PS 34-251 - Rhizobial inoculation in prairie management

Abstract: Restoring mutualisms holds promise for improving the outcomes of ecological restoration, including rare plant reintroductions, but the results of experimental restoration of microbial mutualisms have had mixed results. This may arise from the context-dependence of mutualisms, since the benefits plants gain are dependent on both the identity of their mutualistic partners and the environmental context of their interaction. Legumes may be especially sensitive to variation in mutualistic interactions, since they rely on both mycorrhizal fungi and rhizobia bacteria. Understanding the effects of different strains of rhizobia and how the effects of rhizobia vary across sites can help us identify useful strains for ecological restoration. Our goals were to test how different strains of species-specific rhizobia affect the establishment of Baptisia australis between two field sites and a parallel greenhouse experiment.

We isolated ten strains of rhizobia from soils collected near populations of Baptisia australis, at our field sites located within the Great Parks of Hamilton County, in southwest Ohio. To test effects of rhizobia on legume establishment, we inoculated Baptisia australis seedlings with one of our 10 different strains of rhizobia, an inoculum containing all 10 strains, and a non-inoculated control. Seedlings were raised in a greenhouse for 12 weeks before being transplanted to our two field sites, the Great Parks of Hamilton County and Miami University’s Ecology Research Center. A parallel greenhouse experiment was conducted, with Baptisia australis grown in a pasteurized field soil and sand mix and inoculated with the same rhizobia treatments as the field-transplanted seedlings. We measured stem height and number of leaves once a month for the greenhouse and field experiments and we harvested the greenhouse-grown plants to measure total dry biomass. In the field, we observed both positive and negative responses of Baptisia to different strains of rhizobia, and the best mutualist at one site wasn’t necessarily the best mutualist at the other. Surprisingly, the diversity inoculum did not improve benefits over singular strains. In contrast to the field experiments, we saw little variation in plant growth among rhizobia strains in the greenhouse, suggesting that more realistic environmental contexts may be necessary to understand how mutualisms affect plant growth and restoration outcomes. In practice, our results suggest that inoculating with rhizobia can improve establishment of rare legumes, but the best mutualists will vary among sites. So, restoring mutualisms may depend on locally adapted solutions particular to each restoration site.