Rhizobia mutualists alter competitive dynamics

Background/Question/Methods

Resource mutualisms, such as the interaction between legumes and rhizobia, have the potential to either inhibit or facilitate other species in the community by altering the abiotic and biotic environment. Yet, understanding the role of mutualisms in a community context has typically received less consideration than other species interactions. The legume-rhizobium mutualism can provide the legume a competitive advantage over other species thereby suppressing their growth. Alternatively, by increasing nitrogen availability in the community, the legume-rhizobium mutualism could facilitate the growth of other species by relaxing competition for a limiting resource. Here, I explore how rhizobia alter the competitive interactions between the legume Chamaecrista fasciculata and non-leguminous species in the community. I test this in two separate experiments: the first studies the effects of rhizobia on seven species grown together in early successional mesocosm communities; the second studies how rhizobia alter pairwise competitive outcomes between the legume and other species.

Results/Conclusions

Rhizobia inoculation led to greater biomass production of the legume, C. fasciculata. In mixed-species community mesocosms, Oenothera biennis biomass declined in inoculated pots. This reduction in O. biennis production is correlated with increasing legume biomass, potentially indicating increased competition. Not all species responded negatively when mesocosms were inoculated with rhizobia, however. Bromus kalmii had marginally greater biomass in the presence of rhizobia than in uninoculated pots. Thus, rhizobia provide C. fasciculata the potential to grow larger and outcompete other species, while possibly facilitating others. Specifically, O. biennis may be inhibited by C. fasciculata as a direct competitor for light, while B. kalmii productivity increases could be a result of facilitation from increased nitrogen or decreased competition from other species in the community. These mechanisms are currently being more thoroughly explored with pairwise competition experiments. Overall, rhizobia may serve as a keystone mutualist in communities, altering the competitive dynamics between potentially dominant species in the community.