Interactions between soil microbes and native species drive a diversity-invasibility relationship

Soil microbes can affect both the invasiveness of exotic plants and the invasibility of native plant communities, and native plant species diversity is commonly negatively related to community invasibility. However, it still remains unclear whether soil microbes can influence such a diversity-invasibility relationship. We constructed native plant communities with three levels of species richness (one, three, or six species) in live (un-sterilized) or sterilized soil (i.e., with or without soil microbes) and let them not be invaded by any exotic plant species or invaded by each of three exotic species (Solidago canadensis, Erigeron canadensis and Symphyotrichum subulatum) highly invasive in China. The soils conditioned by the native plant communities not invaded by the exotic species were used as soil microbe inocula to test whether richness-induced differences in soil microbes affect the growth of each of the three invasive species. Diverse plant communities decreased invader biomass and this effect was greater in live than in sterilized soil. Greater invasion resistance in more diverse communities was ascribed to increased complementarity likely caused by plant-soil microbe interactions. We conclude that soil microbes can alter the diversity-invasibility relationship through their interactions with native species richness. Our results highlight the importance to integrate the role of soil microbes in testing the diversity-invasibility hypothesis.