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

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.