Widespread efforts to stem biodiversity loss through restoration are often only partially successful. In prairies, even after adding diverse seed mixes to post-agricultural fields, some species of legumes establish readily (particularly early-successional legume species) while others do not. We hypothesized that missing microbial mutualists could limit the establishment of late-successional, difficult-to-establish legumes in restored prairies, whereas early-successional legumes would form associations with a wide variety of rhizobia in the majority of sites. We tested these hypotheses by growing 12 legume species in soil from either restored or remnant prairies in Michigan and Indiana and isolating 1720 nodulating rhizobia strains. We then evaluated rhizobial mutualist quality in single strain inoculation experiments. We expected that restored prairies would have fewer rhizobia and lower quality rhizobia than remnant prairies for late-successional legume species that rarely establish in restored prairies.
Overall, restored prairies did not produce fewer or lower-quality strains of rhizobia than remnant prairies. Instead, high-quality rhizobia came from both site types. However, for some late-successional legumes that rarely establish in restorations (Amorpha canescens and Astragalus canadensis), remnant prairies were more likely than restored prairies to produce high-quality mutualists. For other legumes that rarely establish (late-successional Dalea candida and mid-successional Desmodium illinoense, Desmodium paniculatum, and Lespedeza virginica), we detected very few high-quality mutualists at our 10 sites, suggesting that missing mutualists could still limit legume establishment in restorations. However, the absence of mutualists cannot explain all variation in legume establishment success: many sites supported high-quality mutualists for other late-successional legumes that establish poorly (Dalea purpurea and Tephrosia virginiana), and we detected almost no high-quality rhizobia for two other legumes that commonly establish in restorations (Baptisia lactea and Lespedeza capitata). This work suggests that missing rhizobial mutualists could explain low establishment of some legumes, but that legume successional status is not a clear predictor of the importance of rhizobia limitation. Instead, we will need to develop species-specific approaches to improving legume establishment in restored prairies.