Harnessing Bacterial-Fungal Interactions to Improve Switchgrass Nitrogen-Use Efficiency
Authors:
Brandon Kristy1,2* ([email protected]), James Moran2, Sarah E. Evans1,2
Institutions:
1W. K. Kellogg Biological Station, Michigan State University–Hickory Corners; 2Department of Integrative Biology, Michigan State University–East Lansing
Abstract
Switchgrass (Panicum virgatum) is a model perennial crop that can be grown on marginal land to sequester carbon (C) into stable soil organic matter and produce sustainable biofuels. Nitrogen (N) fertilizer maintains Switchgrass biomass productivity on marginal land but excessive fertilization offsets potential C sequestration gains by increasing both carbon dioxide (CO2) and nitrous oxide (N2O) emissions. To ameliorate excessive fertilizer, a combination of inoculated and native microbiota can be leveraged to improve Switchgrass N management. Free-living diazotrophs are ubiquitous, nonsymbiotic bacteria that fix atmospheric N into plant-available ammonium. Free-living diazotrophs are estimated to fix ~47 kg N ha-1 in the Switchgrass feedstock system, and their activity is stimulated by Switchgrass root exudation in the rhizosphere. However, free-living N fixation is difficult to predict because it is controlled by soil edaphic conditions. To limit confounding effects from soil native communities on N fixation, researchers quantified the effects of diazotroph inoculation on Switchgrass N using sterile microcosms during a one-week pulse-labeling experiment. Switchgrass seedlings grew in a sterilized, sand-turface (1:1) mixture for 10 weeks under high or low N fertilizer conditions. Subsequently, seedlings were inoculated with either Azotobacter vinelandii DJ (0.6 OD) or sterile, nitrogen-free medium. Directly after diazotroph inoculation, the microcosms were placed into an airtight labeling chamber; the team pulsed 1.0L of 15N2 for seven days before harvesting above ground biomass to quantify 15N enrichment. There was a significant, interactive effect between N fertilization and diazotroph inoculation: diazotroph inoculation increased Switchgrass total N (%) only under low-N conditions, suggesting that inorganic N significantly impacts N fixation activity (p < 0.05). δ15N enrichment (δ15N > 500 ‰) was only identified in Switchgrass inoculated with diazotrophs under low-N conditions (p < 0.05).
In addition to free-living diazotrophs, Switchgrass associate with arbuscular mycorrhizal fungi (AMF). The AMF-symbiont forages for inorganic nutrients beyond the plant’s root zone by extending extraradical hyphae deep into the bulk soil, AMF provide up to 55% of inorganic N requirements under low-N conditions. However, the plant-AMF mutualism does not occur in a vacuum. Bacteria live along the fungus’s hyphae, including free-living diazotrophs. Interactions with N-fixing bacteria could boost the plant’s benefits from AMF symbiosis. Yet, little work has been done to evaluate synergies between N-fixing bacteria residing on AMF. Building on previous GLBRC work on free-living N fixation in bioenergy crops, the team developed split-pot microcosm systems that enable spatially explicit sampling of AMF hyphal bacterial communities in the greenhouse and in the field.
In addition, the team identified methodological considerations for characterizing free-living diazotroph establishment and N-fixation activity on AMF. Finally, the team will share future research endeavors to quantify nutrient benefits from these interactions on the Bioenergy Cropping System Experiment (BCSE) at the Kellogg Biological Station. Ultimately, this research quantifies the contributions of AMF-diazotroph interactions to Switchgrass N health, determining if this alternative source of N can ameliorate excessive fertilization on marginal land.