Abstract: Aims Alpine grassland ecosystems store vast amounts of organic carbon while are fragile. Understanding the responses of the ecosystem carbon storage to the synchronous atmospheric nitrogen deposition and changing precipitation regimes is critical to project the fate of ecosystem carbon budgets under the context of global change. Methods Based on a manipulation field experiment of nitrogen addition (10 g?m–2?a–1) and precipitation change (precipitation reduction by 50% and increase by 50%) in an alpine meadow on the northeastern Qinghai-Xizang Plateau in 2017, the plant biomass, soil organic carbon content (SOCC) and its fractions were observed from 2022 to 2023, in order to explore the response of ecosystem carbon storage to the changes in nitrogen and precipitation. Important findings The results showed that there were little interaction effects of nitrogen addition and precipitation change. The response of vegetation aboveground biomass to the changes in nitrogen and precipitation was functional group-dependent. Nitrogen addition treatment increased the AGB of graminoid and sedge. Decreased precipitation treatment reduced AGB by 27% while increased precipitation treatment impacted AGB insignificantly. Except for sedge, the proportion of functional group AGB against community AGB changed undetectably. The responses of 0–40 cm belowground biomass (BGB) and SOCC to the changes in nitrogen and precipitation were weak and depth- and year-dependent. The root/shoot ratio reduced by 31% in nitrogen addition treatment and increased by 83% in decreased precipitation treatment, respectively. Nitrogen addition treatment increased surface (0–10 cm) mineral-associated organic carbon by 31%. The response ratio (RR) of vegetation AGB was positively related to graminoid. The RR of 0-40 cm BGB was determined by surface and deep (20-40 cm) BGB positively. The RR of 0-40 cm SOCC was equivalently regulated by each layer SOCC. Surface BGB directly impacted the surface POC positively and indirectly impacted the surface MAOC via POC negatively. The vegetation AGB affected the deep MAOC positively and the deep POC negatively. The main effects, rather than the interaction effects of the changes in nitrogen and precipitation, affect AGB significantly while BGB and SOCC undetectably. The differential effects of plant biomass on soil organic carbon fractions are depth-dependent.

Key words: alpine meadow, plant biomass, soil organic carbon, particulate organic carbon, mineral-associated organic carbon, nitrogen addition, precipitation change