Changed plant community composition promotes soil carbon sequestration under long-term silicon and nitrogen addition in an alpine meadow

Changed plant community composition promotes soil carbon sequestration under long-term silicon and nitrogen addition in an alpine meadow | 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 Changed plant community composition promotes soil carbon sequestration under long-term silicon and nitrogen addition in an alpine meadow danghui Xu, Baiying Huang, Yawen Gui, Wenhong Zhou, Yuqi Wu, Wei Mou, and 5 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-8646618/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 o Aims Nitrogen (N) fertilization is recognized for altering plant community composition and reducing species diversity, whereas, silicon (Si) addition enhances plant growth and improves soil nutrient availability. Those changes can profoundly influence ecosystem carbon pools. Nevertheless, how changes in plant community structure mediate soil organic carbon (SOC) storage responses to N and Si addition remains poorly understood. o Methods A nine-year field experiment (2012–2020) was conducted in an alpine meadow with four treatments: no Si and N addition, Si addition, N addition, and combined N + Si addition. o Results SOC significantly increased by 13.1%, 6.7%, and 21.2% under N addition, Si addition, and combined N and Si addition, respectively. Those increases were driven by distinct ecosystem mechanisms: the response to N addition were closely associated with changes in aboveground biomass (AGB), belowground biomass (BGB), and soil heterotrophic respiration ( Rh ); Si addition effects were linked to AGB and Rh ; and the combined treatment’s impact was related to BGB and Rh . This study reveals the governing mechanisms of the SOC pool in an alpine meadow were mediated by changes in the relative biomass of grass (RBG), soil available silicon (SASi), and soil inorganic nitrogen (SIN). o Conclusion Crucially, our findings demonstrate that the simultaneous enhancement of grass biomass, SASi and SIN under combined N and Si treatment lead to greater SOC than that achieved by either nutrient added alone. These findings provide new insights into how plants community composition adjust SOC under long-term Si plus N addition and predictions of ecosystem carbon dynamics under global change. Soil organic carbon Soil available silicon Alpine meadow Nitrogen fertilization Soil heterotrophic respiration Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Introduction The development of livestock husbandry and modern industry has significantly increased soil nitrogen availability through anthropogenic inputs such as fertilization and atmospheric deposition. This elevated N supply stimulates plant growth and alleviates N limitation in terrestrial ecosystems (Kuypers et al. 2018 ; Elrys et al. 2022 ). A frequent outcome of N enrichment is an increase in ecosystem primary productivity accompanied by shifts in plant community composition, typically favoring grass and often at the expense of species diversity (Xu et al. 2018 ; Yan et al. 2021 ). These vegetation changes can, in turn, influence soil organic carbon (SOC) dynamics. Nevertheless, the regulatory mechanisms underlying SOC sequestration, particularly the respective roles played by declining species diversity and increasing grass dominance, remain inadequately understood. Soil organic carbon storage is governed by the balance between carbon (C) sequestration from plant growth and C release from microbial decomposition (Elrys et al. 2022 ; Yang et al. 2022 ). Extensive research suggests that N addition can influence both C gains and C losses (Janssens et al. 2010 ; Song et al. 2019 ). Some studies have suggested that N addition increases aboveground litter production, thereby promoting SOC formation and accumulation (Stevens et al. 2015 ; Schulte-Uebbing et al. 2022 ). However, other have observed that N addition reduce SOC, likely because high N inputs increase protons release and decrease soil microbial diversity, negatively impacting both plants and microbial communities (Geisseler et al. 2017 ; Zhu et al. 2022 ). In contrast, certain studies found that no significant change in SOC following N inputs, which can be attributed to two counteracting mechanisms: while N application increases soil N availability and stimulates microbial biomass and activity, it also elevates proton production, inhibiting microbial growth and function. These opposing effects offset each other, resulting in stable SOC levels (Treseder 2008 ; Zhou et al. 2017 ; Feng et al. 2022 ). Overall, the effect of N addition on SOC appears to be variable and context-dependent, influenced by factors such as plant community composition, soil N status, and site-specific conditions (Du et al. 2018 ; Eastman et al. 2021 ; Lu et al. 2021 ). Silicon (Si), the second abundant element in the earth’s crust, plays a beneficial role in plant growth (Epstein 1994 ; Ma and Yamaji 2006 ). It is also recognized as an important and ubiquitous driver of global change, capable of regulating plant community composition and species diversity (Schaller and Struyf, 2013 ; Xu et al. 2018 , 2020 ). Unlike N, Si addition can favor community composition and plant species by improving light interception and nutrient use efficiency, leading to increased carbon accumulation in plants (Detmann et al. 2012 ; Gong et al. 2006 ; Tamai and Ma 2008 ; Xu et al. 2022a ). Moreover, the rise in soil available silicon (SASi) resulting from Si fertilization promotes microbial growth, thereby altering the decomposition rate of plant litter (Marxen et al. 2016 ; Das et al. 2019 ; Chen et al. 2024 ). In terms of soil properties, Si fertilization can influence soil nutrient content and availability, pH, bulk density, and other related factors, which in turn affect soil carbon stability and soil respiration (Hömberg et al. 2020 ; Kuhla et al. 2021 ; Schaller et al. 2023 ). Given the multifaceted effects of Si on carbon dynamics, we hypothesize that Si fertilization may also influence SOC storage. Predicting responses of SOC sequestration to N and Si addition is inherently challenging, as SOC dynamics are also regulated by plant community structure and soil nutrient availability. Community structure, including species diversity and composition, is generally more sensitive to habitat changes than species richness alone, and can therefore significantly influence SOC sequestration (Spaak et al. 2017 ; Berdugo et al. 2019 ). Changes in habitat conditions often alter the relative biomass of plant functional groups, thereby shifting plant community structure. Consequently, quantifying the contributions of both functional group biomass and species diversity is crucial for understanding SOC storage, as these factors are key determinants of plant community structure (Marxen et al. 2016 ; Yang et al. 2022 ). Increasing evidence suggests that N addition, along with the resulting light limitation, tends to suppress non-grass species and reduce species diversity, while Si addition can partly counteract these effects by alleviating light limitation within the community under N fertilization (Tamai and Ma 2008 ; Song et al. 2018 ; Xu et al. 2018 , 2020 ). Nevertheless, field experiments investigating how Si and N addition jointly alter plant community composition remain scarce. Moreover, the response of SOC storage to combined N and Si addition, particularly in alpine ecosystems, is still poorly understood. The alpine meadows of the Qinghai-Tibetan Plateau (QTP) represent a major vegetation type and play a critical role in SOC storage, both locally and globally. Covering an area of approximately 700,000 km², these meadows store an estimated 4.4 Pg C in the topsoil (1 m depth) (Chen et al. 2013 ; Piao et al. 2019 ). This ecosystem functions as a net carbon sink, as low temperatures limit litter decomposition while ample solar radiation supports plant growth and development (Wang et al. 2019 ). Furthermore, these meadows have long experienced N limitation and are highly sensitive to N inputs, which are projected to increase in this region (Chen et al. 2013 ; Peñuelas et al. 2013 ). Consequently, N fertilization is expected to alter SOC storage, potentially modifying the feedback between the global carbon cycle and atmospheric N deposition in alpine ecosystems. Si addition, which is known to benefit plant growth and ecosystem stability on the QTP, may further modulate SOC storage and its response to atmospheric N deposition (Janssens et al. 2010 ; Quan et al. 2019 ). Although numerous studies have examined the effects of N addition on carbon storage in QTP alpine meadows (Yan et al. 2021 ; Yang et al. 2022 ), the mechanisms through which plant community structure and soil nutrients regulate the SOC pool under combined N and Si addition remain poorly understood. Here, we present results of a nine-year field experiment (2012–2020) conducted in an alpine meadow on the QTP. This study investigates the individual and combined effects of N and Si addition on SOC sequestration, and elucidated how those effects are mediated through changes in plant community composition and soil nutrient availability. The objectives of study were: (1) to investigate the responses of SOC sequestration to Si addition, N addition, and their combined application; and (2) to assess whether and how species diversity, relative biomass of grass (RBG), soil available silicon (SASi), and soil inorganic nitrogen (SIN) regulate SOC dynamics. Based on those objectives, we formulated the following hypotheses: (1) N addition would increase the grass biomass and SIN content, thereby enhancing SOC storage; (2) Si addition would elevate grass biomass and SASi, likewise promoting SOC accumulation; and (3) combined