c4cc53841d107f75.jpg"},{"id":106508008,"identity":"5b81dca8-b633-41cc-8cad-1073593b2c98","added_by":"auto","created_at":"2026-04-09 10:14:12","extension":"jpg","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":4397624,"visible":true,"origin":"","legend":"\u003cp\u003eRelationships of biotic and abiotic variables with the NaOH-Pi, Po, and HCl-Pi content along the gradient of ECM tree dominance. Po, the sum of NaHCO\u003csub\u003e3\u003c/sub\u003e-Po and NaOH-Po; litter CWM-LP and litter CWM-LMn, community-weighted mean litter P and litter Mn, respectively; SOC, soil organic carbon; MBC, microbial biomass carbon; APH activity, acid phosphatase activity; IPSM\u003csub\u003eb\u003c/sub\u003e and OPSM\u003csub\u003eb\u003c/sub\u003e, bacteria involved in inorganic P solubilization and organic P mineralization, respectively; The solid lines denote the fitted regression lines where the relationship is statistically significant, while the dashed lines indicate non-significant relationships. Shaded areas show 95% confidence intervals\u003c/p\u003e","description":"","filename":"image4.jpg","url":"https://assets-eu.researchsquare.com/files/rs-9088640/v1/1d47df0268ff8bc5c9344c46.jpg"},{"id":106507961,"identity":"78a2eeab-9312-4dfc-a7c8-0fbaf1ea3252","added_by":"auto","created_at":"2026-04-09 10:13:52","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":526539,"visible":true,"origin":"","legend":"\u003cp\u003ePath modelling analysis detecting direct and indirect drivers of soil Po (a) and HCl-Pi (b) along the gradient of ECM tree dominance. Standardized path coefficients are the numbers next to the arrows, with the arrow width corresponding to the strength of the association. Positive relationships are shown with red arrows, while negative ones are indicated by black arrows. Solid arrows denote statistically significant relationships (\u003cem\u003ep\u003c/em\u003e \u0026lt; 0.05), whereas dashed arrows signify relationships that are not significant. Litter quality includes variables of litter CWM-LN and litter CWM-LP. Microbial diversity includes variables of fungal richness and bacteria richness. APH activity, acid phosphatase activity; Basal area, basal area of trees. IPSM\u003csub\u003eb\u003c/sub\u003e and OPSM\u003csub\u003eb\u003c/sub\u003e, bacteria involved in inorganic P solubilization and organic P mineralization, respectively\u003c/p\u003e","description":"","filename":"image5.png","url":"https://assets-eu.researchsquare.com/files/rs-9088640/v1/e0ba62e2ab20c9e9eaf3c843.png"},{"id":106507958,"identity":"8e28069f-9d41-4c54-85ce-adff82cbcca4","added_by":"auto","created_at":"2026-04-09 10:13:52","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":5017539,"visible":true,"origin":"","legend":"\u003cp\u003eA conceptual model illustrates that ECM tree dominance regulates soil P pools through coupled changes in P translocation, replenishment, and mobilization. Litter-P, community-weighted mean litter P; IPSM and OPSM, bacteria involved in inorganic P solubilization and organic P mineralization, respectively; Po, the sum of NaHCO\u003csub\u003e3\u003c/sub\u003e-Po and NaOH-Po; APH, acid phosphatase. The arrow width corresponds to the strength of the association\u003c/p\u003e","description":"","filename":"image6.png","url":"https://assets-eu.researchsquare.com/files/rs-9088640/v1/dbb3023f63676acb192f51a6.png"},{"id":108875830,"identity":"3a0884bf-e100-4eba-89a5-1e4deea60f0a","added_by":"auto","created_at":"2026-05-09 14:43:23","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":16804882,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-9088640/v1/d7a4ee09-238b-44d9-9f2b-46c0324ee757.pdf"},{"id":106507962,"identity":"9cbc3d23-a325-43c5-8c3d-27137db70a26","added_by":"auto","created_at":"2026-04-09 10:13:52","extension":"docx","order_by":5,"title":"","display":"","copyAsset":false,"role":"supplement","size":1836502,"visible":true,"origin":"","legend":"","description":"","filename":"Attachment2026.docx","url":"https://assets-eu.researchsquare.com/files/rs-9088640/v1/48298411c4fcc4f199189241.docx"}],"financialInterests":"","formattedTitle":"Decrease of soil total and organic phosphorus with ECM tree dominance in a subtropical mountainous forest","fulltext":[{"header":"Introduction","content":"\u003cp\u003ePhosphorus (P) is a crucial nutrient element in plant physiology and metabolism, playing a key role in plant growth and forest productivity (Du et al. \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e2020\u003c/span\u003e; Lambers et al. \u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e2015\u003c/span\u003e). As a nutrient primarily derived from soil parent material (Porder et al. \u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e2007\u003c/span\u003e; Song et al. \u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e2024\u003c/span\u003e) and not replenished through atmospheric inputs (Hedley et al. \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e1982\u003c/span\u003e), soil P content and availability generally decline with the increase in soil age and weathering degree(Yang and Post, \u003cspan citationid=\"CR63\" class=\"CitationRef\"\u003e2011\u003c/span\u003e). Consequently, P frequently limits forest productivity in highly weathered tropical and subtropical forests (Manu et al. \u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e2024\u003c/span\u003e). In soils, P occurs in multiple forms, including (1) inorganic P (Pi) adsorbed on the surface of minerals or organic matter, (2) organic P (Po) present as nucleic acids, phospholipids, and phytate, (3) primary mineral P retained within parent materials, and (4) residual P occluded within mineral lattices (Turner et al. \u003cspan citationid=\"CR54\" class=\"CitationRef\"\u003e2007\u003c/span\u003e; He et al. \u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e2023\u003c/span\u003e; Yan et al. \u003cspan citationid=\"CR61\" class=\"CitationRef\"\u003e2023\u003c/span\u003e). Most of these P fractions exhibit low solubility and must be transformed into soluble inorganic forms before being taken up by plants. Characterizing soil total P and its various fractions is crucial for elucidating the pathway and capacity of soil P supply, providing key insights into nutrient availability and forest productivity.