Nitrogen (N) and phosphorus (P) additions reduced soil organic carbon (SOC) contents and stocks in alpine meadows on the Tibetan Plateau. However, little is known about microbial mechanisms behind SOC decline. This study investigated the effects of long‐term N and P additions on microbial community composition and SOC decomposition (C mineralization (Cm), mean resistant times for active C pool (MRTa), and slow C pool (MRTs) in alpine meadows. Results showed that the total SOC pool was reduced by 2–9% under N and P additions, of which slow C pool decreased by 3–10%, while active C pool increased by 4–75% compared to the Control. N and P additions shortened MRTs by 34–40% but prolonged MRTa by 30–62%. The relative abundance of four bacterial families was related to Cm or MRTa, while that of most of the fungal families affected SOC decomposition (including Cm, MRTa, and MRTs). N and P additions increased fungal diversity, differentially affected microbial community composition and structure through modifying microbial preference, and increasing the abundance of microbes which are capable of decomposing complex carbohydrate. Soil pH, available N, and total P were main factors determining microbial abundances. Microbial changes due to N and P additions accelerated decomposition of recalcitrant SOC, thus led to declines in slow C pool and total SOC pool but increases in active C pool. Therefore, long‐term N and P additions weaken soil functioning as C pool in alpine meadows.

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 13C 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.

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 additionsinduced 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.

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Fertilizers-induced priming effects of soil organic matter (SOM) decomposition influences net carbon balance and nutrient release. We hypothesize that very strong limitation of plant productivity and microbial activities by nitrogen (N) and phosphorus (P), common in Tibetan meadows, retard SOM decomposition and turnover. Consequently, N and/or P fertilization will induce priming effects of SOM and have implications for carbon balance. Soils from a nine-year fertilization experiment (N alone, P alone, NP together, and control) from a Tibetan alpine meadow were used to investigate priming effect of SOM and carbon balance after addition of 13 C labeled glucose.N and/or P fertilization acidified soil by 0.5 pH unit, decreased SOM content, and increased total and available N, total P. Regardless of fertilization, glucose addition accelerated SOM decomposition with priming effects of 30-60 μg C g À1 soil during 78 days. Alleviation of N and P limitation by N and NP fertilization lowered the priming effect by 17% and 14%, respectively, but P fertilization increased priming effect by 67%. The negative correlation of priming effect intensity with SOM, nitrate or total N, and microbial biomass contents indicated that fertilization-induced differences in soil N and the microbial community are responsible for the priming effects.Positive correlation of carbon balance with total N and ammonium contents suggested that soil N accounts for carbon sequestration. Therefore, long-term N and/or P fertilization accelerate SOM decomposition and reduce SOM storage in alpine meadows, of which P fertilization induces the highest priming effect and the lowest SOM storage.