A long-standing question in ecology is how do seemingly functionally redundant species coexist? Theory has demonstrated that species diversity can be stabilized through negative conspecific density dependence. One mechanism promoting negative conspecific density dependence is negative plant-soil feedback, through which the accumulation of host-specific pathogens in a plant’s rhizosphere reduces the likelihood for conspecific success relative to heterospecific success. While useful, plant-soil feedback may fail to capture the contribution of other important “spheres” to plant performance. Here, using two dominant dune grasses, Schizachyrium scoparium and Calimofolia longiflora, we tested how the rhizosphere and phyllosphere independently and interactively influenced pairwise feedback. Soil and litter samples were collected at Sleeping Bear Dunes National Lakeshore. To test whether the addition of the phyllosphere influenced feedback, we established a greenhouse experiment where we measured the performance of each grass species with conspecific or heterospecific soil, litter, or their combination, plus sterile controls.
Results/Conclusions
In the live soil treatments, plant-soil feedback was positive and significantly different from zero (P= 0.009), suggesting that feedback will lead to eventual exclusion of one of the two species. Both S. scoparium and C. longiflora grew almost twice as large on conspecific soil relative to heterospecific soil. However, the addition of litter altered the direction of plant-soil feedback. In the live soil treatments with the addition of live litter, feedback was negative and significantly different from zero (P= 0.009), suggesting that microbial feedbacks promote coexistence between the two species. Sterile controls of both soil and litter differed significantly from non-sterile treatments, thus validating microbial mediation of biomass change.
Given the switch from coexistence-disrupting to coexistence-mediating feedbacks we observed in our experiment, we suggest that a better understanding of how biotic interaction influence the strength and direction of plant-soil feedback will help generate better predictions for how plant-microbe interactions will influence plant community dynamics.