Impacts of long-term nitrogen addition, watering and mowing on ammonia oxidizers, denitrifiers and plant communities in a temperate steppe

Global changes in nitrogen (N) deposition, precipitation patterns and land use could have an impact on the biogeochemical N cycle mediated by microorganisms. Microbial responses to global changes have been generally evaluated in short-term studies, while the long-term effects are not well understood. In this study, the interactive effects of multiple global change factors on N-cycling microorganisms and their interactions with plants were investigated in a 9-year field experiment with N addition, watering and mowing in a temperate steppe in northern China. Quantitative PCR, terminal restriction fragment length polymorphism (T-RFLP) and clone library were used to analyze the abundance and community structure of ammonia oxidizers and denitrifiers, respectively. Bacterial 16S rRNA gene abundance significantly increased under mowing, but the functional gene abundances including amoA, nirS, nirK and nosZ genes were not affected. Watering significantly affected the ammonia-oxidizing archaea (AOA) community and N addition showed a strong effect on soil ammonia-oxidizing bacteria (AOB) and nosZ-containing denitrifier community structure. No significant interactive effects of mowing, N addition and watering on communities of soil ammonia oxidizers and denitrifiers were observed. Structure equation models suggested that plant biomass and community were significantly correlated with microbial communities and activities. These findings suggest that there are complex interactions between plants and soil microorganisms in grassland ecosystems.