Most mutualistic associations are facultative, and associates can exclude or sanction less cooperative or “cheating” partners that provide a lower benefit to cost ratio. The legume-rhizobia mutualism is a useful model for testing hypotheses about such context dependency due to the intimate nature of resource exchange. Soil nitrogen (N) context influences the threshold at which legumes exclude or sanction N-fixing mutualists, and rhizobia evolve to be less cooperative under elevated N, as carbon rewards from plant hosts diminish under such conditions. In the Northeastern USA, fossil fuel combustion and agricultural activities have resulted in a significant gradient of nitrogen deposition. We tested effects of N deposition on Amphicarpa bracteata, a legume native to eastern North America, and its N-fixing symbiont Bradyrhizobium elkanii in a series of laboratory experiments and field surveys. We used natural abundance of isotopes to determine relative rates of biological N fixation within and among populations along a ~500 kilometer transect.
Results/Conclusions In growth chamber experiments, we found distinct positive effects of rhizobial inoculation on A. bracteata under low N conditions, however when soil N was elevated, plants reduced nodule number and size by >70%. We detected a slight positive relationship between rhizobia and measures of plant fitness under limiting N, and a neutral or negative relationship under elevated N. In field surveys, we did not find the predicted longitudinal N gradient among 20 populations, nor did large scale geographic characteristics significantly predict nodule size, mass, or plant performance. Rather, within-population spatial orientation affected rates of N fixation, with rhizobia along population perimeters contributing more N to plant hosts than in centers. We conclude that A. bracteata modulates its investment in its mutualist depending on fertility context, and that within-population spatial heterogeneity predictably contributes to variation in rhizobial contributions to plant performance.