In grassland ecosystems, N and P fertilization often increase plant productivity, but there is no concensus if fertilization affects soil C fractions. We tested effects of N, P and N+P fertilization at 5, 10, 15 g m−2 yr−1 (N5, N10, N15, P5, P10, P15, N5P5, N10P10, and N15P15) compared to unfertilized control on soil C, soil microbial biomass and functional diversity at the 0–20 cm and 20–40 cm depth in an alpine meadow after 5 years of continuous fertilization. Fertilization increased total aboveground biomass of community and grass but decreased legume and forb biomass compared to no fertilization. All fertilization treatments decreased the C:N ratios of legumes and roots compared to control, however fertilization at rates of 5 and 15 g m−2 yr−1 decreased the C:N ratios of the grasses. Compared to the control, soil microbial biomass C increased in N5, N10, P5, and P10 in 0–20 cm, and increased in N10 and P5 while decreased in other treatments in 20–40 cm. Most of the fertilization treatments decreased the respiratory quotient (qCO2) in 0–20 cm but increased qCO2 in 20–40 cm. Fertilization increased soil microbial functional diversity (except N15) but decreased cumulative C mineralization (except in N15 in 0–20 cm and N5 in 20–40 cm). Soil organic C (SOC) decreased in P5 and P15 in 0–20 cm and for most of the fertilization treatments (except N15P15) in 20–40 cm. Overall, these results suggested that soils will not be a C sink (except N15P15). Nitrogen and phosphorus fertilization may lower the SOC pool by altering the plant biomass composition, especially the C:N ratios of different plant functional groups, and modifying C substrate utilization patterns of soil microbial communities. The N+P fertilization at 15 g m−2 yr−1 may be used in increasing plant aboveground biomass and soil C accumulation under these meadows.

Nutrient additions can increase carbon (C) inputs to soil, but there is no consensus about the response of soil organic C (SOC) storage and C sequestration. For the Tibetan alpine meadows, little is known about the effects and mechanisms of nitrogen (N) and phosphorus (P) addition on SOC stocks. In this study, we applied N and/or P fertilization for 7 years and analyzed soil changes in bulk density, pH, SOC, soil inorganic C (SIC), δ13C, and microbial biomass C (MBC), as well as stocks of SOC, SIC, and MBC for soil to a depth of 40 cm. We found that C:N decreased in 0–20 cm, and pH decreased at both 0–20 cm and 20–40 cm after fertilization. Fertilization with N and/or P decreased SOC stocks in 0–20 cm by 5–12% and SOC stocks from 0 to 40 cm by 3–5%. This SOC stock decline was associated with changes in SOC concentration but not with changes in bulk density. The SIC stock was 18% of total soil C, and was not influenced by either N or P fertilization. Soil δ13C in the 0–20 cm layer was depleted by fertilization with N or N + P, whereas P enriched soil δ13C. Soil MBC was positively correlated with SOC concentration, whereas soil δ13C was negatively correlated with SOC concentration. Soil δ13C, as a proxy of decomposition rate, indicated potentially higher SOC decomposition under N fertilization. These findings suggest that fertilization with N and/or P lowered SOC sequestration in Tibetan alpine meadows.

Core Ideas Litter addition increased C decomposition. Litter properties were more important than abiotic soil properties in controlling C decomposition. N addition would increase decomposition of SOC with added litter and decrease SOC accumulation by altering litter quality. In grassland ecosystems, nitrogen (N)‐induced changes in plant community often affect grassland soil organic carbon (SOC) decomposition and accumulation. However, there is no consensus on the biological and chemical mechanisms that underlie the response of SOC decomposition to N‐addition. In this study, we did a litter addition experiment to study the decomposition of SOC with added litter of six dominant plant species collected from N alone or N combined with phosphorus (N+P) fertilization treatments in a 7‐yr field experiment. The experiment showed that there were no consistent responses of plant chemical properties to N alone or N+P addition. Fertilization altered plants species' dominance in community, in particular the grass Elymus nutans. Litter addition significantly increased C decomposition, with the highest decomposition rates of SOC with added litter of all six plant species in N+P treatment. In general, both plant litter properties of organic C, total N (TN), lignin concentrations, and soil properties of SOC and TN, but not soil microbial biomass C, were the main factors determining cumulative decomposition of SOC with added litter. Litter properties were more important than abiotic soil properties in controlling C decomposition. This study indicated that fertilization with N alone or N+P can increase decomposition of SOC with added litter and lower SOC accumulation through changes in plant species' dominance and chemical properties of litter. In terms of maintaining high SOC and increasing soil C, N fertilization, whether in N alone or N+P, should not be used in these alpine meadows.

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.

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