N and Si addition would jointly alter grass biomass, species diversity, SASi, and SIN, resulting in greater SOC storage than that achieved by Si or N addition alone. Materials and methods Study site This study was conducted at Gansu Gannan Grassland Ecosystem National Observation and Research Station (33°58′N, 101°53′E; 3500 m above sea level), located on the eastern margin of the QTP, China. This site has a mean annual precipitation of 620 mm and an average annual temperature of 1.2°C. The soil and vegetation are characteristic of alpine ecosystems, with the plant community dominated by Elymus nutans and Kobresia capillifolia . Experimental design The experiment area was fenced to exclude yaks and other large herbivores. A long-term experiment was established with a randomized block since April 2011. Each block contained six plots measuring 16 m × 30 m, separated by 2 m-wide walkways that served as buffer zones between adjacent plots. Four treatments were applied in each block: control (CK), nitrogen addition (N: 400 kg N ha⁻¹ year⁻¹ as NH₄NO₃), silicon addition (Si: 40 kg H₄SiO₄ ha⁻¹ year⁻¹ as, equivalent to 14.36 kg Si ha⁻¹ year⁻¹), and combined N + Si addition (NSi), resulting in a total of 24 plots. Aboveground biomass and belowground biomass In August of each even year from 2012 to 2020, coinciding with the peak of the plant biomass, all species aboveground biomass (AGB) was estimated biennially (in 2012, 2014, 2016, 2018 and 2020). Measurements were taken by harvesting all living aboveground plant material of each species within in a randomly placed 0.5 m ×0.5 m subplot and then oven-dried at 65°C for 72 h before weighing. Additionally, plant species diversity was assessed using the Shannon-Wiener index ( H ): $$\:H=-\sum\:_{i=1}^{n}Pilnpi$$ where Pi represents the proportion of total biomass contributed by species i (calculated as the biomass of the species based on individual counts), and n denotes the total number of specie in the community. Since more than 90% of roots were distributed within the 0–20 cm soil layer, belowground biomass (BGB) in this layer was collected biennially (in 2012, 2014, 2016, 2018 and 2020) during the peak biomass period using a soil drill sampler in each subplot. Within each subplot, three soil cores were randomly collected and homogenized into one composite sample. Root samples were then oven-dried at 70°C for 72 h and weighed to estimate of BGB. Additionally, soil from the 0–10 cm layer (after root removal) was passed through a 1-mm mesh sieve and used for the analysis of soil physicochemical parameters. Soil physicochemical parameter Sieved soil samples were air-dried prior to analysis. SOC concentration was determined using the Walkley-Black dichromate oxidation method. SOC storage was calculated as follows: $$\:SOC=SBD\times\:C\%\times\:1000$$ , Where SBD is soil bulk density, C% is SOC concentration. Soil inorganic nitrogen (SIN) content was measured using the Kjeldahl method, while soil available silicon (SASi) was determined by the silicomolybdic blue colorimetric method. Soil heterotrophic respiration Soil heterotrophic respiration ( Rh ) were measured using the deep collar methods with Li-8100 CO 2 fluxes analyzer (LI-COR Inc., Lincoln, NE, USA) (Yan et al. 2021 ; Xu et al. 2022b ). Deep polyvinylchloride (PVC) collars (45 cm height, 10.5 cm inner diameter) were installed in September 2019 by inserting them 37 cm into the soil in each plot. Prior to measurement, all roots and litter were cleared from the collars to ensure pure Rh . Measurements were conducted twice monthly from June to August 2020 between 9:00 and 11:00 AM on sunny days, with a 2-minute sampling duration per collar. Statistical analysis Long-term mean values for SOC, SIN, H , RBG, AGB, and BGB were calculated based on all measurements taken biennially from 2012 to 2020. One-way analyses of variance (ANOVA) with multiple comparison were used to analyze the effects of N addition, Si addition, and combined N and Si addition on these variables. Simple linear regressions were used to examine the relationship of RBG, H , AGB, BGB, SIN, and SASi with SOC. Structural equation modeling (SEM) was employed to analyze direct and indirect relationships among all variables, as well as the pathways through which factors influence SOC storage. All figures were generated using Origin 2020. Results SOC, SIN, and SASi under different treatments Over the nine-year study period, N addition significantly increased SIN and SOC by 35.3% and 13.1%, respectively, compared with the control. Si addition significantly increased SOC by 6.7%. The combined application of Si and N further enhanced SIN and SOC, with increases of 76.2% and 21.2%, respectively. Notably, SOC storage under combined Si and N fertilization was higher than that under the same rate of N applied alone (Fig. 1 a). N addition decreased SASi, Si addition, and combined N and Si addition increased SASi (Fig. 1 c). AGB, BGB, RBG, and H under different treatments Over the nine-year experimental period, N addition exerted contrasting effects on RBG and H . Specifically, N addition significantly increased RBG by 56.5% but decreased H by 8.7% relative to the control (Fig. 2 ). In contrast, Si addition alone significantly enhanced both RBG and H , by 8.8% and 20.0%, respectively. The combined N and Si treatment (NSi) also led to significant increases in RBG and H , 41.7% (Fig. 2 a) and 6.6% (Fig. 2 b), respectively. Regarding AGB and BGB, N addition significantly increased both parameters by 48.9% and 15.7%, respectively, relative to the control. Si addition alone also significantly raised AGB and BGB, by 8.5% and 10.8%, respectively. The combined application of Si and N further amplified these effects, leading to greater increases of 68.6% in AGB and 20.6% in BGB (Fig. 3 ). Heterotrophic respiration under different treatments N addition, Si addition, and combined N and Si addition significantly increased soil heterotrophic respiration by 3.39%, 5.46%, and 8.11%, respectively, compared with the control (Fig. 4 ). Pathways through which experimental treatments influenced SOC Structural equation modeling (SEM) results revealed that under N fertilization, SOC storage was positively associated with AGB, BGB, and Rh (Fig. 5 a). With Si addition alone, SOC increased directly through enhanced AGB, and Rh (Fig. 5 b). Under the combined Si and N treatment, SOC was positively influenced by SASi, SIN, and RBG, which in turn promoted AGB and BGB, BGB and Rh , and AGB, respectively (Fig. 5 c). Discussion Based on a 9-year field experiment with N addition, Si addition, and their combined application, this study reveals distinct mechanisms driving SOC accumulation. N addition significantly enhanced SOC storage primarily by increasing both RBG and SIN, which in turn stimulated AGB. In contrast, Si addition promoted SOC storage mainly by elevating RBG and SASi, thereby boosting AGB, BGB, and Rh . Under the combined N plus Si treatment, SOC accumulation resulted from the synergistic effects of elevated SASi, SIN, and RBG. SASi enhanced AGB and BGB, SIN supported higher AGB and Rh , and RBG further contributed to AGB. Overall, the shifting roles of AGB, BGB, and Rh in governing SOC dynamics were largely attributable to differential changes in RBG, SIN, and SASi induced by each treatment. Our findings highlight the necessity of incorporating plant community composition, particularly RBG, along with associated changes in soil SASi and SIN availability when predicting how Si and N fertilization will affect SOC sequestration. Responses of SOC to N and Si addition Consistent with our first hypothesis, a significant increase in SOC storage was observed over the nine years of field N addition (Fig. 1 a). This result aligns with findings from other N fertilization experiments in alpine meadows (Stevens et al. 2015 ; Zhang et al. 2025 ). The dynamics of SOC sequestration are governed by the balance between carbon inputs from plant litter decomposition and carbon losses through soil respiration (Jackson et al. 2017 ; Yan et al. 2021 ). In N-limited ecosystems such as alpine meadows, N addition typically stimulates plant photosynthetic activity and enhances plant growth (Xu et al. 2018 ), which can in turn promote SOC accumulation. In our study, N fertilization also led to a rise in Rh (Fig. 4 ), a response consistent with reports that low to moderate N addition can elevate soil microbial metabolic rates (Qu et al. 2022 ; Yang et al. 2022 ). Moreover, we detected a significant increase in BGB under N fertilization (Fig. 4 b), further supporting enhanced carbon input to the soil. Taken together, our findings suggest that alpine meadows may function as a net carbon sink under future scenarios of increased N fertilization or atmospheric N deposition. The increase in SOC sequestration under Si addition was primarily driven by elevated AGB and RBG, reflecting enhanced ecosystem carbon uptake. This response was closely associated with increased SASi (Fig. 5 b), in line with our second hypothesis. In terrestrial ecosystems, plants primarily acquire Si from the soil, and higher SASi can raise plant tissue Si concentrations. Elevated Si content promotes more upright leaf and stem growth, increasing light capture and thus enhancing photosynthetic activity (Ma and Yamaji 2006 ; Xu et al. 2018 ). Moreover, Si addition can modify soil pH (Supplementary Fig. 1S), reduce exchangeable aluminum, and ultimately improve N and phosphorus availability (Eneji et al. 2008 ; Lee et al. 2004 ; de Toledo et al. 2021 ; Schaller et al. 2023 ). Collectively, these mechanisms explain how Si addition enhances SOC sequestration through increased SASi and SIN, which subsequently promote AGB and RBG, consistent with the pathways summarized in our conceptual framework (Fig. 5 b). In