Effects of nitrogen and phosphorus additions on decomposition and accumulation of soil organic carbon in alpine meadows on the Tibetan Plateau

Nitrogen (N) and phosphorus (P) additions to grasslands increase aboveground plant biomass and modify plant community composition, thereby affecting plant-derived organic carbon (C) input to soil and soil C cycling and storage. However, the effects of N and P additions on soil organic C (SOC) decomposition and sequestration are not fully understood and their underlying mechanisms are poorly known. This study aimed to explore the mechanisms underlying SOC decomposition and SOC content decline in the topsoil of alpine meadows on the Tibetan Plateau after 9-years of field N and P additions. Stoichiometric characteristics of soil and microorganisms and their effects on microbial decomposition, including priming effects (PEs), C substrates decomposition, and microbial C use efficiency (CUE), were investigated by adding C-13 labeled substrate (glucose or vanillin). Results showed that N and P additions differentially affected the magnitude and direction of PEs and SOC decomposition, accelerated mineralization of glucose and vanillin by 33-45% and 11-45%, respectively, but decreased microbial CUE of glucose and vanillin by 9-15% and 11-48%, respectively. These effects were caused by differential increase in microbial activity and acceleration of microbial decomposition due to N and P additions, and induced lower soil ecological stoichiometric ratios and higher microbial C:N:P ratios. The above effects led to different magnitudes of decomposition and accumulation of SOC and plant-derived C substrate and thus decline in SOC content depending on N and P additions. We found that long-term N and P additions would weaken soil functioning as a C pool for alpine meadows on the Tibetan Plateau.