Response and driving factors of soil microbial diversity related to global nitrogen addition

It is increasingly evident that soil microorganisms play a substantial role in ecosystem processes, which occupy a large part of genetic diversity in terrestrial ecosystems. During the past decades, nitrogen (N) addition induced by human has been regarded as the main driver of global changes to soil microecological processes. Although growing studies demonstrated the loss of biodiversity due to N addition in regional scale, the role of global N addition on soil microbial diversity is lacking. Here, we sought to illuminate the impacts and driving factors of soil microbial diversity in relation to global N addition using a meta-analysis from 1,078 paired observations in 212 studies. We found a negative influence of global N addition on soil microbial diversity, and the reduction of soil microbial diversity was determined by land use types, N fertility types, methods, N addition rates, and duration. In fact, there was a linear decrease of soil microbial diversity along the latitudinal gradient, and N addition-induced acidification was the predominant contributor to the reduction of soil microbial diversity. Both of soil properties (pH, soil organic carbon, soil total N, and soil microbial biomass C) and climatic factors (mean annual temperature and mean annual precipitation) determined soil microbial diversity under global N addition. It was noted that soil fungal diversity was more adaptable to acidic environments, whereas soil bacterial diversity was more adaptable to neutral environment. Most importantly, soil fungal diversity was more sensitive than soil bacterial diversity under global N addition. Our meta-analysis provides a new insight in understanding the key role of global N addition in shaping soil microbial diversity. Thereby, we recommend that future research should be focused on the soil microbial processes using manipulative controlled experiments, which identify the available measures to reduce the loss of soil microbial diversity globally.