Urbanization impacts nitrogen deposition and soil microbiota diversity. However, the impacts of urbanization remain unknown for the legume-rhizobium symbiosis. Annually, legume-rhizobia symbioses contribute over 70 million tons of plant-available nitrogen, with roughly half in agriculture, making this symbiosis critical in global food production. It also represents one of the only ways in which atmospheric nitrogen is made biologically usable, thus strongly influencing nitrogen cycling. How is this symbiosis evolving as the world becomes more urbanized? We developed a field study focused on the white clover, Trifolium repens. We collected 381 samples from three rural and five urban communities in Japan, measured plant biomass, root length, and the number of root nodules, and then constructed linear mixed-effects models in R to determine which parameters explain nodulation and plant growth. Using the best-fit models resulting from model selection, we estimated predicted values for each response variable under rural versus urban status.
Results/Conclusions
The best-fit model for the number of nodules included environment type as a significant factor (rural versus urban). Plant roots from urban environments had significantly more nodules than rural plants given the same below-ground mass (p < 0.05). However, the number of nodules did not affect above-ground mass (p > 0.05). Additionally, below-ground mass did not affect the ratio between above- and below-ground mass (p > 0.05). These findings suggest that urbanization impacts nodulation, but not plant growth in the legume-rhizobium symbiosis. These results could suggest a stronger reliance on nodules for plant growth in clovers from urban communities than in clovers from rural communities. Such a situation might occur if atmospheric nitrogen deposition due to urban air pollution was relatively minor, in contrast to other factors like soil trampling, which may make urban soil conditions worse. Alternatively, these results could suggest that in urban habitats, where nitrogen deposition is 47% higher than in rural habitats, clovers demonstrate weaker discrimination against ineffective rhizosymbionts. The latter could suggest that the legume-rhizobium symbiosis may be somewhat superfluous, and may eventually be lost in urban communities. Root microbiota and soil chemistry may explain the increased nodulation in clovers. We will characterize nodulating rhizobia and abiotic factors that could explain the observed differences in nodulation.Understanding the effects of urbanization on the legume-rhizobium symbiosis may help explain selection pressures against this symbiosis. This work can broaden our application of plant-soil feedback models to plant communities following invasion.