The effect of below-ground plant-microbe interactions and land management on plant-insect interactions

Plants engage with a variety of organisms below- and above-ground, and those interactions range from mutualistic to antagonistic. Below-ground plant mutualists include bacteria and fungi, while above-ground antagonists are herbivorous insects. These interactions are shaped by the land management history of an ecosystem. Our goal was to determine the effects of bison grazing on Monarda fistulosa (wild bergamot) plant growth, vegetative volatile organic compound profiles, and herbivory by the generalist Spdoptera frugiperda (fall armyworm), via the soil microbial community from a remnant tallgrass prairie. To address this goal, we collected soil inside and outside of bison exclosures from the Nachusa Grasslands in northern Illinois, and used it to inoculate M. fistulosa seedlings in a greenhouse study. We found that grazing alters microbial community composition and diversity, especially in the rhizosphere. In addition, we found that although grazing does not impact M. fistulosa plant growth, it does alter M. fistulosa volatile organic compound profiles, measured in real time. Specifically, plants in the ungrazed treatment released more carbon-rich volatiles that have anti-microbial, insect-repelling, and insect-attracting functions. Lastly, we found that the composition of the M. fistulosa volatile organic compound profiles shifted in response to herbivory. However, this change was not mediated by bison grazing legacy effects. Our results indicate that grazing impacts microbial communities in the soil, and its legacy effects may limit resources available for plant-insect-microbe interactions. Future studies are needed to determine the full effects of grazing on plant-insect-microbe interactions, as they could be used to inform land management strategies.