Development of a Low-Input Switchgrass-Microbe System to Improve Energy Ratio and Production of Biofuels.

Switchgrass (SG, Panicum virgatum L.) is a low-input cellulosic feedstock used for biofuel production, that can generate high biomass yields with minimal to no synthetic nitrogen (N) fertilizer. We therefore asked the question: Where is the N coming from? We propose that SG is obtaining its N from an alternative source, perhaps through biological nitrogen fixation (BNF). Studies have investigated N₂-fixation associations with other perennial grasses. However, no study has yet determined if and which N₂-fixing organisms contribute N for SG growth or if the contribution of BNF can increase the efficiency of SG production. The objective of this study is to increase energy production and efficiency of SG biofuels by developing a low-input switchgrass-microbe system, with a focus on the investigation of N₂-fixing SG endophytes as inoculants. Switchgrass rhizomes were collected in Quebec, Canada, from a discontinued biomass trial of 11 SG varieties that had not received management or synthetic fertilizer input for ten years. Over 300 potential N₂-fixing endophytes were isolated on N-free selective solid media. To select for the strains best at increasing plant growth, bacterial screening bioassays were conducted in growth chambers with inoculated and uninoculated plants fertilized with N-free Hoagland’s solution. Identification of these isolates were performed using phylogenetic analysis of the 16S rRNA, with amplification of the nifH gene used as a marker for N₂-fixation capability. Field trials of the best N₂-fixing strains showed that seed inoculation improved yield and stand density in the establishment year, which is a critical year for long-term SG productivity. Harvest results showed that the best strains corresponded to isolates that were collected from rhizomes sampled from the variety plots that produced the greatest biomass. The findings of this project will enable us to develop an environmentally beneficial switchgrass-microbe system that will improve SG energy ratios and lower production costs.