| Switchgrass (SG, Panicum virgatum L.), a temperate perennial grass, was chosen by the US Department of Energy's Herbaceous Energy Crops Program as the 'model' bioenergy crop for further research in North America. Current research on SG for bioenergy feedstock production focuses on improving breeding selection, agronomy and crop physiology, energy potential, and its contribution to mitigating greenhouse gas emissions. However, there is a lack of knowledge regarding plant-microbe interactions with SG, how these associations play a role in its growth and productivity, and their function and potential role in agro-ecosystems. Moreover, as SG has been reported to produce high biomass yields with minimal to no synthetic nitrogen (N) fertilizer, this suggested to us that SG could be obtaining at least some of the N to meet its requirements from plant growth promoting rhizobacteria (PGPR) capable of biological N2-fixation (BNF). The objectives of this research were to determine if: (1) SG associates with PGPR, (2) PGPR we isolated from SG can be used as inoculants capable of promoting SG growth under a controlled environment, and (3) inoculation with PGPR can increase the growth and productivity of SG for biofuel production under a low-N input system. Switchgrass rhizomes were collected in Québec, Canada from a discontinued biomass trial of 11 varieties that had not received N fertilizer or any other management input since 2000. Isolates were chosen on N-free solidified media and screened for their ability to promote plant growth using plant assays conducted in growth chambers. Switchgrass seedlings were inoculated, or not, with batches of mixed isolates and fertilized with N-free Hoagland's solution. Molecular analyses of 16S rRNA gene sequences identified the mixed bacterial inoculum as Paenibacillus polymyxa, a N2-fixing bacterium, and several other PGPR (Pseudomonas, Serratia, and Rahnella spp.) capable of producing auxin and/or solubilizing phosphate. Field trials of inoculated SG seeds were conducted in 2010 on three sites comprising different soil types. The factors tested were the bacterial treatment, either uninoculated control or seed inoculated, and a fertilizer treatment, either 0 or 100 kg N ha−1.;Establishment year results showed that inoculation with a mixed PGPR produced higher tiller density and larger tillers than uninoculated plants, which was the probable cause of the 40% yield increase. This 40% yield increase persisted under N fertilization, at least at the 100 kg N ha−1 rate. Inoculated SG plants also had better N cycling than uninoculated plants, as they contained more N within tillers during anthesis but not after senescence, suggesting a greater amount of N was translocated to below-ground roots and rhizomes of inoculated than uninoculated plants. Greater N storage in roots and rhizomes could mean better early-season regrowth and provide an advantage over weeds. PGPR inoculation also affected the N balance of the harvested biomass by contributing additional non-fertilizer N (ANFN) to SG plants. Interestingly, this bacterial effect was not inhibited in the presence of N fertilizer. The combination of PGPR and nitrogen fertilizer provided a substantial N contribution to SG plants, although the exact amount will require additional research. This investigation showed that SG does associate with PGPR and that PGPR can be effectively utilized as inoculants to enhance SG yields in low-N input systems. This research will help in the development of an environmentally beneficial switchgrass-microbe system, reduces N requirements and has the potential to become a best N management practice. |
