Theory suggests that genetic diversity may influence species coexistence and that species diversity may influence genotype coexistence by altering competitive outcomes among species and genotypes, respectively. However, other coexistence mechanisms such as microbe-mediated plant-soil feedbacks (PSF), may also contribute. Interspecific PSF promotes species coexistence when plants grow better with heterospecific soil microbes than with conspecific microbes, and similarly, intraspecific PSF promotes genotype coexistence when plants grow better with heterogenotypic than with congenotypic microbes. Here, we tested whether genetic diversity influences the strength or direction of interspecific PSF and whether species diversity influences the strength or direction of intraspecific PSF. We found that genetic diversity reduced the capacity for interspecific PSF to promote species coexistence, and, for one study species, species diversity reduced the capacity for intraspecific PSF to promote genotype coexistence. These results suggest that genetic diversity and species diversity may weaken the ability of PSF to promote coexistence.

Genetic diversity and species diversity are typically studied in isolation despite theory showing they likely influence one another. Here, we used simplified communities of one or two populations of one or two species to test whether linkages between genetic and species diversity can be mediated by interactions between plants and their soil microbiota, or microbe-mediated plant-soil feedback (PSF). Interspecific PSF promotes the maintenance of species diversity when plants grow better with heterospecific soil microbes than with conspecific microbes. Similarly, intraspecific PSF promotes the maintenance of genetic diversity when plants grow better with heterogenotypic than with congenotypic microbes. We found that genetic diversity reduced the ability of interspecific PSF to maintain species diversity, and, for one study species, species diversity reduced the ability of intraspecific PSF to maintain genetic diversity. If these patterns occur in more diverse communities, then PSF may cause genetic and species diversity to negatively influence one another.