\u003c/p\u003e \u003cp\u003eMost terrestrial plants can form symbiotic relationships with either arbuscular mycorrhizal fungi (AM) or ectomycorrhizal fungi (ECM) to cope with nutrient limitations (Liu et al. \u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e2018\u003c/span\u003e). Previous studies have shown that AM-associated trees are predominantly distributed in tropical and subtropical forests characterized by low P availability, whereas ECM-associated trees are more common in cold regions with lower nitrogen availability and relatively higher P availability. Consequently, at global and regional scales, AM-dominated forests generally exhibit higher soil P content than ECM-dominated forests, largely due to differences in soil genesis processes (Phillips et al. \u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e2013\u003c/span\u003e). In subtropical montane forests, however, large areas are characterized by the coexistence of AM and ECM tree species (Lin et al. \u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e2017\u003c/span\u003e), and previous studies have reported that soil P availability differs between AM and ECM-dominated stands (Su et al. \u003cspan citationid=\"CR50\" class=\"CitationRef\"\u003e2024\u003c/span\u003e). For example, Lian et al. (\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e2024\u003c/span\u003e) reported higher soil total P (TP) and available P contents under ECM stands compared with AM stands, whereas significantly lower soil TP and available P contents in ECM-dominated stands were also observed (Qi et al. \u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). These inconsistent findings indicate that how AM and ECM-associated trees regulate soil P content and overall P cycling in subtropical forests remains unclear. Given the potential shifts in the distribution of AM and ECM plants under future climate change (Bennett and Classen, 2020; Jo et al. 2019), it is necessary to investigate how mycorrhizal types of tree species shape soil TP content and P fractions to better predict and explain the maintenance of forest productivity.\u003c/p\u003e \u003cp\u003eApart from pedogenic processes, soil TP content is mainly controlled by biological processes, including aboveground P translocation and the replenishment of soil P via litterfall (Turner et al. \u003cspan citationid=\"CR54\" class=\"CitationRef\"\u003e2007\u003c/span\u003e; Yan et al. \u003cspan citationid=\"CR61\" class=\"CitationRef\"\u003e2023\u003c/span\u003e). First, the widely observed negative relationship between aboveground biomass and soil TP suggests that a substantial proportion of P source can be sequestrated in living biomass, and the translocation of P into aboveground vegetation can lead to a decline in soil TP pools (An and Li, \u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e2013\u003c/span\u003e). Meanwhile, long-term litter manipulation experiments have shown that litter addition increases soil TP content, while litter removal results in significant loss of multiple soil P fractions (Turner et al.2024). These findings indicate that litter, through continuous nutrient return, plays an important role in replenishing the soil P pool. Compared to AM trees, ECM trees typically exhibit a more conservative growth and nutrient strategy. Consequently, ECM trees tend to have longer lifespans and greater biomass accumulation (Averill et al. \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e2014\u003c/span\u003e; Shao et al. 2023). In addition, ECM trees tend to produce lower-quality litter, leading to slower decomposition rates and lower nutrient return efficiency (Phillips et al. \u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e2013\u003c/span\u003e). Collectively, these traits can potentially influence soil TP content and its spatial variability.\u003c/p\u003e \u003cp\u003eFine roots and their associated mycorrhizal fungi can mobilize soil P either directly or indirectly through the recruitment of phosphorus-solubilizing microorganisms (PSM). The mechanisms include secreting organic acids to activate adsorbed Pi, by producing phosphatases to mineralize Po, and by releasing protons to hydrolyze mineral P (Hinsinger, \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e2001\u003c/span\u003e; Richardson and Simpson, \u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e2011\u003c/span\u003e). All of these processes have been reported in both types of mycorrhizal trees (Smith and Read, \u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e2010\u003c/span\u003e; Phillips et al. \u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e2013\u003c/span\u003e), thereby potentially influencing soil P fractions. Previous studies have reported that AM-associated trees preferentially rely on Pi sources (Smith et al. \u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e2011\u003c/span\u003e; Hinsinger et al. \u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e2011\u003c/span\u003e), whereas ECM-associated trees tend to utilize Po (Liu et al. \u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e2018\u003c/span\u003e). Thus, ECM-dominated forests generally exhibit higher phosphatase activity than AM-dominated forests and stronger Po mineralization (Cao et al. \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e2023\u003c/span\u003e; Wang et al. \u003cspan citationid=\"CR58\" class=\"CitationRef\"\u003e2023\u003c/span\u003e), whereas AM-dominated forests show greater depletion of inorganic Pi fractions (Phillips et al. \u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e2013\u003c/span\u003e; Smith et al. \u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e2011\u003c/span\u003e). However, whether these P-use preferences can modulate the relative contributions and the intensity of the P mobilization process, and consequently soil P fractions, remains largely unexplored.