When mutualisms matter: Rhizobia effects on plant communities depend on host plant population and soil nitrogen availability

Mutualistic interactions, such as the relationship between legumes and rhizobia, can affect community properties, yet there needs to be greater understanding of when these interactions may be most important to communities. Resource mutualism theory provides predictions based on variation in abiotic and biotic factors. First, there is substantial intraspecific genetic variation in how legumes interact with rhizobia, including variation in legume growth response to rhizobia, number of rhizobia‐housing nodules and nitrogen fixation. Rhizobia likely have greater community impacts when associating with hosts that benefit greatly from rhizobia, produce many nodules and fix a lot of nitrogen. Second, increased soil nutrient availability typically reduces the partner benefits of legume–rhizobium interactions. As a result, the legume–rhizobium resource mutualism may have greater effects on communities in low‐nitrogen environments. We first conducted a common garden study of the annual legume Chamaecrista fasciculata to explore intraspecific variation in focal traits related to competition and mutualistic interactions with rhizobia. Then, we manipulated the presence of rhizobia, nitrogen fertilization and population identity of C. fasciculata in mesocosms simulating native plant communities. We measured plant diversity, assessed changes in community composition and measured soil nutrient availability. We detected significant genetically based intraspecific variation in growth and mutualism‐related traits across C. fasciculata populations. From the experimental mesocosms, we found that although rhizobia consistently reduced diversity, rhizobia most strongly reduced diversity when associating with C. fasciculata populations that were highly dependent on rhizobia and when nitrogen was most limiting. We also detected greater variation among populations in their effects on communities in the absence of rhizobia; the presence of rhizobia dramatically minimized effects of population identity on the surrounding community. Synthesis . These findings show that abiotic environmental factors and intraspecific variation in a dominant host plant can influence the magnitude of mutualism effects on communities and, reciprocally, how the presence of key mutualists can influence the importance of genetically based variation among populations to community outcomes. While prior studies have detected both strong and weak effects of mutualism on plant communities, our study illustrates that mutualism theory may predict when strong mutualism effects are most likely to occur and also shows how theories developed for particular species interactions may make studies investigating the ecological impacts of genetic variation a more predictive science.