Alterations in substrate stoichiometry control the responses of soil diazotrophs to nutrient enrichment

Soil diazotrophs convert atmospheric nitrogen (N) into biologically active N, which is an important N source for plants and microbes in many natural ecosystems. However, responses of diazotrophic communities to long-term N and phosphorus (P) inputs and the primary mediators remain unclear, particularly in semi-arid grasslands. Here we show that the abundance and community composition of diazotrophs responded distinctly to gradients of long-term N and P additions in an Inner Mongolia steppe. Our results showed that moderate to high N inputs (105–280 kg N ha−1 yr−1) significantly reduced both diazotrophic abundance and community diversity. In contrast, low to moderate P additions (20–80 kg P2O5 ha−1 yr−1), along with low N input (24 kg N ha−1 yr−1), significantly increased diazotrophic abundance, while having no significant effects on soil diazotrophic diversity and community structure. Also, long-term N and P inputs led to different co-occurrence patterns in the diazotrophic community, although both of them reduced the stability of diazotrophic co-occurrence network. Structural equation modeling (SEM) analysis further revealed that N additions suppressed the diazotrophic abundance and diversity, corresponding to the decreasing stoichiometric ratio of soil available C to N. Together, these results indicate that nutrient-induced changes in the relative availability of C to N in soil primarily drives the impacts of nutrient enrichment on diazotrophs. These findings suggest that management regimes with low nutrient inputs may provide unique opportunities to increase plant productivity, while sustaining or even enhancing the abundance and activity of soil diazotrophs.