\u003c/p\u003e \u003cp\u003eIn addition to the P- mobilization processes described above, soil properties such as soil organic carbon (SOC), soil microbial biomass (MBC), and soil pH have also been shown to influence soil P fractions. For example, higher SOC provides more adsorption sites, facilitating the retention of P on organic matter (G\u0026eacute;rard, \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e2016\u003c/span\u003e; Richardson et al. \u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e2009\u003c/span\u003e). MBC reflects the size and activity of the soil microbial community and plays a key role in regulating P transformation through microbial uptake, turnover, and enzyme-mediated mineralization processes (Richardson et al. \u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e2009\u003c/span\u003e). Lower soil pH can accelerate the dissolution of primary mineral P (Phillips et al. \u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e2013\u003c/span\u003e). ECM-dominated forests have been reported to exhibit higher SOC content, lower MBC content, and lower pH (Yang et al. \u003cspan citationid=\"CR62\" class=\"CitationRef\"\u003e2025\u003c/span\u003e), which may further influence soil P fractions.\u003c/p\u003e \u003cp\u003eTo explore the influence of mycorrhizal types of trees on soil TP content and P fractions, we established 35 plots with a natural gradient of ECM tree dominance (quantified as the percentage of total basal area contributed by ECM trees) in a subtropical montane forest. Soil TP and different P fractions were determined based on the modified Hedley-P method (Tiessen \u003cspan citationid=\"CR53\" class=\"CitationRef\"\u003e1993\u003c/span\u003e). Plant traits, litter quality, soil physico-chemical properties, and microbial communities were determined to assess their influence on soil P fractions. Due to the stronger above-ground P translocation and lower nutrient return via litterfall in ECM-dominated plots, we hypothesize that (1) soil TP content will decrease with higher ECM tree dominance. Since AM plants may preferentially utilize Pi and ECM plants preferentially utilize Po, we hypothesize (2) soil Pi content will increase, and Po content will decrease with the higher ECM tree dominance. We also hypothesize (3) HCl-Pi content will decrease with increasing ECM tree dominance due to the lower soil pH.\u003c/p\u003e"},{"header":"Materials and Methods","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003eSite description\u003c/h2\u003e \u003cp\u003eThis study was conducted in Badagongshan National Nature Reserve (29.66\u0026deg;\u0026ndash;29.83\u0026deg; N, 109.70\u0026deg;\u0026ndash;110.16\u0026deg; E) in Hunan Province (Fig. \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e). This region features a humid subtropical climate, with an average annual precipitation of 2100 mm and a mean annual temperature of 10.7\u0026deg;C. The soil type is mountainous yellow-brown soil with a loamy texture. The vegetation type is evergreen-deciduous broadleaf mixed forest. The dominant species include \u003cem\u003eCyclobalanopsis multinervis\u003c/em\u003e, \u003cem\u003eCaCarpinus turczaninovii\u003c/em\u003e, \u003cem\u003eCornus kousa\u003c/em\u003e, \u003cem\u003eFagus lucida\u003c/em\u003e, \u003cem\u003eCastanea seguinii\u003c/em\u003e, and \u003cem\u003eSorbus folgneri\u003c/em\u003e.\u003c/p\u003e \u003cp\u003eIn May 2023, 35 plots (20 m \u0026times; 20 m) were established. Each plot was located at similar elevations with identical parent materials. Within each plot, woody plants with a diameter at breast height (DBH) greater than 1 cm were recorded. Mycorrhizal types were assigned for each species based on existing literature and fine root staining analysis (Soudzilovskaia et al. \u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). The ECM tree dominance varied from 2 to 89% across the plots, and reflected a natural mycorrhizal gradient.\u003c/p\u003e \u003c/div\u003e\n\u003ch3\u003eSample collection and processing\u003c/h3\u003e\n\u003cp\u003eWithin each plot, four random points were selected to collect soil samples. Before collecting the mineral soil, we use a ruler to measure the thickness of forest floor. After removing the forest floor, we collected the mineral soil samples from the 0\u0026ndash;10 cm layer using a 5.72 cm diameter soil auger. Samples from the four points within the same plot were combined to form a composite soil sample. Fresh soil samples were sieved through a 2 mm mesh to remove visible plant debris and stones. A portion of the sieved samples was stored at -80\u0026deg;C for microbial sequencing, and another portion was kept at 4\u0026deg;C for the determination of biological parameters, and the remaining samples were air-dried for the analysis of soil physico-chemical properties and P fractions. Data of SOC, total nitrogen (TN), dissolved organic carbon (DOC), dissolved nitrogen (DN), MBC, soil acid phosphatase (ACP) activity, soil pH, and soil texture were obtained from Lu et al. (\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e2025\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eIn autumn 2023, fresh leaf litter was collected from the 28 dominant tree species (accounting for 78% of the total basal area) in the study area. The litter samples were dried in a 65\u0026deg;C to constant weight in the laboratory. After pulverization, litter carbon, nitrogen, P, and manganese (Mn) contents were determined. In this study, community weighted-mean litter carbon \u003cem\u003e(CWM-LC)\u003c/em\u003e, nitrogen \u003cem\u003e(CWM-LN)\u003c/em\u003e, and P \u003cem\u003e(CWM-LP)\u003c/em\u003e contents were calculated to indicate litter quality at community level (Chen et al. \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). Weighted-mean litter Mn (\u003cem\u003eCWM-LMn\u003c/em\u003e) content was calculated as an indicator of rhizosphere carboxylate concentration and used as a proxy of the mobilization of soil adsorbed Pi (Yan et al. \u003cspan citationid=\"CR60\" class=\"CitationRef\"\u003e2025\u003c/span\u003e).