Both intraspecific competition for limiting resources and interactions with species-specific soil microbes are thought to be primary mechanisms causing negative frequency dependence, a process important for the maintenance of species diversity. Although many studies document the existence of negative frequency dependence, few studies explicitly separate out the relative importance of its causal drivers. Here, we couple greenhouse experimentation and community dynamics modeling to untangle the relative importance of competitive interactions from those of plant-microbial feedbacks to evaluate which process serves as a stronger force to species coexistence. We then link these findings to plant community response measured in a mesocom experiment containing plant assemblages differing in both phylogenetic relatedness and presence of soil-borne microbes.
In fully reciprocal greenhouse experiments, we found that the net effects of soil-borne microbes exhibited a high degree of species specificity. Plants grown with their own soil biota performed more poorly than do other plant species grown with those same biota. The resulting negative plant soil feedbacks disappeared when we eliminated soil biota. We also found that plant species pairs that were more closely related exhibit weaker negative feedbacks than did species pairs that were more distantly related. Further, when grown in competition, we found that plant-soil interactions led to stronger negative feedbacks compared to when we isolated only the effects of competitive interations. Finally, when we parameterized community dynamics models with empirical measures of growth responses, we found a significant loss of species diversity in models that only included competitive interactions. Stable coexistence of our six plant species was found in models that incorporated both competitive interactions and plant-microbial feedbacks.