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

1.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.

2.We first conducted a common garden study of the annual l...