\u003c/p\u003e\n\u003ch3\u003eDetermination of soil P fractions\u003c/h3\u003e\n\u003cp\u003eSoil P fractions were extracted using a modified sequential extraction procedure developed by Hedley et al. (\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e1982\u003c/span\u003e) and modified by Tiessen (\u003cspan citationid=\"CR53\" class=\"CitationRef\"\u003e1993\u003c/span\u003e). In brief, 1 g of soil was weighed into a 50-mL centrifuge tube, and sequentially extracted with solutions of increasing strength, including 0.5 mol L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e NaHCO\u003csub\u003e3\u003c/sub\u003e, 0.1 mol L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e NaOH, and 1 mol L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e HCl. Each extraction was performed at 25\u0026deg;C with shaking for 16 h. After extraction, the supernatant was collected by centrifugation and filtered through a 0.45-\u0026micro;m membrane. For NaHCO\u003csub\u003e3\u003c/sub\u003e and NaOH extracts, Pi and total P (Pt) were determined, respectively. Pi was measured by the molybdenum-antimony colorimetric method at 880 nm, and Pt was quantified after digestion with sulfate-sulfuric acid and analyzed using the same colorimetric procedure. Po content was calculated as the difference between Pt and Pi. The HCl extract was directly analyzed for Pi (HCl-Pi) due to the negligible Po content. The extraction residue was washed with deionized water, dried and ground, and then 0.2g of the residue was digested with concentrated H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e-HClO\u003csub\u003e4\u003c/sub\u003e to measure residual P. In this study, NaHCO\u003csub\u003e3\u003c/sub\u003e-Pi represents the most labile Pi fractions. NaOH-Pi represents moderate Pi fractions and is primarily adsorbed on soil minerals or co-precipitated with organic matter. Po represents the sum of NaHCO\u003csub\u003e3\u003c/sub\u003e-Po and NaOH-Po. HCl-Pi corresponds to primary mineral P. Soil TP content was also determined following H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e-HClO\u003csub\u003e4\u003c/sub\u003e digestion using a microwave digestion system (ETHOS ONE, Milestone, Milan, Italy).\u003c/p\u003e\n\u003ch3\u003eAnalysis of soil microbial properties\u003c/h3\u003e\n\u003cp\u003eSoil bacterial and fungal community composition was analyzed using high-throughput sequencing techniques, and DNA was extracted from the frozen soil samples using the MoBio PowerSoil DNA Isolation Extraction Kit (MoBio Laboratories, USA). For bacterial community, the bacterial 16S rRNA gene was amplified using the primer pair: 799F (5' AACMGGATTAGATACCCKG-3') and 1193R (5'-ACGTCATCCCCACCTTCC-3'). For fungal community, the fungal internal transcribed spacer 1(ITS1) region was amplified using the following primer pair: ITS1F (5'-CTTGGTCATTTAGAGGAAGTAA-3') and ITS2-2043R (5'-GCTGCGTTCTTCATCGATGC-3'). The PCR products were purified and sequenced on the same Illumina MiSeq platform (PE-250).\u003c/p\u003e \u003cp\u003eRaw sequencing data were processed using QIIME2 (version 2023.2) for quality control and denoising, and sequences were clustered into operational taxonomic units (OTUs) at 97% similarity. OTUs were then annotated taxonomically using the SILVA (for bacteria) and UNITE (for fungi) databases.\u003c/p\u003e \u003cp\u003eMicrobial groups known to possess functions related to Po mineralization (e.g., phosphatase production) or Pi solubilization (e.g., organic acid secretion) were identified according to (Richardson et al. \u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e2009\u003c/span\u003e) and hereafter referred to as OPSM (organic phosphorus-solubilizing microorganism) and IPSM (inorganic phosphorus-solubilizing microorganism), respectively. The main genera of IPSM and OPSM in bacterial and fungal communities are listed in Table \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e. The relative abundance of IPSM and OPSM was calculated separately for bacterial and fungal communities.\u003c/p\u003e\n\u003ch3\u003eStatistical analyses\u003c/h3\u003e\n\u003cp\u003eLinear regression analysis was employed to examine the effects of ECM tree dominance on the contents of soil TP and P fractions, as well as plant, litter, soil physico-chemical, and microbial properties. Pearson correlation analysis was performed to examine relationships between soil P fractions and all these environmental variables. Additionally, partial least squares path modelling (PLS-PM) was conducted to assess the direct and indirect drivers of soil P fractions. The strengths and directions of these relationships were assessed based on the standardized path coefficients. The predictive power of the model was then evaluated using the coefficient of determination (R\u003csup\u003e2\u003c/sup\u003e) for endogenous latent variables, and the goodness-of-fit of the model was assessed using the Goodness of Fit (GoF) index (Sanchez, 2013). All data processing in this study was performed using R 4.5.2 software.\u003c/p\u003e \u003cdiv id=\"Sec8\" class=\"Section2\"\u003e \u003cdiv id=\"Sec9\" class=\"Section3\"\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003c/div\u003e"},{"header":"Result","content":"\u003ch2\u003ePlant, litter, soil, and microbial properties across the gradient of ECM tree dominance\u003c/h2\u003e\u003cp\u003eECM tree dominance was positively correlated with basal area of trees, thickness of forest floor, and APH activity, and negatively correlated with tree diversity, the contents of litter CWM-LC, CWM-LN, CWM-LP, soil pH, and bacterial and fungi diversities. Moreover, ECM tree dominance had no significant correlations with the contents of litter CWM-LMn, SOC, and MBC (Fig. S2).\u003c/p\u003e\u003cp\u003eThe relative abundance of IPSM and OPSM in bacterial communities averaged 10.4% and 4.2%, respectively, and both increased significantly with increasing ECM tree dominance (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003e). IPSM in bacterial community were primarily represented by \u003cem\u003eCandidatus\u003c/em\u003e and \u003cem\u003eBradyrhizobium\u003c/em\u003e, and OPSM were dominated by \u003cem\u003eBurkholderia\u003c/em\u003e (Figure S3). In contrast, the relative abundance of IPSM and OPSM in fungal community averaged 1.0% and 0.2%, respectively, and both showed no significant relationship with ECM tree dominance (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003e). IPSM in fungal community were dominated by \u003cem\u003ePiloderma\u003c/em\u003e, and OPSM was dominated by \u003cem\u003eTrichoderma\u003c/em\u003e (Figure. S3).