Effects of nitrogen and phosphorus fertilization on soil carbon fractions in alpine meadows on the Qinghai-Tibetan Plateau

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, N ₁₀, 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 N ₁₅) but decreased cumulative C mineralization (except in N ₁₅ 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 N₁₅P ₁₅). 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.