New tools are revealing how feedbacks between plants and their associated microbial communities assist species invasions. Invasive legumes often co-invade an exotic range with rhizobia from their native ranges. Once established, legumes may form positive feedback loops with these microbial mutualists, further promoting invasion success. Here we examine how legume invasions affect soil rhizobial communities, and how these community shifts affect growth of both native and invasive legumes. We further explore how effectively three common restoration techniques return soil rhizobial communities to their native states. For this work, we focus on three invasive legumes—French broom (Genista monspessulana), Spanish broom (Spartium junceum), and Scotch broom (Cytisus scoparius)—in the San Francisco Bay Area.
Results/Conclusions
Our results show that distinct communities of rhizobia associate with native vs invasive legumes in the Bay Area. Further, densities of non-native rhizobia were significantly higher in areas invaded by the focal legumes than in uninvaded areas, providing evidence for strong amplification of familiar rhizobial genotypes by invasive legume hosts. These positive plant-soil feedbacks could help maintain invasive legume populations in their exotic range. However, different invasive removal techniques (herbicide, mowing, hand-pulling) differentially affected soil rhizobial density; hand-pulling most effectively reduced non-native rhizobial densities. Overall, this research points to the importance of mutualisms in driving invasion success as well as restoration efforts.