\u003c/p\u003e\n\u003ch3\u003eSoil TP and P fractions across the gradient of ECM tree dominance\u003c/h3\u003e\n\u003cp\u003eIn this subtropical forest, soil TP ranged from 226.4 to 747.0 \u0026micro;g g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, with NaHCO\u003csub\u003e3\u003c/sub\u003e-Pi, NaOH-Pi, Po, HCl-Pi, and residual P contributing averagely 0.48%, 14.4%, 52.3%, 1.2% and 31.4% of TP, respectively (Figure S5). With the increasing of ECM tree dominance, the contents of NaHCO\u003csub\u003e3\u003c/sub\u003e-Pi, NaOH-Pi, and residual P showed no significant change, while the contents of Po, HCl-Pi, and TP decreased significantly (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cdiv id=\"Sec11\" class=\"Section2\"\u003e \u003ch2\u003eControlling factors of soil TP and P fractions\u003c/h2\u003e \u003cp\u003eSoil TP was negatively correlated with basal area of trees and the thickness of forest floor, and showed a slightly positive correlation with litter CWM-LP content (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eNaOH-Pi content was positively correlated with MBC, SOC, and the relative abundances of IPSM and OPSM in fungi community, but negatively correlated with the thickness of forest floor and the relative abundances of IPSM and OPSM in bacterial community (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e and Fig. S4). The contents of Po, HCl-Pi and residual P were negatively correlated with basal area of trees and the relative abundance of IPSM and OPSM in bacterial community, but positively correlated with MBC, fungal richness and bacterial richness (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e and Fig. S4). Additionally, Po content was positively correlated with litter CWM-LP and negatively correlated with the thickness of forest floor and APH activity. NaHCO\u003csub\u003e3\u003c/sub\u003e-Pi content was positively correlated with MBC, SOC and the relative abundances of IPSM and OPSM in fungi community, and negatively correlated with soil pH. Additionally, NaHCO\u003csub\u003e3\u003c/sub\u003e-Pi content was positively correlated with the contents of NaOH-Pi, HCl-Pi, and residual P (Fig. S6).\u003c/p\u003e \u003cp\u003eSince Po and HCl-Pi contents were also correlated with ECM tree dominance, we further use PLS-PM analysis to examine direct and indirect drivers of Po and HCl-Pi. Litter quality, APH, and microbial diversity could significantly regulate Po concentrations directly, whereas ECM tree dominance could decrease the Po concentrations indirectly by reducing litter quality and by increasing the relative abundance of bacterial OPSM, which then enhances the APH activity. The relative abundance of bacterial IPSM could regulate HCl-Pi content directly, whereas ECM tree dominance could indirectly decrease HCl-Pi content by increasing the relative abundance of bacterial IPSM. The PLS-PM models explained 56.97% and 49.04% of the variance in the Po and HCl-Pi, respectively (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e"},{"header":"Discussion","content":"\u003cdiv id=\"Sec13\" class=\"Section2\"\u003e \u003ch2\u003eEffect of ECM tree dominance on soil TP content\u003c/h2\u003e \u003cp\u003eConsistent with our first hypothesis, soil TP content decreased significantly with the increase in ECM tree dominance (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). Since all soils were collected under similar climatic conditions and parent material, the soil formation processes are expected to be broadly comparable among sites. Therefore, the pronounced variation in TP content highlights the strong influence of biogeochemical processes. Our findings indeed revealed soil TP content significant correlated with basal area of trees, forest floor thickness, and litter CWM-LP, confirming the critical role of biogeochemical processes in shaping soil P dynamics.\u003c/p\u003e \u003cp\u003eFirstly, the negative relationship between TP content and basal area of trees was aligned with previous studies (Qi et al. \u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). Phosphorus, as a key element of plant cell structure, nucleic acids, and energy metabolism-related substances, is largely sequestered within plant biomass (Marschner, \u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e1995\u003c/span\u003e). As biomass increases, large amounts of P are stored in aboveground tissues, resulting in a significant reduction in soil TP content. In this study, plots with higher ECM tree dominance exhibited significantly greater aboveground biomass, indicated by the basal area of trees (Fig. S2). The higher biomass is mainly due to the conservative growth strategy and increased lifespan of ECM tree species, resulting in increased biomass storage (Averill et al. \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e2014\u003c/span\u003e). Therefore, the increase in ECM tree dominance enhanced above-ground P translocation, ultimately leading to a significant decline in soil TP (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eSecondly, soil TP content exhibited a weakly positive correlation with litter P content (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e). This was primarily because litter decomposition serves as a crucial replenishment for soil P pools. Litter with higher P content can release greater amounts of P source during decomposition, thereby strengthening soil P replenishment (Turner et al. \u003cspan citationid=\"CR54\" class=\"CitationRef\"\u003e2007\u003c/span\u003e). In this study, ECM-dominated plots had lower litter CWM-LP content than AM-dominated plots. This finding was consistent with previous studies showing that litter of ECM-associated trees was characterized by significantly lower P content and higher C:P ratios compared with AM-associated trees (Phillips et al. \u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e2013\u003c/span\u003e; Averill et al. \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2019\u003c/span\u003e), resulting in reduced P return via litterfall. Such reductions in litter P inputs can constrain P replenishment to surface soils, thereby accelerating soil TP depletion with increasing ECM dominance.