Biotic plant-soil feedbacks alone do not explain why more diverse plant communities are less invasible

An increase number of studies suggests that more diverse communities of native plants more strongly resist invasion by introduced plants. Here we tested whether biotic plant-soil feedbacks can explain this relationship independently of other factors, via either soil richness, as based on the number of different plant species conditioning the soil; or soil heterogeneity, the degree to which plant-soil feedbacks involving different plant species are spatially separated. No previous study appears to have tested both soil richness and heterogeneity as components of biotic plant-soil feedbacks that might explain why more diverse native plant communities are less invasible.

We conditioned soils with monocultures of six native plant species and grew five introduced plant species individually in sterilized soil inoculated with one, two, or four of the conditioned soils, keeping the conditioned soils separate or mixing them.

Soil richness had little effect on the final dry mass of any introduced species. Higher soil heterogeneity did not decrease final mass in any introduced species and instead increased it in one.

Results suggest that biotic plant-soil feedbacks are not in themselves an important mechanism by which diversity limits invasibility but do not rule out the possibility that such feedbacks play a role in combination with other mechanisms such as abiotic feedbacks or plant competition.