This dissertation addresses the effects of belowground interactions on plant community assembly and the evolution of chemical defenses. In the first three chapters, tall goldenrod (Solidago altissima) is used as a model system to study how microbial interactions influence competitive outcomes and the evolution of chemical defenses in goldenrod roots. In chapter one, I present the results of a greenhouse competition experiment designed to test the relative importance and interactive effects of herbivory and successional shifts in soil microbial communities on the growth and competitive ability of Solidago altissima. The magnitude of plant–soil feedbacks was strongly influenced by both herbivory and competitor species. The strong effects of antagonistic microbial interactions on goldenrod performance and competitive outcomes suggest that goldenrod plants should be under selection to resist these negative plant–soil feedbacks. In chapters two and three, I explore the role of polyacetylenes (a class of fatty acid–derived secondary metabolites produced in the roots of goldenrod) in the mitigation of these antagonistic microbial interactions. In chapter two, I investigated how selection on polyacetylenes is influenced by successional shifts in soil microbial communities. The benefits of polyacetylene production were highly dependent on microbial context, with the strongest benefits occurring in the late successional microbial background, implying that selection on polyacetylenes is driven primarily by microbial interactions rather than allelopathic suppression of competitors. In chapter three, I address potential constraints on the evolution of polyacetylenes associated with the inhibition of the model arbuscular mycorrhizal fungus, Rhizophagus irregularis. The results suggest that ecological costs associated with the suppression of mycorrhizae may constrain investment in chemical defenses. In chapter four, I address the role of belowground plant-plant interactions in mediating associational effects in an agricultural intercropping system. Specifically, I tested the effects of Desmodium intercropping on maize secondary metabolism and herbivore resistance and compared the effects of above- and belowground neighbor cues. The results suggest that soil-borne cues play an important role in mediating the effects of neighboring plants on resistance to insect herbivory in this system. Overall, my dissertation provides insights into the role of rhizosphere interactions in plant community assembly and the evolution of plant secondary metabolism.