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Anthropogenic activities have drastically increased nitrogen (N) deposition, resulting in increased N availability. The continuous increase of N availability may exacerbate phosphorus (P) deficiency, which would limit forest productivity in subtropical forests. Effects of long-time N and N + P additions on SOC turnover in subtropical forests is therefore crucial for understanding the global carbon (C) cycle. The argument of whether N and N + P addition accelerates or slows SOC turnover has been under debate, particularly in P-limited subtropical forests. This study mainly aims to confirm this argument. A ten-year field experiment was conducted in a subtropical evergreen broad-leaved forest in southern Anhui, China. We measured the soil δ13CSOC contents and physicochemical properties under N (100 kg N·ha−1·a−1), N + P (100 kg N ha−1·a−1 + 50 kg P ha−1·a−1) additions, and the control (CK, no N and P additions). We also estimated the β value, which represents the soil organic carbon (SOC) turnover rate, from the slope of the regression between the log10-transformed SOC content and δ13CSOC in soil depth profiles. Our findings revealed that N addition significantly affected soil δ13CSOC compared to CK on both mid-slope and flat ridge sites. The β values responded differently to various treatments and sites. On the mid-slope, the β values did not show significant change with N and N + P additions. On the flat ridge, however, the β value decreased significantly in N and N + P additions, implying an increased SOC turnover rate. In addition, analysis of soil C–N–P stoichiometric ratios and physicochemical properties showed that N and N + P additions could affect β values by modifying soil nutrient content and composition. In general, our findings indicate that N and N + P additions could accelerate the SOC turnover in subtropical forest ecosystems, albeit with close dependence on site-specific factors. [ABSTRACT FROM AUTHOR]

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 (δ13C and δ15N) were investigated in the 0–30 cm layer. Soil C, N and δ13C and δ15N 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); δ13C did not change significantly under N and N + P additions, but the linear slope (β) between δ13C 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 δ15N increased significantly under N and N + P additions, and enriched with the increase of seasonal temperature. Our findings implied that the linear slope of δ13C 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 δ13C and δ15N abundance was affected by the interaction between soil C and N. δ13C 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. [ABSTRACT FROM AUTHOR]