\u003c/p\u003e \u003cp\u003eAdditionally, the forest floor serves as a crucial P \u0026ldquo;reservoir\u0026rdquo; in ecosystems (Peng et al. \u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). Larger forest floor biomass implies that more P is sequestered in undecomposed or partially decomposed litter, delaying its return into the soil P pool. Consequently, forest floor biomass is also considered an important factor regulating soil TP content (Spohn, \u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). Litter from ECM tree species is typically low in nutrients and contains a higher proportion of recalcitrant components (Cheeke et al. \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e2017\u003c/span\u003e), resulting in a slower decomposition rate. Moreover, ECM fungi have a nutrient competition relationship with soil saprophytic microorganisms (Fernandez et al. \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e2020\u003c/span\u003e), further inhibiting litter decomposition. As a result, ECM tree-dominant plots accumulated higher forest floor biomass (Fig. S2), reducing the amount of P returned into the soil and ultimately contributing to lower soil TP content.\u003c/p\u003e \u003cp\u003eBased on the variations of soil P fractions, the decline in soil TP content was mainly driven by reductions in soil Po and HCl-Pi, while NaOH-Pi, and residual P remained largely unchanged across the ECM gradient (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). These results are consistent with our second and third hypotheses, except that the trend in NaOH-Pi was opposite to our expectation.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec14\" class=\"Section2\"\u003e \u003ch2\u003eEffect of ECM tree dominance on soil Po content\u003c/h2\u003e \u003cp\u003eOur results indicated that Po was the predominant P fraction in this subtropical forest (Fig. S5). Since soil Po is largely derived from plant residue as well as microbial biomass and its necromass (Wang et al. \u003cspan citationid=\"CR58\" class=\"CitationRef\"\u003e2023\u003c/span\u003e), variations in litter P content and microbial processes are expected to influence soil Po dynamics. Previous studies have shown that higher litter P content can promote soil Po accumulation (Chi et al. \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). Consistent with this, soil Po content in our study was positively correlated with litter CWM-LP content (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e), suggesting that plant-derived P inputs play an important role in regulating soil Po accumulation (Turner et al. \u003cspan citationid=\"CR54\" class=\"CitationRef\"\u003e2007\u003c/span\u003e; Spohn, \u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). As ECM tree dominance increased, litter CWM-LP content declined (Fig. S2), which might partly explain the low soil Po in the ECM-dominated plots.\u003c/p\u003e \u003cp\u003eSoil microorganisms represent a major pool of Po in soils, and increases in MBC are therefore commonly accompanied by higher soil Po content due to enhanced microbial assimilation and immobilization of P sources (Joergensen et al. 2008). Consistent with previous studies, our results also demonstrated a significant positive relationship between MBC and soil Po content (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e). Moreover, we observed a significant positive relationship between soil microbial diversity and Po content (Fig. S4). Higher microbial diversity may accelerate microbial turnover and necromass production, thereby enhancing the formation and retention of microbially derived Po in soils and ultimately contributing to greater soil Po accumulation (Liang et al. \u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e2017\u003c/span\u003e; Delgado-Baquerizo et al. \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e2016\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eThe mineralization of Po is a key pathway for soil P mobilization (Wang et al. \u003cspan citationid=\"CR58\" class=\"CitationRef\"\u003e2023\u003c/span\u003e), and can potentially reduce soil Po accumulation by accelerating its conversion to inorganic forms. Po mineralization relies on phosphatase activity, which originates mainly from plant roots and PSM (Ragot et al. \u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e2015\u003c/span\u003e). In this study, APH activity was significantly negatively correlated with soil Po content, indicating the enhanced phosphatase activity accelerated Po mineralization. Moreover, APH activity increased with the increasing ECM tree dominance (Fig. S2) and was mainly accompanied by a higher relative abundance of bacterial OPSM (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e), which likely further contributed to the low soil Po observed in the ECM-dominated plots. Previous studies have confirmed that ECM tree species can reduce soil Po pool by regulating microbial activity and enzyme activity associated with Po mineralization (Khan et al. \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e2024\u003c/span\u003e; Rosling et al. \u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e2016\u003c/span\u003e). Notably, compared to PSMs in fungal community, bacterial PSMs exhibited greater relative abundance and functional contribution (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e), suggesting that bacteria may play a more important role in soil Po mineralization. This finding aligns with previous studies indicating that bacteria are the primary source of phosphatases in forest soils (Fraser et al. \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e2015\u003c/span\u003e; Ragot et al. \u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e2015\u003c/span\u003e). Furthermore, we observed a higher abundance of PSM in ECM-dominant plots (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e), suggesting that ECM tree species can recruit PSM more efficiently than AM trees. However, the underlying mechanisms driving differences in PSM abundance between AM and ECM-dominated plots remain unclear and warrant further investigation.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec15\" class=\"Section2\"\u003e \u003ch2\u003eEffect of ECM tree dominance of soil on NaOH-Pi\u003c/h2\u003e \u003cp\u003eSoil NaOH-Pi is another important P fraction. Both SOC and MBC contents exerted a strong positive influence on NaOH-Pi.