Mutualism theory predicts how legumes influence biodiversity-ecosystem function relationships under global change

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Primary productivity, the conversion of solar energy to plant biomass via photosynthesis, is a particularly important ecosystem service that supports most global ecosystems and is positively related to plant biodiversity. This project will increase mechanistic understanding of the biodiversity-productivity relationship to better explain how environmental conditions influence ecosystem services. Ecological theory predicts that differences among plant species underlie variation in the biodiversity-productivity relationship. However, the mechanisms underlying this variation among plant species are rarely experimentally quantified, leaving causal connections between plant variation and the biodiversity-productivity relationship elusive. Plants in the legume (bean) 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 soil, access to atmospheric N via this mutualism can differentiate legumes from non-legumes, thus influencing the productivity-diversity relationship. This project will experimentally examine biotic processes that differentiate legume species from each other and from non-legumes to understand how these differences cause variation in patterns of primary productivity within a long-term biodiversity, CO2, and N manipulation experiment in Minnesota, USA--BioCON. To promote public understanding of science, middle school students will grow and observe legume seeds inoculated with local bacteria under two N treatments to test if bacteria from regions with high soil N are less beneficial than those from regions with low soil N. Undergraduates will be trained in relevant techniques and develop independent projects within the scope of this work. The research and outreach projects will inform management of ecosystem function under current and future global change scenarios, with important implications for human well-being.Niche differences (ND) and relative fitness differences (RFD) among legume species will be assessed by determining the composition and density of, and plant benefit from, the species-specific nodulating rhizobial community (NRC) in soil samples from the ambient condition BioCON biodiversity plots. Structural equation models will relate ND and RFD, and their underlying mechanisms, to relative yield totals calculated from each diversity treatment, to test the hypothesis that higher ND between legumes and non-legumes produces stronger richness-productivity relationships and higher RFD weakens the richness-productivity relationship. Evolution of the NRC in response to enhanced CO2 and/or N, and its consequences for the richness-productivity relationship, will be assessed by determining the composition and density of, and plant benefit derived from, the NRC of each BioCON diversity, CO2, and N treatment combination. Finally, variation among legume species in how CO2- and/or N-availability shifts enforcement of rhizobial cooperation (partner choice and sanctions), and the consequences of this variation for the richness-productivity relationship, will be examined through mixed-inoculation experiments.