Introduced plants become invasive, in part, because they leave behind co-occurring plants, herbivores and soil microbial communities that constrain their growth in their original range. Unleashed from these constraints in their new range, they can spread rapidly among naïve native communities. The story, however, does not always end there. Increasing evidence suggests that under selection pressure from an invasive species, native plants, herbivores, and soil microbial communities sometimes adapt over time to become stronger competitors or consumers of an invading plant. This study asks whether soil microbial communities tend to become more suppressive of an invasive plant the longer they are exposed to it. If so, then these soils could offer a way to introduce biological resistance to an invasive plant in areas where it is still expanding. In greenhouse experiment, we compared the suppressive effect of soil microbial communities from areas where the invasive forb Verbena brasiliensis has long resided with those from areas where it has arrived more recently. While the study found no correlation between residence time and invasive suppression, several of the soil microbial communities significantly reduced the invasive plant’s growth. Three of those inoculum soils were then tested in three “live” background soils from areas where V. brasiliensis was not yet present. The suppressive effects on the invasive plant faded for all but one of the inoculum soils. The results suggest that the accumulation or adaptation of soil pathogens to an invasive plant is not strictly tied to the plant’s residence time. In addition, the suppressive effects of some soil microbial communities do not always translate into new background soil contexts, likely because of unpredictable interactions with the biotic and abiotic properties of the destination soils.
