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

Background/Question/Methods

Big bluestem (Andropogon gerardii) is a dominant grass of the Great Plains that accounts for roughly 70% of the biomass of tallgrass prairies. Its 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, we know little about the role of soil microbes in local adaptation. Soil microbes are reported to play fundamental roles in drought resistance and nutrient uptake. 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 soil samples from six A. gerardii populations from western KS (500 mm rainfall) and Illinois (1200 mm per year). Plants were grown from seed in greenhouse in garden soil with 6 replicates per treatment. We isolated microbes from roots and native soil and cultured in lab in R2A agar broth. We reciprocally inoculated wet and dry microbes (plus mock control) into soil weekly for 12 weeks. Plant form and function were measured weekly for most responses.

Results/Conclusions

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. In looking at the microbe 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 the dry ecotype was matched with its local microbes. Taken together, these results suggest effects of ecotype specific microbe-mediated nutrient availability. These results provided insight into how plants interact with their native microbes and suggest they play an important role in nutrient availability and uptake for A. gerardii. Because A. gerardii is widely used in prairie restoration and as forage for the cattle industry, understanding how A. gerardii interacts with its local soil microbes is crucial. Millions of acres of agricultural land have been restored throughout the plains and cattle grazing is a multi-billion-dollar industry in Kansas alone. These results will help to inform range managers and land conservationists to optimize forage and restoration through use of and matching with beneficial microbes.