Warming mitigates the effects of nitrogen addition on the soil diazotrophic community in a subtropical forest

Current models predict that climate warming and nitrogen (N) deposition will continue to increase in the coming years. However, information regarding how warming and N deposition together impact soil diazotrophs (N-fixing microbes) is still limited. Therefore, the soil N₂-fixation activity and structure of diazotrophic communities were examined in a subtropical forest after three years of warming and N addition. The ecological networks and community assembly were also investigated. We found that N addition decreased soil N₂-fixation activity and diazotrophic abundance and diversity and caused a significant shift in community structure. In contrast, warming had positive effects on soil N₂-fixation activity and diazotrophic abundance, with minor impacts on diazotrophic community composition. The combination of warming and N addition caused a less variation in the diazotrophic community composition and lower associations in the ecological networks than did N addition alone. The assembly of diazotrophic communities was shaped mainly by stochastic processes, with an increase in deterministic processes under N addition. These findings suggest that warming and N addition have differential impacts on soil diazotrophs and that warming might mitigate the changes in soil diazotrophic communities caused by N addition in subtropical forests. Additionally, the results indicate that soil diazotrophic communities are less sensitive to a combination of warming and N addition than to N addition alone. Therefore, the influences of multiple global change factors on soil diazotrophs cannot be overestimated.