Soil microbial communities alter resource allocation in Fagus grandifolia when challenged with a pathogen

Soil microbes are influenced by their environment, and soil pH is well known as a driver of community structure, including within the plant root zone. However, the effect of pH induced changes on root-associated microbial communities for plant growth, resource allocation, and disease resistance is not well understood, especially for long-lived woody plants. In this study, we examined whether soil microbial communities altered by soil pH could affect tree growth, resource allocation, and resistance to a soil-borne pathogen. In a controlled greenhouse setting, we treated Fagus grandifolia saplings with small amounts of forest soil that had been manipulated to alter soil pH and microbial communities. In addition, 1-yr after inoculation with forest soil, half of the trees were also inoculated with the root rot pathogen Phytophthora cinnamomi to induce physiological stress. Tree growth showed no response to treatment with forest microbes; however, P. cinnamomi altered resource allocation, leading to increased ratios of aboveground to belowground biomass for trees treated with forest microbes. Interestingly, trees grown in pasteurized soil had a tendency toward the opposite pattern of reduced ratios of aboveground to belowground biomass. Soil treatment and pathogen inoculation interacted to alter water transport tissues; stems grown with microbes from acidic forest soil had higher vessel density when challenged with P. cinnamomi, while trees grown with microbes from neutral forest soil had higher vessel density in the absence of the pathogen. Our study suggests that the composition of root-associated microbes can affect resource allocation under stressful conditions for long-lived woody plants.