parallel, Si addition significantly stimulated Rh via elevated SASi, underscoring the key role of soil available Si in regulating soil carbon fluxes. This aligns with previous reports that Si can enhance microbial activity and accelerate litter decomposition, thereby increasing soil carbon turnover (Marxen et al. 2016 ; Das et al. 2019 ; Chen et al. 2024 ). Our results show that combined N and Si addition enhanced SOC storage, with a greater positive effect compared to the same amount of N or Si applied alone, consistent with our third hypothesis. The increase in SOC under combined treatment was primarily driven by elevated SASi, SIN and RBG, which were positively correlated with SOC storage through their respective promotion of AGB, BGB and AGB. Although N plus Si addition increased both RBG and SIN (Fig. 1 , Fig. 2 ), with the former negatively affecting Rh and the latter positively affecting Rh , the net effect on Rh was dominated by the negative influence of RGB (Fig. 5 c), thereby contributing to enhanced SOC accumulation. This aligns with previous reports that Si addition can stimulate soil CO₂ flux (Ali et al. 2008 ; de Toledo et al. 2021 ; Eneji et al. 2008 ; Hömberg et al. 2021 ; Lee et al. 2004 ; Schaller et al. 2023 ). Overall, the increases in RBG, SASi, and SIN under combined N and Si addition collectively promoted AGB and BGB, underscoring the pivotal role of these factors in regulating SOC dynamics and potentially activating negative feedback to N fertilization in this alpine region. Responses of RBG, H, SASi and SIN to N and Si addition The increase in RBG and decline in H under N addition can be largely attributed to the stronger competitive capacity of grass in nutrient uptake compared to non-grass species (Yang et al. 2011 ; Wang et al. 2012 ; Yan et al. 2021 ; Elrys et al. 2022 ). In this alpine meadow community, grass typically exhibit greater plant height than non-grass species (Supplementary Fig. 2S), which further enhances their ability to compete for light, thereby likely reinforcing the positive effect on RBG and the negative effect on H . Si plus N addition led to increases in both RBG and SASi (Fig. 1 c, Fig. 2 a). SASi plays a pivotal role in shaping plant community traits: for instance, it promotes more upright leaf and stem growth, thereby enhancing plant competitiveness for light resources (Eneji et al. 2008 ; Xu et al. 2018 , 2020 ). The higher species diversity ( H ) observed under Si addition and under NSi treatment, compared to N addition alone suggests that Si can partially offset the biodiversity loss and ecosystem disturbance induced by N fertilization. Meanwhile, SIN content was higher under NSi treatment than under N addition alone (Fig. 1 b), likely due to Si-induced increases in soil availability of nitrogen, phosphorus, calcium, and water-soluble iron (Lee et al. 2004 ; de Toledo et al. 2021 ), which in turn influences plant community traits. RBG, SASi and SIN mediate N and Si addition effects on SOC Previous studies have suggested that higher plant diversity is generally associated with greater SOC storage, largely because more diverse communities tend to support higher above- and belowground biomass, thereby enhancing carbon accumulation (Tilman et al. 2012 ; Stevens et al. 2015 ; Chen et al. 2018 ; Piao et al. 2019 ; Prommer et al. 2020 ). In contrast, our study found that under N addition, Si addition, and their combined application, species diversity ( H ) had only a minor influence on SOC storage. This discrepancy may be attributed to the relatively complex community composition at our study site, which hosts 20–30 plant species. The species lost under N addition were primarily rare ones, whose low productivity likely resulted in a limited contribution to overall SOC dynamics (Geisseler et al. 2017 ; Zhu et al. 2022 ). Nitrogen fertilization exerted contrasting effects on belowground C inputs. While N addition directly stimulated belowground C allocation, the associated soil N enrichment concurrently suppressed belowground C inputs (Fig. 5 a). The stimulatory effect ultimately outweighed the suppressive effect, leading to a net increase in BGB (Fig. 3 b). Additionally, soil N enrichment resulting from N fertilization promoted AGB and shifted the dominant plant functional group from non-grass toward grass, a change that further enhanced SOC storage. Because grass litter decomposes more slowly than that of sedges and forbs (Du et al. 2018 ; Yan et al. 2021 ; Qu et al. 2022 ; Chen et al. 2024 ), the increased RBG under N addition likely reduced Rh , thereby positively influencing SOC accumulation. Ultimately, the greater C gains coupled with lower C losses under N fertilization resulted in enhanced SOC storage. In this study, Si addition, as well as combined N and Si addition and the corresponding increase in SASi, enhanced both AGB and BGB, thereby promoting SOC storage (Supplementary Fig. S3, Fig. 5 c). These results confirm the third hypothesis of this study. This process was simultaneously influenced by two counteracting mechanisms: increased plant litter decomposition due to elevated SASi, and decreased litter decomposition resulting from a higher RBG (Du et al. 2018 ; Yan et al. 2021 ; Qu et al. 2022 ; Schaller et al. 2023 ). These opposing effects partially offset each other, ultimately contributing to enhanced SOC stability (Schaller and Struyf 2013 ; Kuhla et al. 2021 ). Overall, whereas N addition alone primarily increased SOC quantity, the addition of Si (both alone and in combination with N) improved both the quantity and stability of SOC (Fig. 6 ). This aligns with recent findings that Si fertilization facilitates SOC accumulation by boosting aboveground carbon inputs (Marxen et al. 2016 ; Chen et al. 2024 ). In conclusion, our findings indicate that combined N and Si addition not only increases SOC storage but also improves its stability. Compared to N addition alone, the combined treatment promotes greater SOC accumulation via the synergistic effects of RBG, SASi, and SIN, whereas species diversity appears to play a negligible role in this process. These findings highlight the critical role of plant community composition in SOC storage under anthropogenic influences. It should be noted that although this study elucidates the mechanisms by which N, Si, and their interaction influence SOC dynamics, certain limitations remain. First, while the experiment spanned nine years, SOC turnover operates over longer timescales, and longer-term observations may better reveal its dynamic trends. Second, this study primarily focused on plant community and soil nutrient responses, without quantifying the specific contributions of key biogeochemical processes such as microbial community structure and extracellular enzyme activities. Additionally, interannual variations in environmental factors such as temperature and humidity under field conditions may have introduced some interference to the results. References Ali MA, Lee CH, Kim PJ (2008) Effect of silicate fertilizer on reducing methane emission during rice cultivation. 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Funct Ecol 33:514–523. https://doi.org/10.1111/1365-2435.13256 Xu D, Gao T, Fang X, Bu H, Li Q, Wang X, Zhang R (2020) Silicon fertilization improves plant productivity and soil nutrient availability without changing the grass:legume ratio response to N fertilization. Sci Rep 10:10295. https://doi.org/10.1038/s41598-020-67333-7 Xu D, Gao X, Gao T, Mou J, Li J, Bu H, Zhang R, Li Q (2018) Interactive effects of nitrogen and silicon fertilization on growth of five common plant species and structure of plant community in alpine meadow. CATENA 169:80–89. https://doi.org/10.1016/j.catena.2018.05.017 Xu D, Mohammad AH, Robert H (2022a) Biological function of silicon in a grassland ecosystem. Silicon and Nano-silicon in Environmental. Stress Manage Crop Qual Improv 5:43–54. https://doi.org/10.1016/B978-0-323-91225-9.00018-2 Xu D, Mou W, Wang X, Zhang R, Gao T, Ai D, Yuan J, Zhang R, Fang X (2022b) Consistent responses of ecosystem CO 2 exchange to grassland degradation in alpine meadow of the Qinghai-Tibetan Plateau. Ecol Indic 141:10903. https://doi.org/10.1016/j.ecolind.2022.109036 Yan Y, Quan Q, Meng C, Wang J, Tian D, Wang B, Zhang R, Niu S (2021) Varying soil respiration under long-term warming and clipping due to shifting carbon allocation toward below-ground. Agr For Meteorol 108408:304–305. https://doi.org/10.1016/j.agrformet.2021.108408 Yang Y, Chen X, Liu L, Li T, Dou Y, Qiao J, Wang Y, An S, Chang SX (2022) Nitrogen fertilization weakens the linkage between soil carbon and microbial diversity: A global meta-analysis. Glob Chang Biol 28:6446–6461. https://doi.org/10.1111/gcb.16361 Yang Z, van Ruijven J, Du G (2011) The effects of long-term fertilization on the temporal stability of alpine meadow communities. Plant Soil 345:315–324. https://doi.org/10.1007/s11104-011-0784-0 Zhou Z, Wang C, Zheng M, Jiang L, Luo Y (2017) Patterns and mechanisms of responses by soil microbial communities to nitrogen fertilization. Soil Biol Biochem 115:433–441. https://doi.org/10.1016/j.soilb io.2017.09.015 Zhu X, Zhang Z, Wang Q, Peñuelas J, Sardans J, Lambers H, Liu Z (2022) More soil organic carbon is sequestered through the mycelium-pathway than through the root-pathway under nitrogen enrichment in an alpine forest. Glob Chang Biol 28:4947–4961. https://doi.org/10.1111/gcb.16263 Zhang F, Li H, Zhu J, Wang C, He Y, Zhu J, Yu Q, Zhou H, Li Y, Liang N (2025) Context dependencies in the responses of plant biomass and surface soil organic carbon content to nitrogen addition and precipitation change within alpine grasslands. Agr Ecosyst Environ 381:109475. https://doi.org/10.1016/j.agee.2025.109475 Supplementary Files Fig.S1.jpg Fig.S2.jpg 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. 