Abstract: Soil microbes are key drivers of plant community diversity and dynamics and are thought to mediate plant competition. Invasive plants often cause significant shifts in soil microbial communities in their introduced range. These shifts can impact the fitness of the invader as well as plants in the recipient community and therefore could alter plant-plant competition. To better understand the role that soil microbes play in mediating competition following plant invasion, we grew an invasive forb (Centaurea solstitialis, yellow starthistle, YST) and a grass common in YST-invaded communities (Festuca perennis) alone and in competition in a greenhouse under four soil microbial community treatments. We used two soils that had previously been shown to increase YST growth (High1, High2) and two that had previously decreased YST growth (Low1, Low2) as well as an uninoculated control. Our goals were to 1) determine how YST-conditioned soil microbiomes affected the growth of the competitor species and 2) test whether soil microbes altered the outcome of intraspecific and interspecific competition. We hypothesized that YST responses to soil inocula when grown in competition would be consistent with YST responses when grown alone.
Overall, when grown individually, each species had lower total aboveground biomass when grown in inoculated soil compared to uninoculated controls, but responses to inoculum type varied by species. Contrary to previous experiments, YST did not perform better in either of the two “High” soils compared to “Low” soils when grown alone. However, when competing both intra- and interspecifically, “High1” soil significantly promoted YST growth and reproduction compared to the other live soil inoculation treatments. Under interspecific competition, the grass species performed significantly worse in “High1” soils compared to all other soil treatments. These results suggest that the competitive ability of invasive YST could be enhanced by some soil microbial communities, which is consistent with studies of other invasive plant species. Future work will characterize the composition of the microbial communities associated with each treatment to provide insight into potential mechanisms underlying microbial mediation of plant competition. Understanding the complex interactions between plants and soil microbial communities is critical for predicting plant community dynamics and managing the effects of invasive species.
