COS 75-3 - Biodiversity-ecosystem function relationship drives nitrogen fixation in the legume-rhizobia mutualism

Abstract: Symbiotic nitrogen fixation (SNF), carried out primarily by partnerships between plants in the legume family and rhizobia bacteria, is one of the most important sources of bioavailable nitrogen to terrestrial ecosystems, driving intense interest in the ecological factors that regulate this process. While most work on the regulation of SNF has focused on abiotic drivers such as light, water, and soil nutrient availability, the diversity of rhizobia with which an individual legume partners may play an important but under-recognized role in regulating nitrogen inputs from SNF. Here, we adopt principles from the biodiversity-ecosystem function literature that have been established at the plant-community scale to test the effects of the biodiversity of rhizobial partners on SNF rates within individual legume plants. We grew 1,100 seedlings of two leguminous shrubs in a fully factorial greenhouse experiment with five levels of nitrogen fertilization and six levels of rhizobia diversity (keeping the amount of rhizobia inoculum constant). We assessed the effects of rhizobia diversity on plant growth, biomass allocation, and SNF (using ¹⁵N isotopic tracing) across species and fertilization regimes. We found that rhizobia diversity can increase average SNF rates by up to 90%, that these diversity effects mediate the effects of soil nitrogen availability on SNF, but that rhizobia diversity effects differed substantially between legume species. Increasing rhizobia diversity had significant positive effects on both the biomass and SNF for Ulex europaeus (P < 0.001 for both), but did not impact either biomass or SNF for Spartium junceum (P = 0.86 and P=0.37). For Ulex, the effects of increasing rhizobia diversity were stronger than the effects of our N fertilization treatments. Increasing rhizobia diversity from 1 to 5 rhizobial taxa effectively doubled both biomass and SNF. Alternatively, increasing N fertilization from 0 to 20 gN m^-2 yr^-1 increased Ulex biomass by only 45% and decreased SNF by only 18.8%. Results from our single-rhizobia strain treatments suggests the most likely mechanism for these rhizobia diversity effects is a selection effect – where increasing diversity increases the likelihood of including a particularly high-fixing rhizobial strain. These results provide novel evidence that biodiversity-ecosystem function relationships can occur at scales of an individual plant, and that the effects of rhizobial diversity may be as important as long-established abiotic factors, such as nitrogen availability, in driving terrestrial nitrogen inputs via SNF.