Rhizobia independently adapt to soil and legume host environments, but soil conditions influence the abundance of high-quality partners.
Rhizobia live as free-living microorganisms in the soil and in association with legume hosts. Both environments exert selective pressures on rhizobia, influencing the reproductive success of individual strains (e.g., fitness). The soil, a heterogeneous and fluctuating environment, is often overlooked, and little is known about whether selection in the soil influences the outcomes of the rhizobia-legume mutualism. We exposed a mixture of 68 Sinorhizobium meliloti strains to eight soil treatments (temperature, osmotic, and texture perturbations) and selection by two Medicago plant hosts. We found that cold (4{degrees}C) and warm (32{degrees}C) temperatures, as well as salt addition, had the strongest effects on diversity, community composition, or population size. Strain relative fitness was strongly positively correlated among soil treatments, except for cold. Genome-wide association analysis revealed a complex genetic architecture for soil fitness. In contrast, when comparing rhizobial fitness between soil and host environments, we found minimal strain fitness correlations, suggesting independent genetic bases and habitat-specific adaptations. Lastly, by examining the relationship between rhizobial fitness in the soil and their nitrogen-fixing plant benefits, we found that soil selection influenced the relative abundance of high- and low-quality strains; However, whether these effects were positive or negative for the plant was host dependent. Our results suggest that rhizobial evolution in soil and host are largely independent, but soil selection can alter mutualism benefits. IMPORTANCERhizobia-legume mutualism is crucial for introducing nitrogen into agricultural and natural ecosystems, and rhizobia persistence in the soil is an important component of agroecosystems. However, we know little about how individual strains of rhizobia persist and adapt to this environment, especially in the context of the soils spatial and temporal variations (temperature, moisture, and soil texture). We found that rhizobia similarly adapt to abiotic soil conditions but their reproductive success in the soil is independent from their reproductive success in the host. Intriguingly, we found that certain soil conditions increase (or decrease) the relative abundance of more effective nitrogen-fixing strains. Understanding how rhizobia adapt to diverse environments is crucial for developing effective bioinoculants that maintain high persistence in the soil while are also highly competitive to colonize the host and are beneficial to the plant.