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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-8646618","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":587799506,"identity":"34b9e2c4-8646-441a-a9ee-bdb42ab80849","order_by":0,"name":"danghui Xu","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAArElEQVRIiWNgGAWjYDCCG0D8oQDMNCBeC+MMA1K1MPOQpIXvdo+ZtI1BXWIDe/M2CYaaO4S1SN45liadY8CW2MBzrEyC4dgzwloMbiQfA2rhSWyQyDGTYGw4TIyWxDZpCwOJxAb5N0RrAdrCYGAAtIWHSC2SN9KSLXsMEozbeNKKLRKOEaGF70aO4Y0fFXWy/eyHN974UEOEFjhgAxEJJGgYBaNgFIyCUYAHAAB1hTRuEbcRqAAAAABJRU5ErkJggg==","orcid":"https://orcid.org/0000-0002-0953-4733","institution":"State Key Laboratory of Grassland Agro-ecosystems/School of Life Science, Lanzhou University","correspondingAuthor":true,"prefix":"","firstName":"danghui","middleName":"","lastName":"Xu","suffix":""},{"id":587799507,"identity":"be80e5d3-b7a8-40ee-999a-3020918857c2","order_by":1,"name":"Baiying Huang","email":"","orcid":"","institution":"","correspondingAuthor":false,"prefix":"","firstName":"Baiying","middleName":"","lastName":"Huang","suffix":""},{"id":587799508,"identity":"049f7510-57fd-4bc6-b915-e0a66fbed2ef","order_by":2,"name":"Yawen Gui","email":"","orcid":"","institution":"","correspondingAuthor":false,"prefix":"","firstName":"Yawen","middleName":"","lastName":"Gui","suffix":""},{"id":587799509,"identity":"550f2b34-7b12-4826-8d3b-feeac9765694","order_by":3,"name":"Wenhong Zhou","email":"","orcid":"","institution":"","correspondingAuthor":false,"prefix":"","firstName":"Wenhong","middleName":"","lastName":"Zhou","suffix":""},{"id":587799510,"identity":"ff221f51-76c1-4678-922e-64ad9a7dcb42","order_by":4,"name":"Yuqi Wu","email":"","orcid":"","institution":"","correspondingAuthor":false,"prefix":"","firstName":"Yuqi","middleName":"","lastName":"Wu","suffix":""},{"id":587799511,"identity":"8cd94d4c-cd98-41cb-bf5b-58999aa096a1","order_by":5,"name":"Wei Mou","email":"","orcid":"","institution":"","correspondingAuthor":false,"prefix":"","firstName":"Wei","middleName":"","lastName":"Mou","suffix":""},{"id":587799512,"identity":"17f25ddb-1a50-4ccd-9876-0a01f37b84de","order_by":6,"name":"Zhengwei Ren","email":"","orcid":"","institution":"","correspondingAuthor":false,"prefix":"","firstName":"Zhengwei","middleName":"","lastName":"Ren","suffix":""},{"id":587799513,"identity":"d8766803-8dc9-4820-8366-9d5f0d77dff7","order_by":7,"name":"Zhaoliang Song","email":"","orcid":"","institution":"","correspondingAuthor":false,"prefix":"","firstName":"Zhaoliang","middleName":"","lastName":"Song","suffix":""},{"id":587799514,"identity":"2122acbb-106b-439d-9fa1-c7b82ff3fb36","order_by":8,"name":"Renyi Zhang","email":"","orcid":"","institution":"","correspondingAuthor":false,"prefix":"","firstName":"Renyi","middleName":"","lastName":"Zhang","suffix":""},{"id":587799515,"identity":"cc9c06cb-efba-4c7b-9b74-9c6c52badc7c","order_by":9,"name":"Tianpeng Gao","email":"","orcid":"","institution":"","correspondingAuthor":false,"prefix":"","firstName":"Tianpeng","middleName":"","lastName":"Gao","suffix":""},{"id":587799516,"identity":"f2102ddf-5956-4fa4-bec2-1838a5ca7727","order_by":10,"name":"Xiangwen Fang","email":"","orcid":"","institution":"","correspondingAuthor":false,"prefix":"","firstName":"Xiangwen","middleName":"","lastName":"Fang","suffix":""}],"badges":[],"createdAt":"2026-01-20 08:15:19","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-8646618/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-8646618/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":102498057,"identity":"947c9485-c325-4cc6-a8c4-9be615656984","added_by":"auto","created_at":"2026-02-12 09:57:33","extension":"jpg","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":7637513,"visible":true,"origin":"","legend":"\u003cp\u003eEffects of nitrogen and silicon addition on (a) soil organic carbon storage, (b)soil inorganic nitrogencontent, and (c) soil available silicon content from 2012 to 2020 (measured biennially). Data are shown as mean ±SE.\u003c/p\u003e","description":"","filename":"Fig.1.jpg","url":"https://assets-eu.researchsquare.com/files/rs-8646618/v1/5da21293aaaf4a2596dfc094.jpg"},{"id":102498101,"identity":"2f9c813a-d531-4cf5-a4c4-93f77398acea","added_by":"auto","created_at":"2026-02-12 09:57:39","extension":"jpg","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":165520,"visible":true,"origin":"","legend":"\u003cp\u003eEffects of nitrogen and silicon addition on (a) the relative biomass of grass and (b) Shannon-Wiener index (\u003cem\u003eH\u003c/em\u003e) content from 2012 to 2020 (measured biennially). Data are shown as mean ±SE.\u003c/p\u003e","description":"","filename":"Fig.2.jpg","url":"https://assets-eu.researchsquare.com/files/rs-8646618/v1/c2154fa7d2c035fbbd7a59f1.jpg"},{"id":102498094,"identity":"e2b47663-e364-4403-9b82-0b4acee54a8a","added_by":"auto","created_at":"2026-02-12 09:57:37","extension":"jpg","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":5384335,"visible":true,"origin":"","legend":"\u003cp\u003eEffects of nitrogen and silicon addition on (a) aboveground biomass and (b) belowground biomass from 2012 to 2020 (measured biennially). Data are shown as mean ±SE.\u003c/p\u003e","description":"","filename":"Fig.3.jpg","url":"https://assets-eu.researchsquare.com/files/rs-8646618/v1/85b822e94a8b0a46dc0e92a4.jpg"},{"id":102498114,"identity":"9b8c97e7-451b-44f4-bfe7-7794e5e9b593","added_by":"auto","created_at":"2026-02-12 09:57:45","extension":"jpg","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":3312651,"visible":true,"origin":"","legend":"\u003cp\u003eEffects of nitrogen and silicon addition on soil heterotrophic respiration. Data are shown as mean ±SE.\u003c/p\u003e","description":"","filename":"Fig.4.jpg","url":"https://assets-eu.researchsquare.com/files/rs-8646618/v1/ab476327320afdf1ed3167ad.jpg"},{"id":102498090,"identity":"9114e6c9-7e16-4ae1-95ee-cd616638847c","added_by":"auto","created_at":"2026-02-12 09:57:36","extension":"jpg","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":1103755,"visible":true,"origin":"","legend":"\u003cp\u003eStructural equation models displaying the effect of N addition (a), Si addition (b), and N plus Si addition (c) on SOC sequestration. Numbers on arrows represent standardized coefficients. Red and blue arrows represent positive and negative paths respectively. \u003cem\u003eR\u003c/em\u003e\u003csup\u003e2\u003c/sup\u003e is the proportion of variance explained by models. ***P\u0026lt; 0.001, **P\u0026lt; 0.01, *P\u0026lt; 0.05. The model for N addition had PCMIN/DF=2.376, RMR=0.001, NFI=0.901, IFI=0.943, CFI=0.937 (a); the model for Si addition had PCMIN/DF=2.217, RMR\u0026lt;0.001, NFI=0.944, IFI=0.961, CFI=0.961 (b); and PCMIN/DF=2.141, RMR\u0026lt;0.001, NFI=0.914, IFI=0.956, CFI=0.955 (c).\u003c/p\u003e","description":"","filename":"Fig.5.jpg","url":"https://assets-eu.researchsquare.com/files/rs-8646618/v1/ae4113f1b559c5a4711d0de3.jpg"},{"id":102498091,"identity":"95603519-bc6c-43a0-ac6e-c036bc80f736","added_by":"auto","created_at":"2026-02-12 09:57:36","extension":"jpg","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":2652646,"visible":true,"origin":"","legend":"\u003cp\u003eConceptual model of the dynamic response of soil organic carbon (SOC) storage to nitrogen (N) addition, silicon (Si) addition, and N plus Si addition and the potential mechanisms involved in the plant community and soil nutrient change in the alpine meadow on the Qinghai–Tibetan Plateau. Red arrows indicate an increase, blue arrows indicate a decrease, and grey lines indicate an insignificant change. The thicker the arrow, the greater the magnitude of change. N addition, Si addition, and N plus Si addition Si addition all altered plant community composition, ultimately leading to an increase in both aboveground biomass and belowground biomass. Moreover, soil decomposition (\u003cem\u003eRh\u003c/em\u003e) also increased under all treatments. As a result, the storage of SOC increased under all treatments. Compared to N addition alone, combined N and Si addition leads to greater SOC accumulation via the synergistic effects of plant community composition and soil nutrient condition.\u003c/p\u003e","description":"","filename":"Fig.6.jpg","url":"https://assets-eu.researchsquare.com/files/rs-8646618/v1/02bcc54cb6341f496c11a0a3.jpg"},{"id":105034610,"identity":"2973e967-9de5-451d-8ee4-e3d27952041b","added_by":"auto","created_at":"2026-03-20 07:23:42","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":20757105,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-8646618/v1/e9658384-fd30-420a-9732-408d460ba35f.pdf"},{"id":102498081,"identity":"16337922-1823-4c9b-b0fc-5d3d4b398259","added_by":"auto","created_at":"2026-02-12 09:57:35","extension":"jpg","order_by":11,"title":"","display":"","copyAsset":false,"role":"supplement","size":3120188,"visible":true,"origin":"","legend":"","description":"","filename":"Fig.S1.jpg","url":"https://assets-eu.researchsquare.com/files/rs-8646618/v1/a6b116667bf5fba76fb68fd1.jpg"},{"id":102498098,"identity":"e6ad8494-194d-4d00-abe4-c57252d35f87","added_by":"auto","created_at":"2026-02-12 09:57:38","extension":"jpg","order_by":12,"title":"","display":"","copyAsset":false,"role":"supplement","size":2832782,"visible":true,"origin":"","legend":"","description":"","filename":"Fig.S2.jpg","url":"https://assets-eu.researchsquare.com/files/rs-8646618/v1/2d0beb53b2f69b470f7b1af9.jpg"}],"financialInterests":"","formattedTitle":"Changed plant community composition promotes soil carbon sequestration under long-term silicon and nitrogen addition in an alpine meadow","fulltext":[{"header":"Introduction","content":"\u003cp\u003eThe development of livestock husbandry and modern industry has significantly increased soil nitrogen availability through anthropogenic inputs such as fertilization and atmospheric deposition. This elevated N supply stimulates plant growth and alleviates N limitation in terrestrial ecosystems (Kuypers et al. \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e2018\u003c/span\u003e; Elrys et al. \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). A frequent outcome of N enrichment is an increase in ecosystem primary productivity accompanied by shifts in plant community composition, typically favoring grass and often at the expense of species diversity (Xu et al. \u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e2018\u003c/span\u003e; Yan et al. \u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e2021\u003c/span\u003e). These vegetation changes can, in turn, influence soil organic carbon (SOC) dynamics. Nevertheless, the regulatory mechanisms underlying SOC sequestration, particularly the respective roles played by declining species diversity and increasing grass dominance, remain inadequately understood.