Indirect Interactions Driven by Soil Effects Enable Coexistence Among Competing Plant Species

Reciprocal effects between plants and soil have been proposed as mechanisms that promote coexistence. However, recent theoretical and empirical works have questioned their role in stabilizing coexistence within multispecies communities. In these systems, soil-mediated indirect interactions may play a pivotal but often overlooked role. We investigate these indirect interactions using an experimental system of two competing shrub species grown in soil conditioned by a mediating third tree species. Tree-induced shifts in soil microbial communities, metabolites and nutrients boost growth in the weak competitor species while reducing germination in the dominant competitor. Simulations demonstrate that these shifts in plant performance are sufficient to stabilize coexistence, producing simulated spatial distributions consistent with those observed in natural communities. Our findings underscore the role of soil effects in driving indirect interactions that sustain coexistence in diverse plant communities, highlighting the importance of indirect but also positive and negative multitrophic interactions in maintaining biodiversity.