Historically, plant ecology has focused on competition for resources as the primary structuring element of a community, but recent research promotes the importance of understanding the interactions and subsequent feedbacks between plants, soil, and microorganisms. The manipulation of basic components of feedback systems in plant competition scenarios may provide information concerning the spread and management of several invasive knapweeds in the western US. Three field experiments independently test the effects of soil inoculation, activated carbon (AC) and invasive plant litter on the recruitment and community structure of areas seeded with native species or invasive knapweeds. Experiments with AC and invasive plant litter utilize a split-plot design to isolate potential chemical interference inside the knapweed population compared to the adjacent un-invaded area. In separate experiments, the two areas were seeded with native species or knapweed to determine effects of the treatments on recruitment and plant community structure. Inoculation of knapweed invaded areas with soil from adjacent native plant communities attempts to determine if feedbacks related to the inocula would promote the established invasive plants or the seeded native species. Field studies combining plant-soil feedbacks and competition are essential to address the complex interactions that influence invasive species and assist in the development of innovative management practices.
Results/Conclusions

As expected, the results of the activated carbon studies documented greater exotic species biomass inside of the knapweed populations than outside, but the amount of biomass inside the knapweed populations was significantly greater when treated with AC. In areas sown with Russian knapweed seeds, the vast majority of germination was outside of the knapweed populations, but neither AC or knapweed litter affected seed germination. Potential chemical interference on the germination of native species and knapweed seeds was not affected by the addition of AC, although the treatment may have species specific effects on plant growth. Spotted knapweed inoculated with native soil had decreases in the density of flowering stems and cover, but no effects on the native species. Conversely, a laboratory study with Russian knapweed found increased root biomass when inoculated with soil from an adjacent native plant community. Results of these experiments illustrate the complex and unpredictable responses of both native and invasive plants to treatments that may influence existing plant-soil feedbacks. The preliminary data highlights the difficulties in studying feedback systems and the potential of species-specific responses, even within a group of closely related invasive knapweeds.