\u003c/p\u003e \u003cp\u003eSoil organic carbon storage is governed by the balance between carbon (C) sequestration from plant growth and C release from microbial decomposition (Elrys et al. \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e2022\u003c/span\u003e; Yang et al. \u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). Extensive research suggests that N addition can influence both C gains and C losses (Janssens et al. \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e2010\u003c/span\u003e; Song et al. \u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e2019\u003c/span\u003e). Some studies have suggested that N addition increases aboveground litter production, thereby promoting SOC formation and accumulation (Stevens et al. \u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e2015\u003c/span\u003e; Schulte-Uebbing et al. \u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). However, other have observed that N addition reduce SOC, likely because high N inputs increase protons release and decrease soil microbial diversity, negatively impacting both plants and microbial communities (Geisseler et al. \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e2017\u003c/span\u003e; Zhu et al. \u003cspan citationid=\"CR52\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). In contrast, certain studies found that no significant change in SOC following N inputs, which can be attributed to two counteracting mechanisms: while N application increases soil N availability and stimulates microbial biomass and activity, it also elevates proton production, inhibiting microbial growth and function. These opposing effects offset each other, resulting in stable SOC levels (Treseder \u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e2008\u003c/span\u003e; Zhou et al. \u003cspan citationid=\"CR51\" class=\"CitationRef\"\u003e2017\u003c/span\u003e; Feng et al. \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). Overall, the effect of N addition on SOC appears to be variable and context-dependent, influenced by factors such as plant community composition, soil N status, and site-specific conditions (Du et al. \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e2018\u003c/span\u003e; Eastman et al. \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e2021\u003c/span\u003e; Lu et al. \u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e2021\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eSilicon (Si), the second abundant element in the earth\u0026rsquo;s crust, plays a beneficial role in plant growth (Epstein \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e1994\u003c/span\u003e; Ma and Yamaji \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e2006\u003c/span\u003e). It is also recognized as an important and ubiquitous driver of global change, capable of regulating plant community composition and species diversity (Schaller and Struyf, \u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e2013\u003c/span\u003e; Xu et al. \u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e2018\u003c/span\u003e, \u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). Unlike N, Si addition can favor community composition and plant species by improving light interception and nutrient use efficiency, leading to increased carbon accumulation in plants (Detmann et al. \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e2012\u003c/span\u003e; Gong et al. \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e2006\u003c/span\u003e; Tamai and Ma \u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e2008\u003c/span\u003e; Xu et al. \u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e2022a\u003c/span\u003e). Moreover, the rise in soil available silicon (SASi) resulting from Si fertilization promotes microbial growth, thereby altering the decomposition rate of plant litter (Marxen et al. \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e2016\u003c/span\u003e; Das et al. \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e2019\u003c/span\u003e; Chen et al. \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e2024\u003c/span\u003e). In terms of soil properties, Si fertilization can influence soil nutrient content and availability, pH, bulk density, and other related factors, which in turn affect soil carbon stability and soil respiration (H\u0026ouml;mberg et al. \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e2020\u003c/span\u003e; Kuhla et al. \u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e2021\u003c/span\u003e; Schaller et al. \u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e2023\u003c/span\u003e). Given the multifaceted effects of Si on carbon dynamics, we hypothesize that Si fertilization may also influence SOC storage.\u003c/p\u003e \u003cp\u003ePredicting responses of SOC sequestration to N and Si addition is inherently challenging, as SOC dynamics are also regulated by plant community structure and soil nutrient availability. Community structure, including species diversity and composition, is generally more sensitive to habitat changes than species richness alone, and can therefore significantly influence SOC sequestration (Spaak et al. \u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e2017\u003c/span\u003e; Berdugo et al. \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2019\u003c/span\u003e). Changes in habitat conditions often alter the relative biomass of plant functional groups, thereby shifting plant community structure. Consequently, quantifying the contributions of both functional group biomass and species diversity is crucial for understanding SOC storage, as these factors are key determinants of plant community structure (Marxen et al. \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e2016\u003c/span\u003e; Yang et al. \u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). Increasing evidence suggests that N addition, along with the resulting light limitation, tends to suppress non-grass species and reduce species diversity, while Si addition can partly counteract these effects by alleviating light limitation within the community under N fertilization (Tamai and Ma \u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e2008\u003c/span\u003e; Song et al. \u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e2018\u003c/span\u003e; Xu et al. \u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e2018\u003c/span\u003e, \u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). Nevertheless, field experiments investigating how Si and N addition jointly alter plant community composition remain scarce. Moreover, the response of SOC storage to combined N and Si addition, particularly in alpine ecosystems, is still poorly understood.\u003c/p\u003e \u003cp\u003eThe alpine meadows of the Qinghai-Tibetan Plateau (QTP) represent a major vegetation type and play a critical role in SOC storage, both locally and globally. Covering an area of approximately 700,000 km\u0026sup2;, these meadows store an estimated 4.4 Pg C in the topsoil (1 m depth) (Chen et al. \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e2013\u003c/span\u003e; Piao et al. \u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e2019\u003c/span\u003e). This ecosystem functions as a net carbon sink, as low temperatures limit litter decomposition while ample solar radiation supports plant growth and development (Wang et al. \u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e2019\u003c/span\u003e). Furthermore, these meadows have long experienced N limitation and are highly sensitive to N inputs, which are projected to increase in this region (Chen et al. \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e2013\u003c/span\u003e; Pe\u0026ntilde;uelas et al. \u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e2013\u003c/span\u003e). Consequently, N fertilization is expected to alter SOC storage, potentially modifying the feedback between the global carbon cycle and atmospheric N deposition in alpine ecosystems. Si addition, which is known to benefit plant growth and ecosystem stability on the QTP, may further modulate SOC storage and its response to atmospheric N deposition (Janssens et al. \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e2010\u003c/span\u003e; Quan et al. \u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e2019\u003c/span\u003e). Although numerous studies have examined the effects of N addition on carbon storage in QTP alpine meadows (Yan et al. \u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e2021\u003c/span\u003e; Yang et al. \u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e2022\u003c/span\u003e), the mechanisms through which plant community structure and soil nutrients regulate the SOC pool under combined N and Si addition remain poorly understood.