Plant species coexistence and the persistence of high levels of diversity within plant communities has long posed a challenge to ecologists. Stabilizing negative intraspecific interactions relative to interspecific interactions has traditionally been thought to result from high resource use overlap within species; however, the primacy of resource niche partitioning in explaining coexistence of plant species has been questioned. In the past two decades, it has become well established that the plant microbiome, particularly microbes in soil, play an important role in shaping plant community composition and dynamics. The theoretical framework of plant-soil community feedback offers a useful model for viewing and testing the role of the microbiome in plant community dynamics, as it connects microbial dynamics to the outcome of plant-plant interactions. As tests of plant-soil feedback have become increasingly commonplace, the connection of these tests to the theory has become tenuous in some cases. I will review and extend the plant-soil feedback theory and the conditions for microbiome mediation of plant-plant dynamics. And I will then demonstrate the value of this framework in analyzing the outcome of plant-plant interactions.
Results/Conclusions
Microbial feedback involves two steps: first, the composition of the microbial community changes in response to plant species identity. This change in soil community composition could be a result of alterations in density or composition of the pathogens, soil mutualists, or other components of the soil community and are expected from microbial trade-offs in overcoming plant defenses or variation in symbiont compatibility. Secondly, the change in composition of the soil community alters the relative fitness of individual plant species through both direct feedbacks on fitness of their host species and indirect feedbacks feedbacks on fitness of competing hosts. Long-term coexistence requires that effects on heterospecifics are more positive than effects on conspecifics as measured in a full factorial test experiment.
We present a test of the relative importance of plant-soil feedbacks and interspecific competition using a pair of co-occurring grass species. We had previously demonstrated negative feedback between these species and this negative feedback is due to both host-specific pathogens and host-specific changes in rhizosphere composition. CWe show that this negative feedback is sufficient and essential to the long-term coexistence of these species. These results are consistent with soil microbial dynamics, and the dynamics of soil pathogens in particular, being more important than resource competition for plant species coexistence.