How does inoculation with species-specific rhizobia affect survival and growth of rare legumes?

Background/Question/Methods

Many plant species are reliant on microbial mutualists, such as rhizobia bacteria. The absence of these soil microbes may inhibit the ability plants have to succeed in ecological restorations. There may be some potential for soil microbes to recover in degraded landscapes, however direct intervention and reintroduction of these microbes may improve the establishment of rare plant species. My goal is to test whether the absence of species-specific rhizobia mutualists explains why some plants establish poorly in restorations. I created parallel experiments in the greenhouse and in a post-agricultural field to bring theoretical research into an applied setting. I focused on Lespedeza capitata, Dalea purpurea, and Amorpha canescens as study plant species because they are rare and difficult to re-establish into restorations. Each plant species was grown with live soil prior to the project setup so that I could produce cultures of species-specific rhizobia. I planted seedlings of each plant species with or without species-specific rhizobia into our field plot. In the greenhouse, I grew each plant species in either sterile soil or live soil collected from the same plot as the field experiment. In each soil treatment, I tested the effects of species-specific rhizobia bacteria inoculum on plant growth.

Results/Conclusions

In the greenhouse, I found that Lespedeza capitata and Amorpha canescens formed more nodules in live soil than sterile soil. Rhizobia inoculation further increased nodulation and dry biomass of Lespedeza and Amorpha in live soil. After one growing season in the field, each plant species had a negligible response to rhizobia inoculation. However, our study plant species typically invest in growing roots as seedlings, so first-year biomass may be a poor indicator of their fitness. Survival into future growing seasons may be a better representation of the effect of inoculation. In the greenhouse, these results show that beneficial microbes may be otherwise missing from restorations. However, this mutualistic interaction may take more than one growing season to show clear benefits to plant growth, or have effects on other metrics of plant success, such as long-term survival, that are not yet clear in our short-term experiment.