Soil chemistry influences microbial communities and nitrogenase gene abundance in the rhizospheres of Miscanthus X giganteus and Panicum virgatum

Background/Question/Methods: Miscanthus x giganteus and Panicum virgatum are promising bioenergy feedstock crops suitable for the temperate zone. While we have previously shown that Miscanthus benefits from nitrogen fixation by associative diazotrophs, the effects of plant species and soil chemistry on community structure and nitrogen fixation potential of microbiomes associated with these plants are currently not well understood. We characterized rhizosphere and endophytic microbial communities of both crops from eight research sites in Illinois with a range of soil chemistry parameters. Total bacterial and diazotrophic communities were assessed in the rhizosphere and endophytic compartment using fragment length analysis of the ribosomal intergenic spacer and the nifH gene respectively. nifH gene abundance in the rhizosphere was measured with quantitative polymerase chain reaction. Microbial and diazotrophic taxa in the M. X giganteus rhizosphere at each site were identified by sequencing the 16S rRNA gene and nifH gene respectively. 

Results/Conclusions: Multivariate statistical analyses of these data show that while bacterial and diazotrophic endophytic communities differed substantially between plant species, site-to-site variation predominated differences between rhizosphere communities. nifH abundance in the rhizosphere was positively correlated with abundance of ammonia and bioavailability of iron in the soil but negatively correlated with abundance of other soil nutrients including total nitrogen and nitrates. Indicator species analysis revealed enrichment of Proteobacteria, specifically Deltaproteobacteria, in sites with greater nifH abundance. This study demonstrates that associative diazotrophs could contribute to the sustainability of bioenergy feedstock production by enhancing nitrogen availability in low-fertility soils, especially with improved iron bioavailability via site selection and/or soil amendments.