Effects of seven-year nitrogen and phosphorus additions on soil microbial community structures and residues in a tropical forest in Hainan Island, China

Microbial residue retention is critical for soil organic carbon (SOC) accumulation in forests. However, the changes in soil microbial residues under atmospheric nitrogen (N) and phosphorus (P) deposition in tropical forests remain unclear. Here, we measured soil phospholipid fatty acids, enzyme activity, and amino sugars (biomarkers of microbial residues) in a seven-year N and P addition experiment in a secondary tropical montane rainforest. Our results showed that five years of nutrient additions showed no significant effect on microbial community structure but the effect became significant at the seventh year. In N addition plots, a decrease in the arbuscular mycorrhizal fungal biomass resulted in declines in the contribution of fungal residues to SOC and the ratio of fungal to bacterial residues. The fungi:bacteria ratio increased in P and N&P addition plots while the fungal residue:bacterial residue ratio was decreased by P addition, which may be related to an imbalance in microbial residue decomposition. Both extracellular enzyme activities and SOC showed minor changes under nutrient additions. Our results suggest that changes in microbial community structure and enzyme activity induced by N and P deposition may alter the accumulation and composition of microbial residues in SOC in N-rich tropical forests.