Switchgrass establishment and seeding year production can be improved by inoculation with rhizosphere endophytes

Nitrogen-fixing bacteria (NFB) and other plant growth promoting rhizobacteria (PGPR) have been used to increase growth and yield of corn and sugarcane, two graminaceous crops utilized as bioenergy feedstocks. Switchgrass (SG, Panicum virgatum L.), a native perennial grass of North America, is also a desirable bioenergy crop because it can be grown on marginal lands and thus does not necessarily displace food production. Yet no investigations have been conducted on the use of NFB and/or PGPR as inoculants to increase SG growth and productivity. Studies reporting high biomass yields from SG with minimal to no fertilizer nitrogen (N) input prompted us to hypothesize that SG is obtaining part of its N from an alternative source, perhaps through biological nitrogen fixation from root/rhizome associated PGPR. We isolated bacteria from SG rhizomes that had not received fertilizer N input for over ten years and which increased plant growth in the absence of the fertilizer N under growth chamber conditions. The bacteria were identified as a strain of Paenibacillus polymyxa, a N2-fixing bacterium, and several other PGPR capable of solubilizing phosphate and/or producing auxins. Our objective was to test this PGPR inoculum under field conditions with low-N inputs. Here, we present the findings regarding SG productivity in the establishment year. Switchgrass seeds inoculated with the PGPR culture had greater tiller density, and taller tillers resulting in 40% higher yield than uninoculated plants. PGPR have the potential to be effectively utilized as inoculants to enhance SG yields in a low-N input production system.