Ecological theory predicts that niche and relative fitness differences among plant species underlie variation in the biodiversity-productivity relationship. However, the mechanisms by which plant species differ are rarely experimentally quantified, leaving causal connections between plant diversity and productivity elusive. Plants in the legume family are infected by beneficial soil bacteria called rhizobia, which fix atmospheric nitrogen (N) in exchange for carbon (C) from their plant hosts. As most plants obtain N from the soil, access to atmospheric N via this mutualism can differentiate legumes from non-legumes, thus influencing the productivity-diversity relationship. However, legume species differ in their ability to enforce cooperation from their rhizobial partners. We experimentally examined the mutualism-based processes that differentiate three legume species (Amorpha canescens, Lespedeza capitata, Lupinus perennis) from each other and from non-legumes to understand how these differences cause variation in patterns of primary productivity within the BioCON experiment at Cedar Creek LTER, Minnesota.
Over 17 years of experimental diversity manipulations, the three BioCON legume species differed in monoculture aboveground net primary productivity (ANPP) and contribution to polyculture ANPP. To identify the mutualistic processes underlying this functional response, we grew each of the three legumes in soil obtained from monocultures and polycultures from which each legume was historically present or absent. We found that all three legumes nodulated poorly in soil obtained from monocultures where they were historically absent, better in soil obtained from their own monocultures, and best in soil obtained from plots where all three legumes were historically present. However, the benefit obtained from these rhizobial mutualists differed across the three legumes, with L. perennis obtaining similar benefit (as measured by individual plant growth) across all legume diversity treatments, A. canescens obtaining greatest benefit in soils obtained from plots with all legume species present, and L. capitata obtaining greatest benefit in soils obtained from its own monocultures. These results are indicative of synergistic effects among legumes in the structuring of the soil rhizobial community, and may result in differences in ANPP at the plot level. Overall, this research will inform management of ecosystem function under current and future global change scenarios, with important implications for human well-being.