Cooperation Under Changing Conditions: Tests of Mutualism Theory in Legume-Rhizobium Systems

Abstract

Cooperation between leguminous plants and nitrogen-fixing rhizobium bacteria is a critical component of global nitrogen cycling. However, evolutionary and mutualism theory predict that increased soil nitrogen will disrupt this mutualism. I explored the effects of soil nitrogen on legume-rhizobium mutualism with a combination of greenhouse and field studies. First, I grew field-collected plants and soil microbes from across a natural soil nitrogen gradient with three levels of nitrogen fertilizer to study how soil nitrogen contributes to local adaptation. Although plants from high-nitrogen sites were more plastic in their allocation of resources to rhizobia than plants from low-nitrogen sites, I only found local adaptation of rhizobia to high-nitrogen sites; there was no evidence for plant local adaptation to N. Second, I used a field experiment to study the effects of the 2021 emergence of Brood X cicadas, which should result in a natural nitrogen pulse, on wild legumes through changes in maternal effects and soil microbial communities. I found that decaying cicadas affected multiple generations of plants: seeds from plants amended with cicadas were more likely to germinate, and soil microbial communities from cicada-addition plots accelerated early seedling growth. Finally, I experimentally evolved soil microbial communities in the greenhouse to investigate the direct and indirect (light and host availability) pathways by which nitrogen fertilization of plant communities can lead to a decline in microbial mutualism, and whether the mutualism decline observed in the field can be reversed by ceasing fertilization. I found that no single factor caused strong mutualism decline, but that any combination of two or three factors caused soil microbes to be less beneficial to plant growth. However, soil microbes from nitrogen-addition field plots did not become more beneficial to plants after evolving in low-nitrogen greenhouse conditions. Together, these results demonstrate how the complexities of real-world conditions complicate the predictions of simple theoretical frameworks and highlight the importance of considering the broader biotic community context in studies of evolutionary ecology