Friday, August 16, 2019: 8:20 AM
M111, Kentucky International Convention Center
Darian Smercina, Department of Plant, Soil and Microbial Sciences, Michigan State University, East Lansing, MI, Sarah Evans, Kellogg Biological Station and Dept of Integrative Biology, Michigan State University, Hickory Corners, MI, Alan W Bowsher, Department of Microbiology & Molecular Genetics, Michigan State University, East Lansing, MI, Maren L. Friesen, Crop and Soil Sciences/Plant Pathology, Washington State University, Pullman, WA, James R. Cole, Center for Microbial Ecology and Department of Plant, Soil, and Microbial Sciences, Michigan State University, East Lansing, MI, David Hoyt, EMSL, Pacific Northwest National Laboratory, Richland, WA, Kirsten S. Hofmockel, Earth and Biological Sciences Directorate, Pacific Northwest National Laboratory, Richland, WA and Lisa K. Tiemann, Plant, Soil and Microbial Sciences, Michigan State University, East Lansing, MI
Background/Question/Methods: Free-living nitrogen fixation (FLNF), an important terrestrial nitrogen (N) source, likely occurs in the rhizosphere where carbon (C) is readily available. Switchgrass (
Panicum virgatum), an important bioenergy crop, harbors a diverse rhizosphere community of free-living N-fixing bacteria. It is becoming increasingly clear that diazotrophs are present and actively fixing N in the switchgrass rhizosphere, yet it is not known if or how fixed N is exchanged between diazotrophs and switchgrass and if this is coupled with root exudation. Using results from a greenhouse study of switchgrass grown in three different Michigan marginal land soils, we assessed switchgrass-associated diazotroph community composition (
nifH gene sequencing) and N-fixation rates in response to different levels and legacies of N addition. We also examined switchgrass rhizosphere metabolite composition in response to N availability and inoculation with a diazotroph,
Azotobacter vinelandii (AV)
, using data from hydroponically grown switchgrass. Building from these previous studies, we are now using a novel, sterile growth system to assess interactions between switchgrass and known diazotrophs and test the “C for N exchange” hypothesis by pairing multiple techniques including fluorescent
in situ hybridization (FISH), NanoSIMS via
13C and
15N labelling, and assessment of the rhizosphere metabolome via NMR and LC-MS.
Results/Conclusions: We find that switchgrass cultivates a distinct and consistent diazotroph community regardless of long-term or short-term N additions or initial diazotroph community composition. In this community, we consistently find members of the genus Azotobacter. Despite a lack of response to N by the diazotroph community, switchgrass root exudate chemistry is driven more strongly by N availability than by diazotroph presence. Exudates were carbohydrate dominated (58.9% of identified compounds; p < 0.0001) under high N conditions, but organic acid dominated (40.1% of identified compounds; p = 0.004) under low N. AV inoculation only marginally increased free amino acid (p = 0.067) and nucleic acid (p = 0.070) concentrations. To examine switchgrass-diazotroph interactions further, we grew switchgrass, inoculated with known diazotrophs, in a sterile system. We used NanoSIMS images of switchgrass roots labeled via introduction of 99 atom% 13C-CO2 to show direct uptake of subsequent 13C labeled root exudate by AV. Addition of 98 atom% 15N2 will allow us to simultaneously visualize if N-fixation by AV is coupled to C uptake. Development of diazotroph species-specific FISH probes allowed us to visualize establishment of diazotroph populations on switchgrass roots, confirming potential for the “C for N exchange” hypothesis.
