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) (N(5), N(10), N(15), P(5), P(10), P(15), N(5)P(5), N(10)P(10), and N(15)P(15)) 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 N(5), N(10), P(5), and P(10) in 0–20 cm, and increased in N(10) and P(5) while decreased in other treatments in 20–40 cm. Most of the fertilization treatments decreased the respiratory quotient (qCO(2)) in 0–20 cm but increased qCO(2) 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 N(5) in 20–40 cm). Soil organic C (SOC) decreased in P(5) and P(15) in 0–20 cm and for most of the fertilization treatments (except N(15)P(15)) in 20–40 cm. Overall, these results suggested that soils will not be a C sink (except N(15)P(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.