Neighboring plants can alter one another’s belowground microbial associations, yet because most plant-microbial studies are conducted on isolated plants, little is known about the mechanisms and consequences of neighbor-induced changes in microbial communities. Root exudates, or suites of labile carbon compounds, play a central role in plants’ microbial recruitment, nutrient acquisition, and signaling with neighbors. Therefore, we hypothesized that neighbor-induced changes in root exudates may be one mechanism by which neighboring plants alter each other’s microbial communities. We used a greenhouse experiment and root exudate soil incubations to investigate how plant neighbor identity and competitiveness influences a focal plant’s rhizosphere bacterial community and root exudate profiles; if neighbor-induced changes in root exudates underlie shifts in the rhizosphere bacterial community structure and function; and if changes in rhizosphere communities affect future plant growth. We grew a focal plant, Switchgrass (Panicum virgatum), alone, with a conspecific neighbor, or with one of three native prairie species (Andropogon gerardii, Rudbeckia hirta, Koeleria macrantha), and evaluated how neighbor plants altered the focal plants’ root exudate profiles (untargeted metabolomics) and rhizosphere bacterial community structure (16S amplicon) and function (soil nitrogen (N), carbon (C) availability, N-fixation, soil feedback effect on seedling growth).
Results/Conclusions:
In the greenhouse experiment, neighbor identity explained 54% of the variation in focal plant root exudate composition and 20% of the variation in bacterial community composition. The focal plant root exudates and bacterial community shifted most when grown with a competitive neighbor (R.hirta). Under strong neighbor competition switchgrass exudates contained 13 times more malic acid than the other neighbor treatments. When we incubated soils with different exudates, the soils conditioned with more malic acid had a distinct bacterial community, greater dissolved organic carbon (C), total soil N, and microbial biomass N, but they had little effect on germination and seedling growth. Therefore, because competition induced greater malic acid exudation and malic acid additions altered bacterial community structure and increased soil C and N, we predict that plants may compete with neighbors by exuding malic acid to increase soil C and N mineralization, perhaps through recruiting distinct microbial communities. These results demonstrate that neighbor-specific changes in exudates is one mechanism by which neighbors influence one another’s microbiomes, and that these changes may be greatest with strong competition. Finally, these results suggest that neighbor-driven changes in a plant’s microbial interactions could exacerbate issues with scaling studies on isolated plants to