Autotrophic and symbiotic diazotrophs dominate nitrogen-fixing communities in Tibetan grassland soils

Biological nitrogen fixation, conducted by soil diazotrophs, is the primary nitrogen source for natural grasslands. However, the diazotrophs in grassland soils are still far from fully investigated. Particularly, their regional-scale distribution patterns have never been systematically examined. Here, soils (0-5 cm) were sampled from 54 grasslands on the Tibetan Plateau to examine the diazotroph abundance, diversity, and community composition, as well as their distribution patterns and driving factors. The diazotroph abundance was expressed as nifH gene copies, measured using real-time PCR. The diversity and community composition of diazotrophs were analyzed through MiSeq sequencing of nifH genes. The results showed that Cyanobacteria (47.94%) and Proteobacteria (45.20%) dominated the soil diazotroph communities. Most Cyanobacteria were classified as Nostocales which are main components of biological crusts. Rhizobiales, most of which were identified as potential symbiotic diazotrophs, were also abundant in approximately half of the soil samples. The soil diazotroph abundance, diversity, and community composition followed the distribution patterns in line with mean annual precipitation. Moreover, they also showed significant correlations with prokaryotic abundance, plant biomass, vegetation cover, soil pH values, and soil nutrient contents. Among these environmental factors, the soil moisture, organic carbon, available phosphorus, and inorganic nitrogen contents could be the main drivers of diazotroph distribution due to their strong correlations with diazotroph indices. These findings suggest that autotrophic and symbiotic diazotrophs are the predominant nitrogen fixers in Tibetan grassland soils, and highlight the key roles of water and nutrient availability in determining the soil diazotroph distribution on the Tibetan Plateau.