PS 45-195 - Do wet and dry ecotypes of a dominant grass grow better with their native soil microbes?

Abstract: Big bluestem (Andropogon gerardii) is a dominant grass of the Great Plains that accounts for roughly 70% of the biomass of tallgrass prairies. Its broad distribution across a steep rainfall gradient in the Great Plains has given rise to locally adapted wet and dry ecotypes. Abiotic factors, such as rainfall, in the formation of ecotypes has been well-studied. However, much less is known about the role of biotic factors, such as soil microbiomes, in such local adaptation. Soil microbiomes are fundamental in drought resistance and nutrient uptake and are expected to vary like A. gerardii along the Midwest precipitation gradient. We investigated how local soil microbes affect A. gerardii growth and whether specific plant ecotypes are matched to local soil microbes. We predicted that each ecotype would grow better when grown with its native microbes. We collected seed and rhizosphere soil samples from six A. gerardii populations from western KS (500 mm rainfall) and Illinois (1200 mm per year). We isolated microbes from roots and native soil, cultured them in R2A agar broth, and reciprocally inoculated wet and dry microbes (plus mock control) weekly into garden soil where plants were grown for 12 weeks with 6 replicates per treatment. Plant form and function were measured weekly for a variety of responses. Results showed distinct differences between ecotypes and soil microbiomes. Population markers show that ecotypes are genetically differentiated and that wet ecotypes are more variable than dry ecotypes. Metabarcode (v4 of the 16s) sequencing showed overlap among the wet and dry soil inocula as well as some notable differences among them. Clostridium, Paenibacillus, and Lysinibacillus were abundant in the dry soil inoculum but almost entirely absent from the wet inoculum. Repeat sequencing over time indicated that inocula were compositionally stable over time. We found that ecotypes differed in biomass, leaf area and width, and height. Wet ecotypes produced more biomass, greater leaf area, and were taller than the dry ecotype. For inoculation effects, physiological traits, such as chlorophyll absorbance, a proxy for photosynthesis, were enhanced in ecotypes growing with their local microbes. Even more notable, the dry ecotype produced ~30% more biomass when it was matched with its local microbiome. Taken together, these results suggest ecotype specific and microbe-mediated effects on nutrient availability. These results will help to inform range managers and land conservationists to optimize forage production and restoration through the use of and matching with beneficial microbes.