Plant competition alters the temporal dynamics of plant-soil feedbacks

1. Most studies on plant-soil feedback (PSF) and plant competition measure the feedback response at one moment only. However, PSFs and competition may both change over time, and how PSF and competition interact over time is unclear. 2. We tested the temporal dynamics of PSF and interspecific competition for the forb Jacobaea vulgaris and the grass Holcus lanatus. We grew both species indigvidually and in interspecific competition in soil that was first conditioned in the greenhouse by J. vulgaris, by H. lanatus or without plant growth. For a period of 11 weeks, we harvested plants twice a week and analysed the fungal and chemical composition of the different soils at the end of the first and second growth phase. 3. During the second growth phase, when grown in isolation, both species produced more biomass in heterospecific conditioned soil than in conspecific conditioned soil. Young J. vulgaris exhibited a strong negative conspecific feedback, but this effect diminished over time and became neutral in older plants. In contrast, when grown in competition, the negative conspecific feedback of J. vulgaris exacerbated over time. Older H. lanatus plants benefited more from heterospecific conditioning when competing with J. vulgaris, then when grown isolated. 4. Fungal community composition and soil chemistry differed significantly between soils but this was mainly driven by differences between plant-conditioned and unconditioned soils. Remarkably, at the end of the second growth phase, fungal community composition was not explained by the legacy of the species that had been grown in the soil most recently, but still reflected the legacy of the first growth phase. We reexamined plant growth during a third growth phase. Biomass of J. vulgaris was still influenced by the treatments imposed during the first phase, while H. lanatus responded only to the plant growth treatments imposed during the second phase. 5. Synthesis. Our study shows that the direction and magnitude of PSF depends on plant age and competition, and also on soil legacy effects of earlier plant growth. These results highlight the need to incorporate dynamic PSFs in research on plant populations and communities.