Diazotrophic abundance and community structure associated with three meadow plants on the Qinghai-Tibet Plateau.

Symbiotic diazotrophs form associations with legumes and substantially fix nitrogen into soils. However, grasslands on the Qinghai-Tibet Plateau are dominated by non-legume plants, such as . Herein, we investigated the diazotrophic abundance, composition, and community structure in the soils and roots of three plants, non-legume and and the legume , using molecular methods targeting gene. Diazotrophs were abundantly observed in both bulk and rhizosphere soils, as well as in roots of all three plants, but their abundance varied with plant type and soil. In both bulk and rhizosphere soils, showed the highest diazotroph abundance, whereas had the lowest. In roots, and showed the highest and the lowest diazotroph abundance, respectively. The bulk and rhizosphere soils exhibited similar diazotrophic community structure in both and , but were substantially distinct from the roots in both plants. Interestingly, the root diazotrophic community structures in legume and non-legume were similar. Diazotrophs in bulk and rhizosphere soils were more diverse than those in the roots of three plants. Rhizosphere soils of were dominated by Actinobacteria, while rhizosphere soils and roots of were dominated by Verrumicrobia and Proteobacteria. The root diazotrophs were dominated by Alphaproteobacteria. These findings indicate that free-living diazotrophs abundantly and diversely occur in grassland soils dominated by non-legume plants, suggesting that these diazotrophs may play important roles in fixing nitrogen into soils on the plateau.