Background/Question/Methods Sustainable production of biofuel crops requires low anthropogenic inputs in order to maximize the net energy gain, as well as minimize greenhouse gas emissions. Nitrogen is typically the most limiting nutrient for plant growth, and inputs of N fertilizer account for a major portion of fossil fuel use in agricultural systems. Diazotrophic bacteria can play a role in relieving nitrogen demands in plants, thereby reducing or eliminating the need for fertilizer application. C4 grasses such as maize, Miscanthus, and switchgrass harbor bacterial assemblages that can colonize the rhizosphere and plant tissues and enhance plant growth without causing disease. We have isolated several bacteria from the roots and stems of Miscanthus x giganteus and Panicum virgatum with nitrogen-reducing capabilities based on the presence of the diagnostic gene for nitrogen fixation, nifH. All isolates belong to the Enterobacteriaceae. Our research explored the hypothesis that these populations may contribute to the low nitrogen requirements of these biofuel crops. Significant growth benefit was demonstrated following application of these strains to a model grass grown in an N-free medium. To optimize this beneficial plant-microbe interaction, we also explored the ecological drivers that govern colonization and activity of plant-associated bacteria in Miscanthus. Culture-independent DNA ‘fingerprinting' approaches based on the 16S-23S rRNA intergenic spacer region were employed to compare bacterial communities associated with long-term native plots of Miscanthus in Taiwan. Results/Conclusions Microbial communities colonizing the rhizomes and rhizosphere of Miscanthus are influenced by soil chemistry and fertility. Significantly different microbial assemblages were also observed in different Miscanthus species. Results suggest that agronomic practices may be able to influence the microbial assemblages that colonize this potential biofuel crop. Understanding how microbial diversity and composition correlate with environmental variables will enable growers to effectively utilize microbes for sustainable crop production.