Chemoheterotrophic diazotrophs contribute to nitrogen incorporation in a semi-arid desert

To explore the non-cyanobacteria diazotrophic populations and their microbial interactions, and further evaluate the N-fixing activities of chemoheterotrophic groups in deserts, we collected soil samples from the Mu Us Desert of northern China, and performed amplicon sequencing, targeting the functional gene nifH combined with the dark incubation with 15N2 labelling under laboratory conditions. A total of 4455 operational taxonomic units were clustered from 786,778 high-quality nifH reads, 96.57% of which were classified as Proteobacteria. Network analyses showed that three putative keystone taxa (i.e. the genera Azospirillum, Bradyrhizobium, and Mesorhizobium) were mostly responsible for connecting other diazotrophic populations, and that 98.03% of the associations between any two nodes were positive. The 15N2 labelling experiment showed that the average N-fixing rates added 0.03 mg N kg−1 day−1 as soil total N and 0.006 mg N kg−1 day−1 as total dissolved N over 28 days. Our results highlight the interactions of non-cyanobacteria diazotrophs and the potential of the chemoheterotrophic types in transforming atmospheric N2 into bio-available ammonium in desert soils.