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Ecological consequences of urbanization on a legume–rhizobia mutualism

Mutualisms are key determinants of community assembly and composition, but urbanization can alter the dynamics of these interactions and associated effects on ecosystem functions. Legume-rhizobia mutualisms are a model interaction to evaluate the ecological and ecosystem-level effects of urbanization, particularly urban-driven eutrophication and nitrogen (N) deposition. Here, we evaluated how urbanization affected the ecology of the mutualism between white clover (Trifolium repens) and its rhizobial symbiont (Rhizobium leguminosarum symbiovar trifolii) along an urbanization gradient. We found that the abundance of rhizobium nodules on white clover decreased with urbanization. White clover acquired N from mixed sources of N fixation and uptake from the soil for the majority of the urbanization gradient, but white clover primarily acquired N from the soil rather than N fixation by rhizobia at the urban and rural limits of the gradient. Importantly, we identified soil N as a critical nexus for urban-driven changes in the white clover-rhizobium mutualism. Taken together, our results demonstrate that urbanization alters the ecological consequences of a legume-rhizobium mutualism, with direct and indirect effects of the urban landscape on an ecologically-important mutualistic interaction.