The difference in the ability of plants to obtain resources has been used to predict the competition outcomes. Competitive interactions between plants can be influenced by trophic interactions. If mycorrhizae increase the growth of inferior competitors, then it can prevent competitive exclusion. I examined the effect of mycorrhizae on competitive interactions and found that inferior competitors had slow growth in the absence of mycorrhizae, but greater growth with mycorrhizae. By providing greater growth responses to inferior versus superior competitors, mycorrhizae promoted coexistence. In a separate experiment, I looked at the effects of whole soil biota on competitive ability. Coexistence can occur if dominant competitors experience negative feedback and or if inferior competitors experience positive feedback. I examined Plantago lanceolata L. specific feedback effects on competitive ability of 21 co-occurring species. I found that feedback effects did not affect hierarchies, and that positive feedback did not improve species competitive ability. / NSERC (Natural Science and Engineering Research Council), University of Guelph, Queen Elizabeth II Graduate Scholarship for Science and Technology