Nitrogen (N) and phosphorus (P) additions to grasslands increase aboveground plant biomass and modify plant community composition, thereby affect plant-derived organic carbon inputting to soil and soil C cycling and storage. However, the effects of nutrient additions on SOC decomposition and soil C sequestration have no census and their underlying mechanisms are poorly understood. This study aimed to explore the mechanisms underlying SOC decomposition and SOC content decline in the topsoil of Tibetan alpine meadows after nine-year field N and P additions. Soil and microbial stoichiometric characteristics were measured and priming effects (PEs), substrate decomposition, as well as microbial C use efficiency (CUE) by adding 13C labeled substrate (glucose or vanillin) were analyzed. N and P additions differentially affected the magnitude and direction of PEs and SOC decomposition, accelerated substrate mineralization of glucose by 33-45% and that of vanillin by 11-45%, but decreased microbial CUE of glucose by 9-15% and that of vanillin by 11-48%. This was associated with the N and P additions-induced lower soil ecological stoichiometric ratios and higher microbial C:N:P ratios compared with Control. Therefore, these comprehensive effects of N and P additions on decomposition of SOC and plant-derived C substrates reduced SOC sequestration and thus SOC content. Long-term N and P additions would weaken soil functioning as C pool of Tibetan alpine meadows.