Plant-soil feedbacks (PSF), particularly those occurring between plants and soil microorganisms, are receiving increasing attention as a process that interacts with aboveground competition to affect plant species coexistence. To date, theoretical studies on PSF have been developed independently of the theoretical framework known as modern coexistence theory, which has dominated the literature on species coexistence over the past two decades. This framework proposes that coexistence is mediated by two types of mechanisms: equalizing forces, which reduce fitness differences, and stabilizing forces, which increase niche differences between species. Applying this framework to PSF studies may help to clarify how PSF modifies the effects of aboveground competition on species coexistence. To our knowledge, however, no such attempt has been made. To this end, we built a general model of two plant species interacting via both aboveground competition and PSF. We varied the strengths of intra- and inter-specific aboveground competition to quantify the stabilizing and equalizing forces that were driven by aboveground competition under different PSF scenarios. We use this analysis to study how PSF may modify the effects of aboveground competition on plant species coexistence.
First, under the scenario in which feedback strengths were identical for all conspecific and heterospecific PSF, we found results consistent with predictions from classic models. For example, strong intra-specific competition of the competitively superior species facilitated stable coexistence via both stabilizing and equalizing forces. Second, under the scenario in which both plant species experience stronger negative conspecific PSF than heterospecific PSF, the stabilizing and destabilizing forces driven by all aboveground competition coefficients were weaker compared to the first scenario. Finally, when only one species is affected by negative conspecific and heterospecific PSF, only the stabilizing and destabilizing forces driven by the impacts of aboveground competition on that species increased. These results clarify how aboveground competition and PSF may interact to affect species coexistence. For example, if during invasion both resident and invader are affected by host-specific pathogens (as assumed by the Janzen-Connell hypothesis), aboveground competition may not play a major role as a stabilizing force. In contrast, if the invader is less affected by pathogens (as assumed by the enemy release hypothesis), stabilizing forces arising from aboveground competition impacting the invader may play an important role. Efforts to test these predictions with empirical data from greenhouse experiments are currently underway.