Despite the known importance of soil microbes and their influence on soil processes and plant health, studies on plant-soil microbe interactions at the plant species level are difficult to generalize to additional species or community types. A mechanistic understanding of plant-soil interactions is therefore needed to determine how plants and soil microbes interact at scales that are relevant to plant community and ecosystem-scale processes. One useful method for identifying ecological similarity across plant species involves a comparative approach using plant traits and/or phylogenetic relatedness. We conducted a greenhouse experiment with seven congeneric grasses (genus Poa) and their native soils to examine how plants influence microbial community composition and whether phylogenetic signal plays a role in plants’ control on microbial community composition in the rhizosphere. Seeds of each species were planted in native soil as well as a homogenized soil from all seven populations. We also applied a nitrogen (N) treatment to determine the relationship between plant and microbial responses to environmental change. Rhizosphere microbial community composition for each plant species (using next generation 16S and ITS gene sequencing for bacteria and fungi) was compared to phylogenetic distances among plant species as well as plant traits and responsiveness to nitrogen addition.
We found that microbial community composition was not more similar on the roots of closely related species compared to more distantly related species within the genus Poa. While plants affected soil microbial community composition similarly among plant species, the resulting soil communities in the presence of plants were significantly different from microbial communities in soils without plants present. There was no consistent relationship between plant growth responses and microbial community responses to added N across plant species, but plant growth was correlated with the magnitude of change in microbial community composition due to N for individual plant species. This work suggests that deeper radiations should be explored (family level or other genera) to better understand the degree to which it is appropriate to generalize about specificity in plant-soil microbe interactions.