What is the Impact of the Rhizosphere Microbiome on Switchgrass Nitrogen Status?

Nitrogen (N) transformations are largely mediated by microorganisms that can carry out specific functions (e.g., N-fixation or N-mineralization). The microbiome associated with the switchgrass rhizosphere is therefore a critical factor that influences the overall N-cycle in perennial biofuel cropping systems. While N is commonly thought to limit productivity in terrestrial systems emerging research suggests the aboveground biomass yields in unfertilized switchgrass plots closely match yields in fertilized plots.

Mechanisms driving similar biomass yields in fertilized vs. unfertilized switchgrass plots are currently not well understood, but are crucial to understanding the sustainability of switchgrass as a candidate feedstock to biofuel production. ​We will test the hypothesis that the switchgrass rhizosphere microbiome plays a critical role in alleviating plant N-limitation in unfertilized switchgrass plots. By coupling molecular techniques to characterize the rhizosphere microbiome and biogeochemical assays to understand the pools and fluxes of N in switchgrass plots over multiple field sites and growing seasons we will gain a better understanding of the influence of the rhizosphere microbiome on N-transformations and ultimately plant N-availability.

Does the Chemistry and Rate of Root Exudation Influence Rhizosphere Nitrogen Transformations?

Microbially mediated N-transformations in the rhizosphere that influence plant N-availability are energetically costly and require an organic substrate as an energy source. Both soil organic carbon and root exudates are potential energy sources for the rhizosphere microbiome, but the relative importance of these two organic carbon sources to microorganisms involved with N-transformations is poorly understood. We will test the hypothesis that root exudation rates and chemistry will vary as a function of plant N status and rhizosphere N-availability. We predict plant allocation of organic carbon to root exudates will increase as N-availability decreases and higher exudation rates will be associated with higher rates of N-fixation in the rhizosphere. Additionally, we will use controlled greenhouse experiments to identify organic compounds in root exudates that vary as a function of plant N-status to assess their influence on microbial communities and N-fixation and N-mineralization rates.

How Are Switchgrass Functional Traits Influenced by Nitrogen Availability and Microbiome Interactions?

We will leverage the six fields sites (each of which contains the same switchgrass genotype) to examine environmental and biologically (e.g., microbe-plant interactions) mediated impacts on switchgrass functional traits driven by changes in gene expression. Greenhouse experiments will be used to understand the influence of N-availability and the rhizosphere microbiome on the switchgrass transcriptome, along with ensuing impacts on productivity and functional traits.

Are Plant-Rhizosphere Linkages Generalizable Across Temporal and Spatial Scales?

Our field sites are located throughout Michigan and Wisconsin and make use of the ongoing GLBRC Marginal Lands Experiment plots. These sites capture a range of environmental conditions from soil texture and chemistry to differences in annual temperature and precipitation. The six field sites will allow us to identify both general and site specific links between switchgrass functional traits, the rhizosphere microbiome and N-cycling. Additionally, the three year field campaign will allow us to understand and account for year-to-year variability necessary to reach more general conclusions.