Ecological consequences of urbanization on a legume-rhizobia mutualism

David Murray-Stoker, Department of Ecology & Evolutionary Biology, University of Toronto, Toronto, ON, Canada, David Murray-Stoker and Marc T. J. Johnson, Department of Biology, University of Toronto Mississauga, Mississauga, ON, Canada, David Murray-Stoker and Marc T. J. Johnson, Centre for Urban Environments, University of Toronto Mississauga, Mississauga, ON, Canada

Background/Question/Methods
Urbanization is a major driver of ecosystem change at local and global scales, consistently altering the ecological setting in terms of both biotic and abiotic factors. Elucidating how urbanization affects the ecological consequences of species interactions is important for understanding the drivers of biodiversity and ecosystem change in urban environments. 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) by sampling 49 populations along an urbanization gradient. Our study focused on three primary questions: (Q1) does rhizobia nodulation vary along an urbanization gradient? (Q2) How does the source of plant nitrogen (i.e., from soil or gaseous N2 fixed by rhizobia) change along the urbanization gradient? And (Q3) how do urban landscape features influence the interactions between soil N, plant N, and rhizobia nodulation?
Results/Conclusions
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: urbanization altered the landscape, changing the amount of green and impervious surface cover surrounding each white clover population, and these effects cascaded onto soil N, nodule density, and the source of nitrogen used by white clover. Taken together, our results demonstrate that urbanization alters the ecology of a legume-rhizobium mutualism, with direct and indirect effects of the urban landscape plant-soil-microbe interactions.