\u003c/p\u003e \u003cp\u003eHere, we present results of a nine-year field experiment (2012\u0026ndash;2020) conducted in an alpine meadow on the QTP. This study investigates the individual and combined effects of N and Si addition on SOC sequestration, and elucidated how those effects are mediated through changes in plant community composition and soil nutrient availability. The objectives of study were: (1) to investigate the responses of SOC sequestration to Si addition, N addition, and their combined application; and (2) to assess whether and how species diversity, relative biomass of grass (RBG), soil available silicon (SASi), and soil inorganic nitrogen (SIN) regulate SOC dynamics. Based on those objectives, we formulated the following hypotheses: (1) N addition would increase the grass biomass and SIN content, thereby enhancing SOC storage; (2) Si addition would elevate grass biomass and SASi, likewise promoting SOC accumulation; and (3) combined N and Si addition would jointly alter grass biomass, species diversity, SASi, and SIN, resulting in greater SOC storage than that achieved by Si or N addition alone.\u003c/p\u003e"},{"header":"Materials and methods","content":"\u003cp\u003eStudy site\u003c/p\u003e \u003cp\u003eThis study was conducted at Gansu Gannan Grassland Ecosystem National Observation and Research Station (33\u0026deg;58\u0026prime;N, 101\u0026deg;53\u0026prime;E; 3500 m above sea level), located on the eastern margin of the QTP, China. This site has a mean annual precipitation of 620 mm and an average annual temperature of 1.2\u0026deg;C. The soil and vegetation are characteristic of alpine ecosystems, with the plant community dominated by \u003cem\u003eElymus nutans\u003c/em\u003e and \u003cem\u003eKobresia capillifolia\u003c/em\u003e.\u003c/p\u003e \u003cp\u003eExperimental design\u003c/p\u003e \u003cp\u003eThe experiment area was fenced to exclude yaks and other large herbivores. A long-term experiment was established with a randomized block since April 2011. Each block contained six plots measuring 16 m \u0026times; 30 m, separated by 2 m-wide walkways that served as buffer zones between adjacent plots. Four treatments were applied in each block: control (CK), nitrogen addition (N: 400 kg N ha⁻\u0026sup1; year⁻\u0026sup1; as NH₄NO₃), silicon addition (Si: 40 kg H₄SiO₄ ha⁻\u0026sup1; year⁻\u0026sup1; as, equivalent to 14.36 kg Si ha⁻\u0026sup1; year⁻\u0026sup1;), and combined N\u0026thinsp;+\u0026thinsp;Si addition (NSi), resulting in a total of 24 plots.\u003c/p\u003e \u003cp\u003eAboveground biomass and belowground biomass\u003c/p\u003e \u003cp\u003eIn August of each even year from 2012 to 2020, coinciding with the peak of the plant biomass, all species aboveground biomass (AGB) was estimated biennially (in 2012, 2014, 2016, 2018 and 2020). Measurements were taken by harvesting all living aboveground plant material of each species within in a randomly placed 0.5 m \u0026times;0.5 m subplot and then oven-dried at 65\u0026deg;C for 72 h before weighing. Additionally, plant species diversity was assessed using the Shannon-Wiener index (\u003cem\u003eH\u003c/em\u003e):\u003cdiv id=\"Equa\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equa\" name=\"EquationSource\"\u003e\n$$\\:H=-\\sum\\:_{i=1}^{n}Pilnpi$$\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003ewhere Pi represents the proportion of total biomass contributed by species i (calculated as the biomass of the species based on individual counts), and n denotes the total number of specie in the community.\u003c/p\u003e \u003cp\u003eSince more than 90% of roots were distributed within the 0\u0026ndash;20 cm soil layer, belowground biomass (BGB) in this layer was collected biennially (in 2012, 2014, 2016, 2018 and 2020) during the peak biomass period using a soil drill sampler in each subplot. Within each subplot, three soil cores were randomly collected and homogenized into one composite sample. Root samples were then oven-dried at 70\u0026deg;C for 72 h and weighed to estimate of BGB. Additionally, soil from the 0\u0026ndash;10 cm layer (after root removal) was passed through a 1-mm mesh sieve and used for the analysis of soil physicochemical parameters.\u003c/p\u003e \u003cp\u003eSoil physicochemical parameter\u003c/p\u003e \u003cp\u003eSieved soil samples were air-dried prior to analysis. SOC concentration was determined using the Walkley-Black dichromate oxidation method. SOC storage was calculated as follows:\u003cdiv id=\"Equb\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equb\" name=\"EquationSource\"\u003e\n$$\\:SOC=SBD\\times\\:C\\%\\times\\:1000$$\u003c/div\u003e\u003c/div\u003e,\u003c/p\u003e \u003cp\u003eWhere SBD is soil bulk density, C% is SOC concentration.\u003c/p\u003e \u003cp\u003eSoil inorganic nitrogen (SIN) content was measured using the Kjeldahl method, while soil available silicon (SASi) was determined by the silicomolybdic blue colorimetric method.\u003c/p\u003e \u003cp\u003eSoil heterotrophic respiration\u003c/p\u003e \u003cp\u003eSoil heterotrophic respiration (\u003cem\u003eRh\u003c/em\u003e) were measured using the deep collar methods with Li-8100 CO\u003csub\u003e2\u003c/sub\u003e fluxes analyzer (LI-COR Inc., Lincoln, NE, USA) (Yan et al. \u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e2021\u003c/span\u003e; Xu et al. \u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e2022b\u003c/span\u003e). Deep polyvinylchloride (PVC) collars (45 cm height, 10.5 cm inner diameter) were installed in September 2019 by inserting them 37 cm into the soil in each plot. Prior to measurement, all roots and litter were cleared from the collars to ensure pure \u003cem\u003eRh\u003c/em\u003e. Measurements were conducted twice monthly from June to August 2020 between 9:00 and 11:00 AM on sunny days, with a 2-minute sampling duration per collar.\u003c/p\u003e \u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003eStatistical analysis\u003c/h2\u003e \u003cp\u003eLong-term mean values for SOC, SIN, \u003cem\u003eH\u003c/em\u003e, RBG, AGB, and BGB were calculated based on all measurements taken biennially from 2012 to 2020. One-way analyses of variance (ANOVA) with multiple comparison were used to analyze the effects of N addition, Si addition, and combined N and Si addition on these variables. Simple linear regressions were used to examine the relationship of RBG, \u003cem\u003eH\u003c/em\u003e, AGB, BGB, SIN, and SASi with SOC. Structural equation modeling (SEM) was employed to analyze direct and indirect relationships among all variables, as well as the pathways through which factors influence SOC storage. All figures were generated using Origin 2020.\u003c/p\u003e \u003c/div\u003e"},{"header":"Results","content":"\u003cp\u003eSOC, SIN, and SASi under different treatments\u003c/p\u003e \u003cp\u003eOver the nine-year study period, N addition significantly increased SIN and SOC by 35.3% and 13.1%, respectively, compared with the control. Si addition significantly increased SOC by 6.7%. The combined application of Si and N further enhanced SIN and SOC, with increases of 76.2% and 21.2%, respectively. Notably, SOC storage under combined Si and N fertilization was higher than that under the same rate of N applied alone (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ea). N addition decreased SASi, Si addition, and combined N and Si addition increased SASi (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ec).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eAGB, BGB, RBG, and H under different treatments\u003c/p\u003e \u003cp\u003eOver the nine-year experimental period, N addition exerted contrasting effects on RBG and \u003cem\u003eH\u003c/em\u003e.\u003c/p\u003e \u003cp\u003eSpecifically, N addition significantly increased RBG by 56.5% but decreased \u003cem\u003eH\u003c/em\u003e by 8.7% relative to the control (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). In contrast, Si addition alone significantly enhanced both RBG and \u003cem\u003eH\u003c/em\u003e, by 8.8% and 20.0%, respectively. The combined N and Si treatment (NSi) also led to significant increases in RBG and \u003cem\u003eH\u003c/em\u003e, 41.7% (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ea) and 6.6% (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eb), respectively.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eRegarding AGB and BGB, N addition significantly increased both parameters by 48.9% and 15.7%, respectively, relative to the control. Si addition alone also significantly raised AGB and BGB, by 8.5% and 10.8%, respectively. The combined application of Si and N further amplified these effects, leading to greater increases of 68.6% in AGB and 20.6% in BGB (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eHeterotrophic respiration under different treatments\u003c/p\u003e \u003cp\u003eN addition, Si addition, and combined N and Si addition significantly increased soil heterotrophic respiration by 3.39%, 5.46%, and 8.11%, respectively, compared with the control (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003ePathways through which experimental treatments influenced SOC\u003c/p\u003e \u003cp\u003eStructural equation modeling (SEM) results revealed that under N fertilization, SOC storage was positively associated with AGB, BGB, and \u003cem\u003eRh\u003c/em\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ea). With Si addition alone, SOC increased directly through enhanced AGB, and \u003cem\u003eRh\u003c/em\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eb). Under the combined Si and N treatment, SOC was positively influenced by SASi, SIN, and RBG, which in turn promoted AGB and BGB, BGB and \u003cem\u003eRh\u003c/em\u003e, and AGB, respectively (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ec).