A growing body of research indicates that interactions between plants and soil microorganisms can have a strong influence on plant community dynamics. By altering the taxonomic composition of the soil microbiota with which they must interact, plants can affect their own fitness and that of their competitors in a process known as "plant-soil feedback." Theoretical and empirical work demonstrates that plant-soil feedback can affect the local abundance of plants, drive plant community succession, and influence the outcome of plant competitive exclusion vs. coexistence, and this dynamic may play an important role in plant invasion. The mechanisms behind plant-soil feedback in invaded systems can range from direct interaction with pathogens, parasites, mutualists and commensals in the soil to alterations of nutrient cycling rates through feedback to microbial decomposer communities. These mechanisms all involve different scales of response (in space and time). How can the implicit consideration of scale better inform our understanding of plant-soil feedback? Here I utilize DNA-based, whole-community fingerprinting of soil bacterial and fungal communities to uncover associations between microbial communities and invasive plants at a variety of spatial scales.
A broad study of an Asian legume across its invaded North American range revealed consistent and statistically significant shifts in soil bacterial and fungal communities in relation to the relative population density the invasive plant at each site. However, the invader-associated component of microbial beta diversity was a minor part of a broader, complex response of microbial community composition to many different dimensions of environmental variability related to (e.g.) geography, climate, soil texture and chemistry, and ecosystem type. At the scale of 100-1000 m2, bacterial and fungal communities from heavily invaded sites were significantly different from those of uninvaded sites, but this response was restricted to a handful (~20) microbial taxa. At the localized scale of individual plant root balls, only fungal communities showed significant patterns of turnover related to plant species. Thus, certain fungal taxa may be interacting directly with native and invasive plants on fine spatial scales, while other fungi and soil bacteria may be responding to indirect changes occurring at larger scales. Highlighting different scales of microbial response to invasive plants can help refine hypotheses, predictions, and management strategies related to microbial involvement in plant invasions and ecological restoration.