Soil Organic Carbon Turnover Response to Nitrogen and Phosphorus Additions in Eastern China: Evidence from Stable Carbon Isotopes

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 & delta;C-13(SOC) contents and physicochemical properties under N (100 kg N & BULL;ha(-1)& BULL;a(-1)), N + P (100 kg N ha(-1)& BULL;a(-1) + 50 kg P ha(-1)& BULL;a(-1)) additions, and the control (CK, no N and P additions). We also estimated the & beta; value, which represents the soil organic carbon (SOC) turnover rate, from the slope of the regression between the log10-transformed SOC content and & delta;C-13(SOC) in soil depth profiles. Our findings revealed that N addition significantly affected soil & delta;C-13(SOC) compared to CK on both mid-slope and flat ridge sites. The & beta; values responded differently to various treatments and sites. On the mid-slope, the & beta; values did not show significant change with N and N + P additions. On the flat ridge, however, the & beta; 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 & beta; 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.