Soil microbes deal with the nitrogen deposition enhanced phosphorus limitation by shifting community structure in an old-growth subtropical forest

Elevated atmospheric nitrogen (N) deposition potentially enhances the degree of phosphorus (P) limitation in tropical and subtropical forests. However, it remains elusive that how soil microorganisms deal with the N deposition-enhanced P limitation. We collected soils experienced 9 years of manipulative N input at various rates (0, 40, and 80 kg N ha−1 y−1) in an old-growth subtropical natural forest. We measured soil total and available carbon (C), N and P, microbial biomass C, N and P, enzyme activities involved in C, N and P acquisition, microbial community structure, as well as net N and P mineralization. Additionally, we calculated element use efficiency and evaluated microbial homeostasis index. Our findings revealed that N input increased microbial biomass C:P (MBC:P) and N:P (MBN:P) ratios. The homeostasis indexes of MBC:P and MBN:P were 0.68 and 0.75, respectively, indicating stoichiometric flexibility. Interestingly, MBC:P and MBN:P correlated significantly with the fungi:bacteria ratio (F:B), not with N and P use efficiencies, net N and P mineralization, and enzyme C:P (EEAC:P) and N:P (EEAN:P) ratios. Furthermore, EEAC:P and EEAN:P correlated positively with F:B but did not negatively correlate with the C:P and N:P ratios of available resources and microbial biomass. The effects of N deposition on MBC:P, MBN:P and EEAN:P became insignificant when including F:B as a covariate. These findings suggest that microbes flexibly adapted to the N deposition enhanced P limitation by changing microbial community structure, which not only alter microbial biomass C:N:P stoichiometry, but also the enzyme production strategy. In summary, our research advances our understanding of how soil microorganisms deal with the N deposition-enhanced soil P limitation in subtropical forests.