Changes in community assembly processes and co-occurrence networks of soil diazotrophs along an elevational gradient in Tibetan alpine meadows

Shifts in soil microbial community composition along elevational gradients have been extensively studied; yet, the assembly process and co-occurrence network of soil microbes underlying these shifts remain to be explored, especially for diazotrophs that contribute to the most nitrogen input in natural ecosystems. In this study, we examined how assembly processes and co-occurrence networks of soil diazotrophs changed along an elevational gradient with four elevation levels ranging from 3200 m to 3800 m using high-throughput sequencing of the nifH gene in Tibetan alpine meadows. Our analyses showed that alpha-diversity of soil diazotrophs was lower at low elevations (i.e., 3200 m and 3400 m) relative to high elevations (i.e., 3600 m and 3800 m). Moreover, dominant diazotrophic phyla shifted from Actinobacteria at low elevations to Proteobacteria at high elevations. The change in diazotrophic community composition along the elevation gradient was related to the differences in soil temperature, moisture, NH4+-N concentration and pH. Species rank-abundance distribution models showed that soil diazotrophic communities were mainly assembled by deterministic processes at 3200 m, 3600 m and 3800 m elevations, and by stochastic processes at the 3400 elevation. Additionally, the complexity of diazotrophic co-occurrence networks increased with increasing elevations. Collectively, our findings highlight the change in soil diazotrophic communities along the elevation gradient and the high environmental sensitivity of soil diazotrophs, and suggest that global change could have consequences for soil diazotrophic community compo-sition and structure in montane ecosystems.