Mutualism variation in the nodulation response to nitrate.

The evolution of mutualisms under novel selective pressures will play a key role in ecosystem responses to environmental change. Because fixed nitrogen is traded in plant–rhizobium mutualisms, increasing N availability in the soil is predicted to alter coevolution of these interactions. Legumes typically decrease the number of associations (nodules) with rhizobia in response to nitrate, but the evolutionary dynamics of this response remain unknown. We grew plant and rhizobium genotype combinations in three N environments to assess the coevolutionary potential of the nodule nitrate response in natural communities of plants and rhizobia. We found evidence for coevolutionary genetic variation for nodulation in response to nitrate (G × G × E interaction), suggesting that the mutualism response to N deposition will depend on the combination of partner genotypes. Thus, the nitrate response is not a fixed mechanism in plant–rhizobium symbioses, but instead is potentially subject to natural selection and dynamic coevolution.