Ecology of nitrogen-fixing bacteria associated with a perennial grass biofuel crop

Background/Question/Methods Sustainability is a key economic and environmental issue in bioenergy feedstock production. 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. Minimal application of nitrogen fertilizer is desirable in order for feedstock production to be economically and energetically viable, but relying solely on soil N to support feedstock production is not a sustainable long-term strategy. Sustainable production of biofuel crops requires low anthropogenic inputs in order to maximize the net energy gain, as well as minimize greenhouse gas emissions. Diazotrophic bacteria can play a role in relieving nitrogen demands in plants, 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. Although the presence of diazotrophic endophytes has been well documented in a number of non-leguminous plant species, the ecological factors that govern endophyte abundance and activity are not well understood at this time. The goal of our investigation is to identify plant and environmental conditions (soil type, water availability, etc.) that influence endophytic and rhizosphere bacteria associated with Miscanthus and favor the development of nitrogen-fixing endophyte populations. Molecular methods were used to detect nitrogen-fixing bacteria in native and agricultural plots of Miscanthus in Taiwan and Illinois.

Results/Conclusions Diazotrophs were ubiquitously detected in both endophyte and rhizosphere samples of Miscanthus. Nitrogen-fixing Enterobacteriaceae were isolated from Miscanthus. Comparisons of bacterial assemblages from replicate plants and different locations reveal that endophyte and rhizosphere communities are non-random collections of bacteria that display consistent patterns of composition. Plant species and endophytic localization were the two most important factors shaping plant-associated bacterial community composition in both native and agricultural Miscanthus. Environmental factors such as pH, soil moisture, organic matter, and calcium concentration were also important influences on community composition. Abundance of nitrogen-fixing genes was significantly greater in the rhizosphere of unfertilized Miscanthus than in the maize rhizosphere at the same field sites (p<0.0001), indicating that cultivation of Miscanthus influences the diazotroph community in these plots. Understanding how microbial diversity and composition correlate with environmental variables will enable agronomic practices that effectively utilize beneficial microbes for sustainable crop production.