Effects of nitrogen deposition on soil organic carbon fractions in the subtropical forest ecosystems of S China

Experiments were conducted between 2003 and 2008 to examine how N additions influence soil organic C (SOC) and its fractions in forests at different succession stages in the subtropical China. The succession stages included pine forest, pine and broadleaf mixed forest, and old-growth monsoon evergreen broadleaf forest. Three levels of N (NH4NO3)-addition treatments comprising control, low-N (50?kg N ha1 y1), and medium-N (100?kg N ha1 y1) were established. An additional treatment of high-N (150?kg N ha1 y1) was established in the broadleaf mixed forest. Soil samples were obtained in July 2008 for analysis. Total organic C (TOC), particulate organic C (POC, > 53 mu m), readily oxidizable organic C (ROC), nonreadily oxidizable organic C (NROC), microbial biomass C (MBC), and soil properties were analyzed. Nitrogen addition affected the TOC and its fractions significantly. Labile organic-C fractions (POC and ROC) in the topsoil (010?cm) increased in all the three forests in response to the N-addition treatments. NROC within the topsoil was higher in the medium-N and high-N treatments than in the controls. In the topsoil profiles of the broadleaf forest, N addition decreased MBC and increased TOC, while no significant effect on MBC and TOC occurred in the pine and mixed forests. Overall, elevated N deposition increased the availability of labile organic C (POC and ROC) and the accumulation of NROC within the topsoil irrespective of the forest succession stage, and might enhance the C-storage capacity of the forest soils.