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e"},{"header":"Discussion","content":"\u003cp\u003eBased on a 9-year field experiment with N addition, Si addition, and their combined application, this study reveals distinct mechanisms driving SOC accumulation. N addition significantly enhanced SOC storage primarily by increasing both RBG and SIN, which in turn stimulated AGB. In contrast, Si addition promoted SOC storage mainly by elevating RBG and SASi, thereby boosting AGB, BGB, and \u003cem\u003eRh\u003c/em\u003e. Under the combined N plus Si treatment, SOC accumulation resulted from the synergistic effects of elevated SASi, SIN, and RBG. SASi enhanced AGB and BGB, SIN supported higher AGB and \u003cem\u003eRh\u003c/em\u003e, and RBG further contributed to AGB. Overall, the shifting roles of AGB, BGB, and \u003cem\u003eRh\u003c/em\u003e in governing SOC dynamics were largely attributable to differential changes in RBG, SIN, and SASi induced by each treatment. Our findings highlight the necessity of incorporating plant community composition, particularly RBG, along with associated changes in soil SASi and SIN availability when predicting how Si and N fertilization will affect SOC sequestration.\u003c/p\u003e \u003cp\u003eResponses of SOC to N and Si addition\u003c/p\u003e \u003cp\u003eConsistent with our first hypothesis, a significant increase in SOC storage was observed over the nine years of field N addition (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ea). This result aligns with findings from other N fertilization experiments in alpine meadows (Stevens et al. \u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e2015\u003c/span\u003e; Zhang et al. \u003cspan citationid=\"CR53\" class=\"CitationRef\"\u003e2025\u003c/span\u003e). The dynamics of SOC sequestration are governed by the balance between carbon inputs from plant litter decomposition and carbon losses through soil respiration (Jackson et al. \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e2017\u003c/span\u003e; Yan et al. \u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e2021\u003c/span\u003e). In N-limited ecosystems such as alpine meadows, N addition typically stimulates plant photosynthetic activity and enhances plant growth (Xu et al. \u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e2018\u003c/span\u003e), which can in turn promote SOC accumulation. In our study, N fertilization also led to a rise in \u003cem\u003eRh\u003c/em\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e), a response consistent with reports that low to moderate N addition can elevate soil microbial metabolic rates (Qu et al. \u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e2022\u003c/span\u003e; Yang et al. \u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). Moreover, we detected a significant increase in BGB under N fertilization (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eb), further supporting enhanced carbon input to the soil. Taken together, our findings suggest that alpine meadows may function as a net carbon sink under future scenarios of increased N fertilization or atmospheric N deposition.\u003c/p\u003e \u003cp\u003eThe increase in SOC sequestration under Si addition was primarily driven by elevated AGB and RBG, reflecting enhanced ecosystem carbon uptake. This response was closely associated with increased SASi (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eb), in line with our second hypothesis. In terrestrial ecosystems, plants primarily acquire Si from the soil, and higher SASi can raise plant tissue Si concentrations. Elevated Si content promotes more upright leaf and stem growth, increasing light capture and thus enhancing photosynthetic activity (Ma and Yamaji \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e2006\u003c/span\u003e; Xu et al. \u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e2018\u003c/span\u003e). Moreover, Si addition can modify soil pH (Supplementary Fig.\u0026nbsp;1S), reduce exchangeable aluminum, and ultimately improve N and phosphorus availability (Eneji et al. \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e2008\u003c/span\u003e; Lee et al. \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e2004\u003c/span\u003e; de Toledo et al. \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e2021\u003c/span\u003e; Schaller et al. \u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e2023\u003c/span\u003e). Collectively, these mechanisms explain how Si addition enhances SOC sequestration through increased SASi and SIN, which subsequently promote AGB and RBG, consistent with the pathways summarized in our conceptual framework (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eb). In parallel, Si addition significantly stimulated \u003cem\u003eRh\u003c/em\u003e via elevated SASi, underscoring the key role of soil available Si in regulating soil carbon fluxes. This aligns with previous reports that Si can enhance microbial activity and accelerate litter decomposition, thereby increasing soil carbon turnover (Marxen et al. \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e2016\u003c/span\u003e; Das et al. \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e2019\u003c/span\u003e; Chen et al. \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e2024\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eOur results show that combined N and Si addition enhanced SOC storage, with a greater positive effect compared to the same amount of N or Si applied alone, consistent with our third hypothesis. The increase in SOC under combined treatment was primarily driven by elevated SASi, SIN and RBG, which were positively correlated with SOC storage through their respective promotion of AGB, BGB and AGB. Although N plus Si addition increased both RBG and SIN (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e, Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e), with the former negatively affecting \u003cem\u003eRh\u003c/em\u003e and the latter positively affecting \u003cem\u003eRh\u003c/em\u003e, the net effect on \u003cem\u003eRh\u003c/em\u003e was dominated by the negative influence of RGB (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ec), thereby contributing to enhanced SOC accumulation. This aligns with previous reports that Si addition can stimulate soil CO₂ flux (Ali et al. \u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e2008\u003c/span\u003e; de Toledo et al. \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e2021\u003c/span\u003e; Eneji et al. \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e2008\u003c/span\u003e; H\u0026ouml;mberg et al. \u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e2021\u003c/span\u003e; Lee et al. \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e2004\u003c/span\u003e; Schaller et al. \u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e2023\u003c/span\u003e). Overall, the increases in RBG, SASi, and SIN under combined N and Si addition collectively promoted AGB and BGB, underscoring the pivotal role of these factors in regulating SOC dynamics and potentially activating negative feedback to N fertilization in this alpine region.\u003c/p\u003e \u003cp\u003eResponses of RBG, H, SASi and SIN to N and Si addition\u003c/p\u003e \u003cp\u003eThe increase in RBG and decline in \u003cem\u003eH\u003c/em\u003e under N addition can be largely attributed to the stronger competitive capacity of grass in nutrient uptake compared to non-grass species (Yang et al. \u003cspan citationid=\"CR50\" class=\"CitationRef\"\u003e2011\u003c/span\u003e; Wang et al. \u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e2012\u003c/span\u003e; Yan et al. \u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e2021\u003c/span\u003e; Elrys et al. \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). In this alpine meadow community, grass typically exhibit greater plant height than non-grass species (Supplementary Fig.\u0026nbsp;2S), which further enhances their ability to compete for light, thereby likely reinforcing the positive effect on RBG and the negative effect on \u003cem\u003eH\u003c/em\u003e.\u003c/p\u003e \u003cp\u003eSi plus N addition led to increases in both RBG and SASi (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ec, Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ea). SASi plays a pivotal role in shaping plant community traits: for instance, it promotes more upright leaf and stem growth, thereby enhancing plant competitiveness for light resources (Eneji et al. \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e2008\u003c/span\u003e; Xu et al. \u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e2018\u003c/span\u003e, \u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). The higher species diversity (\u003cem\u003eH\u003c/em\u003e) observed under Si addition and under NSi treatment, compared to N addition alone suggests that Si can partially offset the biodiversity loss and ecosystem disturbance induced by N fertilization. Meanwhile, SIN content was higher under NSi treatment than under N addition alone (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eb), likely due to Si-induced increases in soil availability of nitrogen, phosphorus, calcium, and water-soluble iron (Lee et al. \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e2004\u003c/span\u003e; de Toledo et al. \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e2021\u003c/span\u003e), which in turn influences plant community traits.