Interspecific competition of early successional plant species in ex-arable fields as influenced by plant–soil feedback

Plant–soil feedback can affect plants that belong to the same (intraspecific feedback) or different species (interspecific feedback). However, little is known about how intra- and interspecific plant–soil feedbacks influence interspecific plant competition. Here, we used plants and soil from early-stage ex-arable fields to examine how intra- and interspecific plant–soil feedbacks affect the performance of 10 conditioning species and the focal species, Jacobaea vulgaris. Plants were grown alone and in competition in both conditioned and control soils. Overall, plant–soil feedback of the 10 plant species influenced the competitiveness of J. vulgaris more strongly than their own competitiveness. However, effects depended on species combination: competitiveness of J. vulgaris was significantly enhanced by interspecific plant–soil feedback from Anthoxanthum odoratum, Agrostis capillaris, and Trifolium dubium, and significantly decreased by interspecific feedback from Achillea millefolium. Intraspecific feedback from Taraxacum officinale and A. odoratum decreased their competitiveness with J. vulgaris. There was a positive relationship between the strength of interspecific feedback and competitiveness of J. vulgaris in conditioned soil. Multiple linear regression showed that the competitiveness of J. vulgaris in conditioned soil was determined by interspecific feedback and competitiveness of neighbour plants. The positive relationship between interspecific feedback and competitiveness in control soil suggests that the soil feedback effect of the competing species on J. vulgaris can build up quickly during competition. We conclude that the effect of plant–soil feedback on interspecific competition may be due to either legacy effects of plant species previously colonizing the soil, or immediate interspecific feedback of the competing plant species via the soil. Therefore, our results suggest that plant–soil feedback can influence interspecific plant competition through a multitude of intra- and interspecific plant–soil interactions both from predecessors, and from the currently competing plant species