Interactions between plants and soil microbes play an important role in structuring terrestrial ecosystems by influencing plant growth and competitive ability. Changes in plant-microbe interactions alter interactions between plant species, which could affect plant community composition. 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 climate change, including changing water availability, could alter the outcome of plant-microbe interactions, which would in turn affect interactions among plant species. Furthermore, if non-native species benefit from alterations in plant-microbe interactions, climate change could exacerbate the existing threat that non-native species pose to biodiversity. 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. To test for an interaction between water availability and soil microbes, plants were grown in either live or sterile soil and were provided ambient, reduced, or increased amounts of water. Within these treatments, we tested the outcome of pairwise interactions between native and non-native species on three target species – native grass Schizachyrium scoparium, native forb Rudbeckia hirta, and non-native forb Plantago lanceolata – grown in the presence of either a conspecific or a heterospecific. At the end of the experiment, aboveground biomass was collected, dried, and weighed, and root samples were taken to determine AMF colonization.
Results/Conclusions
The collected biomass was analyzed with a nested ANOVA to correct for the interdependency of the two plants sharing a pot. Overall results show that there was a significant interaction effect on biomass between the three treatments (soil type, watering, and pairwise interaction, p=0.03), and that the pairwise interaction was also a major determinant of biomass (p<0.0001). The full model was broken down by target species to determine how the treatment effects differed between the three species. Across all three species, the competitor was a significant determinant of biomass (p<0.001 for all three species). Schizachryium scoparium, a dominant native grass in the Texas coastal prairie, performed significantly better in sterile soil than in live soil (p=0.02). For the native forb, R. hirta, watering treatment (p=0.003) and competitor (p<0.001) were the only significant factors affecting biomass, but the three-way interaction between treatments was almost significant (p=0.07). The three way interaction between treatments had a significant effect on P. lanceolata biomass (p=0.001).