The potential nitrogen-fixing bacterial diversity in the rhizospheric soil of the native switchgrass (Panicum virgatum L.) from Tall Grass Prairies of Northern Oklahoma was studied using a partial region of nitrogenase structural gene-nifH. Eleven clone libraries constructed from nifH amplicons gave 407 good-quality sequences. More than 70% of sequences showed similarity of nifH with uncultured bacteria (< 98%). The dominance of sequences affiliated with Deltaproteobacterial nifH was observed, followed by Betaproteobacterial nifH sequences. The nifH gene library was dominated by the genera Geobacter, Rhizobacter, Paenibacillus, and Azoarcus. Sequences affiliated with rhizobia, such as Bradyrhizobium, Methylocystis, Ensifer, etc., were also in the rhizosphere in small numbers. From Deltaproteobacteria, five genera, namely Geobacter, Pelobacter, Geomonas, Desulfovibrio, and Anaeromyxobacter, contributed to 48% of the total sequences suggesting the dominance of group Deltaproteobacteria in the rhizosphere of native switchgrass. Considering the percent similarity of the nifH sequences with cultivated bacteria, this study demonstrated the presence of novel bacterial species in switchgrass rhizospheric soil from Tall Grass Prairie.
