Interactions between plants and soil microbes may alter the relative importance of intraspecific and interspecific plant competition in a changing climate

Abstract

Interactions between plants and soil microbes play an important role in structuring terrestrial ecosystems by influencing plant growth and competitive ability. Studies have shown that abiotic conditions such as varying nutrient levels or environmental stress can alter the direction and magnitude of plant-microbe interactions. Given this context-dependency, it is possible that the effects of changing climates, including changing water availability, could alter the outcome of plant-microbe interactions, which could in turn affect interactions among plant species. We tested whether water availability mediated the effect of soil microbes on pairwise plant interactions in the Texas coastal prairie using a controlled greenhouse experiment with three plant species: Schizachyrium scoparium, Rudbeckia hirta and Plantago lanceolata. To test for an interaction between water availability and soil microbes, plants were grown in either live or sterile soil treatments and with high, medium and low water availability. We found that the presence of soil microbes generally increased the strength of intraspecific competition relative to interspecific competition, but this effect depended on water availability. In the presence of microbes, as water availability decreased the strength of intraspecific competition generally increased. Our results suggest that soil microbes may play a role in stabilizing coexistence by increasing conspecific negative density dependence, especially in drier environments.