Environment dependence of rhizobial relative fitness in the legume-rhizobia symbiosis

Spatial and temporal variation in resource availability, population density, and composition likely affect the ecology and evolution of symbiotic interactions. We examined how host genotype, Nitrogen addition, rhizobial density, and community complexity affected a legume-rhizobia ( Medicago truncatula - Ensifer meliloti ) mutualism. Host genotype had the strongest effect on the size, number, and rhizobial composition of root nodules (the symbiotic organ). By contrast, the effect of small changes in N-availability and the complexity of the inoculum community (2, 3, 8, or 68 strains) were minor. Higher inoculum density resulted in a nodule community that was less diverse and more beneficial but only in the more selective host. With the less selective host, higher density resulted in more diverse and less beneficial nodule communities. Density effects on strain composition deserve additional scrutiny as they can create eco-evolutionary feedback and have translational relevance for overcoming establishment barriers in bio-inoculants. Short Abstract The environmental context of the nitrogen-fixing mutualism between leguminous plants and rhizobial bacteria varies over space and time. The understudied environmental variable of rhizobial density had a larger effect on the relative fitness of 68 rhizobia ( Ensifer meliloti ) strains in nodules than the addition of low-levels of nitrogen or community complexity. ### Competing Interest Statement The authors have declared no competing interest.