\u003c/p\u003e \u003cp\u003eFirst, organic matter contains abundant functional groups, such as carboxyl, phenolic, and hydroxyl groups, which can directly bind phosphate through ligand exchange or indirectly enhance Pi retention by forming organo\u0026ndash;mineral complexes, thereby enhancing the stability of NaOH-Pi and reducing its mobility and leaching risk (G\u0026eacute;rard, \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e2016\u003c/span\u003e; Guppy et al. \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e2005\u003c/span\u003e; Kaiser and Guggenberger, \u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e2000\u003c/span\u003e). Previous studies in temperate and tropical forest soils have shown that higher SOC is associated with great concentrations of NaOH-Pi due to enhanced organo\u0026ndash;mineral interactions and stronger P sorption capacity (Hou et al. \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e2014\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eSecond, soil microorganisms produce a wide range of extracellular phosphatases that hydrolyze Po compounds, and the released Pi can be rapidly adsorbed onto soil mineral surface or co-precipitated with organic matter, potentially enhancing soil NaOH-Pi pool (B\u0026uuml;nemann et al. \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e2012\u003c/span\u003e). In this study, MBC was significantly positively correlated with NaOH-Pi (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e), suggesting that microbial-mediated Po mineralization likely contributed to the formation and maintenance of NaOH-Pi pool. Similar relationships have been reported in forest and grassland soils, where higher MBC and activity are associated with greater Pi sorption capacity and more inorganic P fractions (Richardson and Simpson, \u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e2011\u003c/span\u003e).\u003c/p\u003e \u003cp\u003ePrevious studies have shown that soil acidity plays a dominant role in regulating NaOH-Pi content by modifying the abundance, surface charge, and reactivity of iron and aluminum (Fe/Al) oxides (Zarif et al. \u003cspan citationid=\"CR64\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). Under acidic conditions, increased protonation of mineral surfaces enhances phosphate sorption through ligand exchange, thereby increasing the proportion of NaOH-Pi (Hinsinger, \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e2001\u003c/span\u003e; G\u0026eacute;rard, \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e2016\u003c/span\u003e; Walker and Syers, \u003cspan citationid=\"CR57\" class=\"CitationRef\"\u003e1976\u003c/span\u003e; Barrow, \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e2017\u003c/span\u003e). However, in the present study, soil pH showed no significant relationship with NaOH-Pi. This discrepancy was likely attributable to the uniformly strong acidity of soils (mean pH 4.15) across the study area. Under this condition, minerals form stable iron-aluminum-phosphate complexes via coordination exchange mechanisms (Sanyal and De Datta, \u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e1991\u003c/span\u003e), which chemically protect the NaOH-Pi pool and make it less susceptible to soil pH (Cross and Schlesinger, \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e1995\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eSince ECM tree dominance showed no significant effects on SOC and MBC contents (Fig. S3), NaOH-Pi content had no significant correlation with ECM tree dominance. Some previous studies have suggested that AM trees preferentially utilize Pi sources (Lian et al. \u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e2024\u003c/span\u003e), which could potentially reduce soil Pi pool. However, in our study, litter CWM-LMn, an indicator of rhizosphere carboxylate concentration and a proxy of the mobilization of soil adsorbed Pi, showed no significant correlation with ECM tree dominance (Fig. S2). Additionally, the relative abundance of IPSM, an indicator of microorganisms capable of mobilizing inorganic P, was positively, rather than negatively, correlated with ECM tree dominance (Fig. S2). Thus, in our study, we did not observe preferential Pi utilization by AM plants. Overall, ECM tree dominance did not influence soil Pi dynamics.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec16\" class=\"Section2\"\u003e \u003ch2\u003eEffect of ECM tree dominance on soil primary mineral P content\u003c/h2\u003e \u003cp\u003ePrimary mineral P accounted for only 1.2% of TP in this subtropical forest, indicating an advanced degree of soil weathering.\u003c/p\u003e \u003cp\u003eWe observed a significant negative relationship between basal area of trees and HCl-Pi content (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e), consistent with previous studies (Tie et al. \u003cspan citationid=\"CR52\" class=\"CitationRef\"\u003e2024\u003c/span\u003e). Greater basal area of trees indicates intensified root growth and mycorrhizal fungal activities, which can accelerate the weathering of primary phosphate minerals through the release of organic acids and protons (Kohler et al. \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e2015\u003c/span\u003e; van Hees et al. \u003cspan citationid=\"CR56\" class=\"CitationRef\"\u003e2006\u003c/span\u003e), thereby reducing the soil primary mineral P pool.\u003c/p\u003e \u003cp\u003eThe relative abundance of IPSM also showed a negative role on soil primary mineral P content. This pattern likely reflects the ability of IPSM to produce low-molecular-weight organic acids (Tian et al. \u003cspan citationid=\"CR51\" class=\"CitationRef\"\u003e2021\u003c/span\u003e). These acids can chelate metal cations (e.g., Ca\u0026sup2;⁺, Fe\u0026sup3;⁺, and Al\u0026sup3;⁺) and destabilize mineral P-bearing phases, thereby facilitating the dissolution and depletion of mineral P. The dissolution of primary mineral P can further be reinforced under acidic soil conditions, as lower pH enhances mineral solubility and weakens the structural stability of primary P minerals (Hinsinger, \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e2001\u003c/span\u003e; G\u0026eacute;rard, \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e2016\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eIn our study, the basal area of trees and the relative abundance of IPSM increased, and soil pH decreased markedly with increasing ECM tree dominance (Fig. S2). Together, these coordinated changes resulted in a significant decline in mineral P along the ECM tree dominance gradient (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eOverall, we propose a conceptual framework illustrating how ECM tree dominance reshapes