Density-dependent plant-soil feedbacks of two plant species affected by plant competition

Both plant-soil feedbacks (PSF) and plant competition drive plant community assembly, but their interactive effects have rarely been investigated, and the role of community composition in modulating these interactions is unknown. We conducted a fully reciprocal experiment with two plant species (Polygonum criopolitanum and Carex thunbergii) and their associated soil biota communities to untangle the relative importance of PSF and competition. The two species were grown either in monoculture or in mixed plantings, with various densities of each plant's neighbor, and either in the presence or absence of the soil biota associated with either species. When grown individually, each plant species showed a neutral PSF in the presence of its own soil biota, while feedbacks in competition were predominantly negative. P. criopolitanum produced more biomass in the presence of soil biota than in autoclaved soil, but the opposite was observed for C. thunbergii. In competition, both plant species were suppressed by its presence of their own soil biota, and neither had a competitive advantage, resulting in a significant negative pairwise PSF in most cases. C. thunbergii also showed stronger per capita effects than did P. criopolitanum. The predication on species coexistence largely depended on the presence of soil biota, the soil origin, and the plant species ratio. When planted in equal proportion, P. criopolitanum and C. thunbergii only coexisted in the presence of soil biota of P. criopolitanum, while C. thunbergii usually excluded P. criopolitanum under all other conditions. When the density of the neighboring plant increased, either species was excluded in the presence of their own soil biota. However, in the absence of a soil biota, there was a priority effect, giving an advantage to the species that was planted first. Our study revealed that the strength and direction of these interactions were altered by interspecific competition at the community level, and both the identity and the density of the competitors must be taken into account in interpreting plant community assembly processes.