Nitrogen and Phosphorus Additions Impact Stability of Soil Organic Carbon and Nitrogen in Subtropical Evergreen Broad-Leaved Forest

Atmospheric nitrogen (N) deposition can have a series of effects on forest ecosystems. As N deposition increases N availability in forest ecosystems, soil phosphorus (P) becomes a limiting factor for forest productivity. However, it is not clear whether long-term N deposition can affect on soil carbon (C) and N stability, and whether the addition of P can moderate the impact of N deposition. A five-year experiment of N and N + P additions was conducted in a subtropical evergreen broad-leaved forest in eastern China. The seasonal dynamics of soil C and N and the abundance of their stable isotopes (δ 13 C and δ 15 N) were investigated in the 0–30 cm layer. Soil C, N and δ 13 C and δ 15 N abundances had significant vertical distribution characteristics. Soil C and N contents increased significantly under N and N + P additions, especially in the topsoil (0–10 cm); δ 13 C did not change significantly under N and N + P additions, but the linear slope (β) between δ 13 C and soil organic carbon (SOC) and C/N indicated that N + P addition slowed down the decomposition rate and enhanced the stability of SOC. Soil δ 15 N increased significantly under N and N + P additions, and enriched with the increase of seasonal temperature. Our findings implied that the linear slope of δ 13 C and SOC (β) and C/N can serve as potential indicators of soil C stability under N and N + P additions in subtropical forests. The soil δ 13 C and δ 15 N abundance was affected by the interaction between soil C and N. δ 13 C abundance showed no significant response of the stability of SOC to N and N + P additions. It suggests that the further study is needed to reveal their response mechanisms to long-term N deposition in subtropical forest ecosystem .