Effects of interspecific competition on plant-soil feedbacks generated by long-term grazing

Grazing by large herbivores leads to changes in soil properties which can in turn modify plant performance. However, little is known about how competition among plant species alters the strength and direction of grazing-induced plant-soil feedbacks (PSFs). In a previous monoculture experiment, we found that the intensity of sheep grazing generated consistent abiotic feedback effects and species-specific biotic feedback effects. To test if and how interspecific plant competition modifies the PSFs observed in our previous monoculture experiment, five naturally-occurring plant species (Artemisia capillaris, Dodartia orientalis, Lespedeza davurica, Oxytropis racemosa, and Stipa bungeana) were grown in mixed communities in sterilized and unsterilized soils from plots of four grazing intensities (0, 2.7, 5.3, and 8.7 sheep/ha). Further, the five plant species were grown in mixtures in treatment-specific soil following inoculation with arbuscular mycorrhizal fungi (AM) and/or a mixture of pathogenic Fusarium species (FU), to test the contribution of these two fungal groups to biotic PSFs. All plant species experienced net neutral to positive abiotic and biotic PSFs. Compared with our previous monoculture experiment, we found that the presence of interspecific competition did not change the abiotic PSF effects generated with increasing grazing intensity, but shifted the direction and strength of the biotic PSFs generated within each grazing intensity. The presence of pathogenic Fusarium-only significantly decreased the proportional biomass of the dominant species A. capillaris and S. bungeana, and the presence of AM fungi-only significantly increased the proportional biomass of the subordinate species D. orientalis and O. racemosa. Our study strongly suggests that in grazed ecosystems interspecific competition and biotic PSFs interact to drive plant community dynamics, and also that soil mutualists such as AM fungi have the potential to promote plant species coexistence by facilitating the performance of subordinate species.