Interactions between plants and soil microbes can strongly influence plant diversity and community dynamics. Soil microbes may promote plant diversity by driving negative frequency-dependence in plant populations, or may cause species exclusion by providing one species an average fitness advantage over others. Past empirical research motivated by an influential plant-soil feedback model has found that plants often grow less vigorously in soil harboring a microbial community cultivated by conspecific individuals than by heterospecific individuals, indicating that these feedbacks can stabilize coexistence. However, existing research has generally neglected that these same interactions can also generate average fitness differences among plants, which can drive species exclusion. Here we use theory to develop metrics that quantify microbially mediated plant fitness differences, and ask how accounting for these effects can change our understanding of how microbes influence plant diversity. We also explore whether microbial effects on plant-plant interactions shift along fertility gradients with a framework that integrates both plant resource uptake and plant-microbial interactions.
Results/Conclusions
We show that in the classic plant-soil feedback model, soil microbes can generate fitness differences that favor plant species exclusion when they disproportionately harm (or favor) one plant species over another. By exploring a range of scenarios with the model, we also show that these fitness differences may also favor coexistence if they equalize competitive fitness differences, or if they generate intransitive dominance hierarchies among plants. We find that when microbes do not directly mediate plant resource uptake, microbial control over coexistence increases with increasing fertility. Finally, we show how the metrics we develop here can quantify microbially mediated fitness differences in empirical settings by conducting a two-phase plant-soil feedback experiment among 15 species pairs of California annual plants. In all, our analysis provides a more complete foundation for understanding the coexistence consequences of plant-microbe interactions, and suggests avenues for future research to understand how these ubiquitous interactions shape plant diversity.