In this presentation, I will describe results from my research group on interactions between plants and microbes and how these interactions can influence plant species abundance within communities. The results focus on a number of studies conducted in Southern Ontario, Canada using local and foreign plants, as well as several local and foreign mycorrhizal and parasitic fungi. The data indicate that the strongest negative feedbacks are exhibited when native plants are coupled with native microbes, whereas the neutral and positive feedbacks are found when one or both of the partners are foreign. Overall, rare plants display the strongest negative feedback, whereas abundant plants typically show neutral feedback responses. Feedback associated with fungal parasites is neutral to negative, whereas in association with mycorrhizal fungi it is positive to negative, depending on the source of the plants and fungi. The combined effects of mycorrhizal and parasitic fungi are not additive, but rather negative effects seem to mask any positive effects. Overall, these results indicate that interactions with soil microbes can contribute to plant species abundance within communities. At the end of the presentation, I will also spend a few minutes developing a synthesis of the various presentations at this symposium.