soil P pools through coupled changes in P translocation, replenishment, and mobilization (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e). Soil TP declined in ECM-dominated plots primarily due to enhanced aboveground P translocation and reduced P replenishment via litterfall. Specifically, soil Po decreased with increasing ECM tree dominance, driven by both lower Po inputs from litterfall and elevated APH activity, which accelerates Po mineralization. In addition, mineral P was progressively depleted as ECM tree dominance increased, likely due to promoted mineral P dissolution via organic acid release and enhanced soil acidification. In contrast, NaOH-Pi remained relatively stable across the ECM tree dominance gradient. Generally, mobilized P from NaOH-Pi, Po, and HCl-Pi contributes to the labile P pool, known as NaHCO\u003csub\u003e3\u003c/sub\u003e-Pi. We further observed that NaHCO\u003csub\u003e3\u003c/sub\u003e-Pi content was positively correlated with all P fractions except Po (Fig. S6), further highlighting the influence of different P fractions on soil P availability. Moreover, although Po content was low in ECM-dominated plots, enhanced phosphatase activity can maintain Po mineralization and thus sustain soil P supply for plants.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e"},{"header":"Conclusion","content":"\u003cp\u003eOur study demonstrated that increasing dominance of ECM tree species leads to a significant decline in soil TP content in the subtropical mountainous forest. The reduction in TP was mainly caused by the enhanced aboveground P translocation and reduced P replenishment via litterfall. The decline in TP was mainly reflected in reductions of Po and primary mineral P, whereas NaOH-Pi and residual P remained relatively stable. The decline in Po was primarily driven by higher Po mineralization and lower Po replenishment from litterfall, while the decline in primary mineral P was mainly due to acid-driven dissolution. Our findings highlight that shifts in tree mycorrhizal dominance can substantially alter soil P fractions by regulating both plant-derived P inputs and microbially mediated P mobilization.\u003c/p\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. No author has any financial or non-financial interests that are directly or indirectly related to the subject matter of this research.\u003c/p\u003e \u003c/p\u003e\u003ch2\u003eFunding\u003c/h2\u003e \u003cp\u003eThis work was financially supported by the National Natural Science Foundation of China (Grant No. 32371736; Grant No. 32171599).\u003c/p\u003e\u003ch2\u003eAcknowledgments\u003c/h2\u003e \u003cp\u003eWe acknowledge with gratitude the assistance and support provided by the Administration of Badagongshan National Nature Reserve. Special thanks are extended to Wei Guorong and Qin Biwu from Tianpingshan Station of Badagongshan for their valuable help in sample plot selection and the sampling process.\u003c/p\u003e\u003ch2\u003eData availability statement\u003c/h2\u003e \u003cp\u003eThe raw and processed data supporting the findings of this study are available from the corresponding author (Q. 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Forests 11:1274. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.3390/f11121274\u003c/span\u003e\u003cspan address=\"10.3390/f11121274\" 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":"Mycorrhizal type, Hedley P fractionation, acid phosphatase, phosphorus-solubilizing microorganism, subtropical forest, Badagongshan Mountain","lastPublishedDoi":"10.21203/rs.3.rs-9088640/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-9088640/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003ch2\u003eAims\u003c/h2\u003e \u003cp\u003eArbuscular mycorrhizal (AM) and ectomycorrhizal (ECM) plants differ in phosphorus uptake strategies and litter quality, but their effects on soil total P (TP) and P fractions along natural dominance gradients remain unclear, especially in P-limited subtropical forests. This study aimed to clarify how tree mycorrhizal associations regulate soil P dynamics.\u003c/p\u003e\u003ch2\u003eMethods\u003c/h2\u003e \u003cp\u003eWe established 35 plots across an AM and ECM tree dominance gradient. Soil P fractions were determined using the modified Hedley method. Plant traits, litter quality, soil properties, and microbial communities were analyzed to identify driving mechanisms.\u003c/p\u003e\u003ch2\u003eResults\u003c/h2\u003e \u003cp\u003eSoil TP, organic P (Po), and primary mineral P (HCl-Pi) decreased significantly with increasing ECM tree dominance. TP content was negatively related to the basal area of trees and the thickness of forest floor, and positively correlated with community-weighted-mean litter P content. Reduced TP was mainly driven by stronger aboveground P translocation and lower litter P input. Declines in Po and HCl-Pi were attributed to accelerated mineralization and dissolution mediated by soil enzymes, phosphorus-solubilizing microorganisms (PSM), and low pH.\u003c/p\u003e\u003ch2\u003eConclusions\u003c/h2\u003e \u003cp\u003eIncreasing ECM dominance accelerates soil P depletion and may exacerbate P limitation, but ECM plants can sustain P availability and forest productivity via enhanced organic P mineralization. This study highlights the key role of mycorrhizal fungi in regulating soil P cycling and alleviating P limitation in subtropical forests.\u003c/p\u003e","manuscriptTitle":"Decrease of soil total and organic phosphorus with ECM tree dominance in a subtropical mountainous forest","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2026-04-09 10:13:05","doi":"10.21203/rs.3.rs-9088640/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":"6c97dd20-3ee2-467f-9b66-6327765cc4dd","owner":[],"postedDate":"April 9th, 2026","published":true,"recentEditorialEvents":[{"type":"decision","content":"Reject after review","date":"2026-05-09T10:42:44+00:00","index":"","fulltext":""}],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[],"tags":[],"updatedAt":"2026-05-09T14:43:10+00:00","versionOfRecord":[],"versionCreatedAt":"2026-04-09 10:13:05","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-9088640","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-9088640","identity":"rs-9088640","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}