Independent variations of plant and soil mixtures reveal soil feedback effects on plant community overyielding

1. Recent studies have shown that the positive relationship between plant diversity and plant biomass ('overyielding') can be explained by soil pathogens depressing productivity more in low than in high diverse plant communities. However, tests of such soil effects in field studies were constrained by experimental limitations to manipulate soil community composition independent of plant community composition. Here, we report of an experiment where feedback effects to plants were tested for both plant and soil monocultures and mixtures. 2. Our results demonstrate that overyielding is the result of plant species in mixture being more growth-limited by 'own' soil biota than by soil biota of other plant species. This effect disappeared when the soils had been sterilized by gamma-irradiation. Mixing plants themselves did not result in overyielding except when grown in the soil of one of the species (Leucanthemum vulgare), where growth of one species disproportionally increased in mixture compared to monoculture. 3. Soil nutrient availability could not explain differences in growth between the non-sterilized soils. Therefore, our results suggest that plant species-specific soil biota rather than the plants have contributed to the plant community overyielding. 4. Species biomass ranking in mixtures highly differed between non-sterilized soils of different histories of soil conditioning, whilst the ranking was more consistent in sterilized soil. Sterilized soils of different origin differed significantly in nutrient availability. These results suggest that shifts in competitive hierarchies depend on plant species-specific interactions influenced by soil biota and cannot be induced by mineral nitrogen. 5. Synthesis. Our results show that overyielding in four plant species mixtures can be due to species-specific interactions between plants and their specific soil biota. Neither mixing the plant species alone nor the differential responses of species to mineral nitrogen influenced community productivity, but mixing soil biota did.