Impacts of nitrogen-fixing rhizobia on resilience to herbivore pest damage and drought stress in soybean

Increasing variability in field conditions, and its subsequent effects on insect pest complexes, poses a significant challenge for crop management strategies in agricultural systems. Rhizobia bacteria are facultative legume mutualists that infect the roots of their host plant and form root nodules. From these nodules, rhizobia fix atmospheric nitrogen into plant-available ammonia utilized for plant growth. This study aimed to test the role of bacterial mutualists in driving resilience and stability of ecosystem functioning in soybean cropping systems. In addition, we tested the hypothesis that altering the nodulating rhizobia community will directly impact soybean yield through increased N availability and indirectly mediate plant trait responses to herbivore and drought stress. We performed greenhouse feeding assays in which we reared the generalist legume pest, Chrysodeixis includens, on a commercial variety of soybean. We manipulated the soil rhizobial community under drought and well-watered regimens using 24 rhizobia strains. We quantified herbivore damage on soybean foliage, herbivore growth rates, root nodulation, and plant fitness traits throughout the vegetative stages of soybean, when soybeans are generally most susceptible to pest damage that impacts yield. We propose that understanding how soil microbial communities mediate plant-insect interactions under variable field conditions offers novel insight into how management strategies can promote stability of soybean yields. Future work will address the effects of rhizobia strain diversity in mediating soybean-rhizobial interactions. In particular, we will examine the functional role of rhizobia diversity as a buffer against leaf-chewing herbivory and drought stress in greenhouse and field settings.