Background/Question/Methods Genetic diversity and species diversity are typically studied in isolation despite theory showing they likely influence one another. Linkages between genetic and species diversity have so far been considered primarily through the mechanism of competition, but other types of species interactions like the interaction between plants and their soil microbiota, or plant-soil feedback (PSF), also may contribute. 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. To test whether linkages between genetic and species diversity can be mediated by PSF, we conducted a two-generation greenhouse experiment in which we manipulated genetic and species diversity in the first generation. We manipulated species diversity by training the soil with pairs of plants that were either two individuals of the same species or one individual of each of two species, and we manipulated genetic diversity within each species by training the soil with two individuals from the same population or one individual from each of two populations. We then measured the resulting impact on the strength of inter- and intraspecific PSF, respectively. Results/Conclusions Interspecific PSF was significantly more positive (or less negative) in the high genetic diversity treatment (conditioned by two populations) than in the low genetic diversity treatment (conditioned by one population), indicating that genetic diversity may weaken the ability of PSF to maintain species diversity (Phase II species × Phase I species × Phase I genetic diversity: P = 0.02). We also found that, for one of our two study species, intraspecific PSF was marginally significantly more positive (or less negative) in the high species diversity treatment (conditioned by both Coreopsis lanceolata and Echinacea purpurea) than in the low species diversity treatment (conditioned by Coreopsis lanceolata alone), indicating that species diversity may weaken the ability of intraspecific PSF to maintain genetic diversity (Phase II population × Phase I population × Phase I species diversity: P = 0.052). If these patterns hold for other plant species and are exemplary of more diverse communities, then our work illustrates that genetic and species diversity may be negatively linked through microbe-mediated plant-soil feedback.
