Plant Root Exudates and Rhizosphere Bacterial Communities Shift with Competitive Neighbor

Abstract

Plants simultaneously interact with each other and soil microbial communities, yet most studies on plant-microbial interactions are performed on isolated, individual plants. A plant’s neighborhood context can alter its belowground signaling. In particular, signals in the form of root exudates help plants interact with neighbors, abiotic conditions and soil microbes. Therefore, we predicted that root exudates would also play a central role in neighbor-induced shifts in rhizosphere communities. We conducted a greenhouse experiment to test this and determine how the rhizosphere bacterial community of a focal plant, Panicum virgatum, changed when beside different neighbor plants, and whether these shifts were mediated by neighbor-induced changes in root exudation. We found that neighbor did affect both focal plant exudates and rhizosphere community, and that changes were largest when the focal plant was beside the most competitive neighbor, Rudbeckia hirta, which reduced both focal plant growth and nitrogen uptake. Multiple mechanisms mediated neighbor’s impacts on rhizosphere assembly, including neighbor-induced changes in root exudates during nitrogen-limitation and microbial spillover from roots of larger neighbors. Using an additional soil incubation, we also found that neighbor-induced changes in exudates can have even greater effects on soil nutrient cycling than on microbial assembly. Overall, we show that plant neighbors alter rhizosphere processes through changes in root exudates, particularly during strong competition. This work demonstrates that mechanisms driving rhizosphere assembly may differ in mixed-species communities, thus emphasizing a need for microbiome studies that extend beyond isolated plants.