Plant-soil feedbacks contribute to stabilized coexistence in a nutrient limited mountain meadow

Background/Question/Methods Negative plant-soil feedback occurs when plant species grow less in soil cultivated by conspecifics than in soil cultivated by their heterospecific competitors, and is one possible mechanism that enhances species coexistence. However, its contribution relative to other plant interactions in stabilizing species coexistence is poorly understood. Here, we investigate the degree to which plant-soil feedback explains conspecific and heterospecific interactions in a phosphorus limited mountain grassland on an acidic soil. In this system, we set up an experiment in which each of seven focal species grew alone and with conspecific or heterospecific competition at different competitor densities. After one year of plant growth, we fit low density growth rates for each of the seven species and interaction coefficients for all pairs. In a coupled greenhouse experiment, we grew the same species in sterilized field soil inoculated with soil from field monocultures conditioned by each of the seven focal species, and in soil without inoculum.

Results/Conclusions We found six of the seven species experienced primarily negative interactions with neighbors, while one species showed primarily positive effects of neighbors. Of those showing competitive effects of neighbors, only one, Festuca heterophylla, showed much stronger effects of conspecific neighbors than of heterospecific neighbors. This species was also one of two that tended to show lower shoot biomass when growing in soil cultivated by conspecifics than when growing in soil cultivated by heterospecifics. The other species showing a similar negative feedback, Carex pilulifera, was generally facilitated by neighbors in the field experiment, but less so by conspecific neighbors. The remaining species grew no better on conspecific than heterospecific soils, consistent with the relative strengths of their interaction coefficients in the field. Our results show that plant-soil feedback can play a very important role in stabilizing coexistence in this system.