Decreasing soil microbial diversity is associated with decreasing microbial biomass under nitrogen addition

While aboveground biodiversity has been widely studied, how microbial biodiversity responds to increasing nitrogen (N) deposition is still unclear. Here we conducted a meta-analysis to investigate the responses of soil microbial diversity and composition to N addition. Overall, we found N addition decreased both soil microbial diversity and the relative abundance of Actinobacteria and Nitrospirae, although the effect may vary among different ecosystems. The effect size on microbial Shannon index was positively correlated with the changes in soil microbial biomass under N addition. The initial soil conditions, the duration of treatment, the N addition rate and changes in soil organic carbon under N addition all affected the effect sizes of N addition on microbial Shannon index, while changes in soil pH played a minor role. Overall, our results suggest that the losses of microbial diversity with increasing N deposition rate would alter ecosystem functions and may have profound feedbacks to global climate change.