\u003c/p\u003e \u003cp\u003eRBG, SASi and SIN mediate N and Si addition effects on SOC\u003c/p\u003e \u003cp\u003ePrevious studies have suggested that higher plant diversity is generally associated with greater SOC storage, largely because more diverse communities tend to support higher above- and belowground biomass, thereby enhancing carbon accumulation (Tilman et al. \u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e2012\u003c/span\u003e; Stevens et al. \u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e2015\u003c/span\u003e; Chen et al. \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e2018\u003c/span\u003e; Piao et al. \u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e2019\u003c/span\u003e; Prommer et al. \u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). In contrast, our study found that under N addition, Si addition, and their combined application, species diversity (\u003cem\u003eH\u003c/em\u003e) had only a minor influence on SOC storage. This discrepancy may be attributed to the relatively complex community composition at our study site, which hosts 20\u0026ndash;30 plant species. The species lost under N addition were primarily rare ones, whose low productivity likely resulted in a limited contribution to overall SOC dynamics (Geisseler et al. \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e2017\u003c/span\u003e; Zhu et al. \u003cspan citationid=\"CR52\" class=\"CitationRef\"\u003e2022\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eNitrogen fertilization exerted contrasting effects on belowground C inputs. While N addition directly stimulated belowground C allocation, the associated soil N enrichment concurrently suppressed belowground C inputs (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ea). The stimulatory effect ultimately outweighed the suppressive effect, leading to a net increase in BGB (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eb). Additionally, soil N enrichment resulting from N fertilization promoted AGB and shifted the dominant plant functional group from non-grass toward grass, a change that further enhanced SOC storage. Because grass litter decomposes more slowly than that of sedges and forbs (Du et al. \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e2018\u003c/span\u003e; Yan et al. \u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e2021\u003c/span\u003e; Qu et al. \u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e2022\u003c/span\u003e; Chen et al. \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e2024\u003c/span\u003e), the increased RBG under N addition likely reduced \u003cem\u003eRh\u003c/em\u003e, thereby positively influencing SOC accumulation. Ultimately, the greater C gains coupled with lower C losses under N fertilization resulted in enhanced SOC storage.\u003c/p\u003e \u003cp\u003eIn this study, Si addition, as well as combined N and Si addition and the corresponding increase in SASi, enhanced both AGB and BGB, thereby promoting SOC storage (Supplementary Fig. S3, Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ec). These results confirm the third hypothesis of this study. This process was simultaneously influenced by two counteracting mechanisms: increased plant litter decomposition due to elevated SASi, and decreased litter decomposition resulting from a higher RBG (Du et al. \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e2018\u003c/span\u003e; Yan et al. \u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e2021\u003c/span\u003e; Qu et al. \u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e2022\u003c/span\u003e; Schaller et al. \u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e2023\u003c/span\u003e). These opposing effects partially offset each other, ultimately contributing to enhanced SOC stability (Schaller and Struyf \u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e2013\u003c/span\u003e; Kuhla et al. \u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e2021\u003c/span\u003e). Overall, whereas N addition alone primarily increased SOC quantity, the addition of Si (both alone and in combination with N) improved both the quantity and stability of SOC (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e). This aligns with recent findings that Si fertilization facilitates SOC accumulation by boosting aboveground carbon inputs (Marxen et al. \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e2016\u003c/span\u003e; Chen et al. \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e2024\u003c/span\u003e).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eIn conclusion, our findings indicate that combined N and Si addition not only increases SOC storage but also improves its stability. Compared to N addition alone, the combined treatment promotes greater SOC accumulation via the synergistic effects of RBG, SASi, and SIN, whereas species diversity appears to play a negligible role in this process. These findings highlight the critical role of plant community composition in SOC storage under anthropogenic influences. It should be noted that although this study elucidates the mechanisms by which N, Si, and their interaction influence SOC dynamics, certain limitations remain. First, while the experiment spanned nine years, SOC turnover operates over longer timescales, and longer-term observations may better reveal its dynamic trends. Second, this study primarily focused on plant community and soil nutrient responses, without quantifying the specific contributions of key biogeochemical processes such as microbial community structure and extracellular enzyme activities. Additionally, interannual variations in environmental factors such as temperature and humidity under field conditions may have introduced some interference to the results.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eAli MA, Lee CH, Kim PJ (2008) Effect of silicate fertilizer on reducing methane emission during rice cultivation. Biol Fert Soils 44:597\u0026ndash;604. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1007/s00374-007-0243-5\u003c/span\u003e\u003cspan address=\"10.1007/s00374-007-0243-5\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eBerdugo M, Maestre FT, K\u0026eacute;fi S, Gross N, Bagousse-Pinguet YL, Solivers S (2019) Aridity preferences alter the relative importance of abiotic and biotic drivers on plant species abundance in global drylands. 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Agr Ecosyst Environ 381:109475. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.agee.2025.109475\u003c/span\u003e\u003cspan address=\"10.1016/j.agee.2025.109475\" 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":"Soil organic carbon, Soil available silicon, Alpine meadow, Nitrogen fertilization, Soil heterotrophic respiration","lastPublishedDoi":"10.21203/rs.3.rs-8646618/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-8646618/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003e\u003cstrong\u003eo \u003c/strong\u003e\u003cem\u003e\u003cstrong\u003eAims\u003c/strong\u003e\u003c/em\u003e\u003cstrong\u003e \u003c/strong\u003eNitrogen (N) fertilization is recognized for altering plant community composition and reducing species diversity, whereas, silicon (Si) addition enhances plant growth and improves soil nutrient availability. Those changes can profoundly influence ecosystem carbon pools. Nevertheless, how changes in plant community structure mediate soil organic carbon (SOC) storage responses to N and Si addition remains poorly understood.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eo \u003c/strong\u003e\u003cem\u003e\u003cstrong\u003eMethods\u003c/strong\u003e\u003c/em\u003e\u003cstrong\u003e \u003c/strong\u003eA nine-year field experiment (2012–2020) was conducted in an alpine meadow with four treatments: no Si and N addition, Si addition, N addition, and combined N + Si addition.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eo \u003c/strong\u003e\u003cem\u003e\u003cstrong\u003eResults\u003c/strong\u003e\u003c/em\u003e\u003cstrong\u003e \u003c/strong\u003eSOC significantly increased by 13.1%, 6.7%, and 21.2% under N addition, Si addition, and combined N and Si addition, respectively. Those increases were driven by distinct ecosystem mechanisms: the response to N addition were closely associated with changes in aboveground biomass (AGB), belowground biomass (BGB), and soil heterotrophic respiration (\u003cem\u003eRh\u003c/em\u003e); Si addition effects were linked to AGB and \u003cem\u003eRh\u003c/em\u003e; and the combined treatment’s impact was related to BGB and \u003cem\u003eRh\u003c/em\u003e. This study reveals the governing mechanisms of the SOC pool in an alpine meadow were mediated by changes in the relative biomass of grass (RBG), soil available silicon (SASi), and soil inorganic nitrogen (SIN).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eo \u003c/strong\u003e\u003cem\u003e\u003cstrong\u003eConclusion\u003c/strong\u003e\u003c/em\u003e Crucially, our findings demonstrate that the simultaneous enhancement of grass biomass, SASi and SIN under combined N and Si treatment lead to greater SOC than that achieved by either nutrient added alone. These findings provide new insights into how plants community composition adjust SOC under long-term Si plus N addition and predictions of ecosystem carbon dynamics under global change.\u003c/p\u003e","manuscriptTitle":"Changed plant community composition promotes soil carbon sequestration under long-term silicon and nitrogen addition in an alpine meadow","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2026-02-12 09:54:16","doi":"10.21203/rs.3.rs-8646618/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":"47258371-f833-455e-bc11-001acb3dfed4","owner":[],"postedDate":"February 12th, 2026","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[],"tags":[],"updatedAt":"2026-03-18T14:06:47+00:00","versionOfRecord":[],"versionCreatedAt":"2026-02-12 09:54:16","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-8646618","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-8646618","identity":"rs-8646618","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}