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 consensus 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. [ABSTRACT FROM AUTHOR]