Inconsistent responses of soil microbial community structure and enzyme activity to nitrogen and phosphorus additions in two tropical forests

Soil microorganisms play an important role in biogeochemical cycles in terrestrial ecosystems. Increasing nitrogen (N) and phosphorus (P) deposition are likely to regulate microbial growth by altering soil nutrient availability in tropical forests, yet their impacts on microbial community structure and function between primary forests and secondary forests are not well understood. To investigate how nutrient availability affects microbial community structure and function in tropical forests, we measured soil phospholipid fatty acids and enzyme activities in a seven-year N and P fertilization experiment in two tropical montane rainforests, China. In N addition plots, fungal biomass, arbuscular mycorrhizal fungal biomass and fungi to bacteria ratio (F/B) decreased in the secondary forest, but had moderate changes in the primary forest. In P and N plus P addition plots, microbial biomass showed minor changes, but the F/B increased significantly in both forests. However, hydrolytic enzyme activities did not show a significant change in the secondary forest, while they decreased significantly in the primary forest. Microbial P limitation in the primary forest decreased under P addition and N plus P addition. Our findings suggest inconsistent responses of microbial community structure and enzyme activity to N and P additions in tropical soils depending on forest type.