Plant-soil feedbacks are being increasingly recognized as important for plant coexistence and community structure across a range of systems. However, theoretical predictions are relatively lacking as to how plant-soil feedbacks will interact with other important mechanisms that mediate plant community dynamics and diversity. Exogenously-generated soil heterogeneity, niche relationships among competing plants, and differences between plants in their feedback traits all have the potential to interactively affect the impacts that plant-soil feedbacks have on community structure. We use spatially explicit simulation-based models to explore how species-specific plant-soil feedbacks affect the relationship between soil heterogeneity and plant diversity when species coexist via niche partitioning. We compare cases in which a single focal species: 1) exhibits either positive, negative, or no plant-soil feedback, 2) competes best at either low or high soil heterogeneity, and 3) has peak fitness in the absence of feedbacks that is either higher than, lower than, or equal to that of its competitors.
Strongly positive feedbacks increase the potential for competitive dominance by the focal species. This can change the relationship between heterogeneity and diversity from a smooth increase to either hump-shaped (if the focal species performs better under high heterogeneity) or initially flat with an accelerating increase (if homogeneity favors the focal species). Strongly negative feedbacks reduce the potential for focal species dominance. This tended to lead to higher diversity than equivalent positive-feedback treatments when the distribution of soil types favored the focal species, and lower diversity when it instead favored other species. This often translated to flatter relationships between heterogeneity and diversity.