Rate-specific responses of prokaryotic diversity and structure to nitrogen deposition in the Leymus chinensis steppe

Serious nitrogen (N) deposition in terrestrial ecosystems causes soil acidification and alters the structure and function of microbial community. We hypothesized that the responses of different prokaryotic groups (e.g. more sensitive to N and more sensitive to pH) to N deposition would be largely dependent on deposition rates. In this study, we investigated the responses of soil prokaryotic community structure and stability after 13-year N deposition in the semi-arid Leymus chinensis steppe in Inner Mongolia, China. Our results demonstrated that prokaryotic community structure changed at low N deposition rate of 1.75 g N m-2 yr-1; however, dramatic changes in microbial abundance, respiration quotient, prokaryotic diversity occurred at N deposition rates > 5.25 g N m-2 yr-1 when pH dropped below 6.0. The two patterns indicated the difference in driving forces for different microbial properties. The N-driven and pH-driven processes are likely most important mechanisms determining the responses of bacterial community to N. Some copiotrophic/oligotrophic bacteria, e.g. Proteobacteria and Acidobacteria, changed their relative abundances with N deposition continuously even at low rate, indicating that they are more sensitive to N directly. Some bacterial groups significantly change their relative abundance at high N deposition rate when pH drop <6.0, e.g. Verrucomicrobia, and Armatimonadetes, indicating that they are mores sensitive to pH below 6.0. N deposition altered prokaryotic community structure through enrichment of copiotrophic bacteria (species adjustment) at low N deposition rates and through enrichment of nitrophilous taxa and siginificant loss of diversity at high N rates. This study also demonstrated that high N addition diminished the stability of prokaryotic community structure and activity through reduction in species diversity and bacterial interaction.