Negative biotic soil-effects enhance biodiversity by restricting potentially dominant plant species in grasslands

• We tested plant size and local abundance in relation to biotic soil-effects.
• Large but non-dominant plants experienced the most negative biotic soil-effects.
• Biotic-soil effects especially restrict potentially highly competitive grasses.
• Biotic soil effects are especially pronounced in less intensively managed grasslands.
• Negative biotic soil-effects enhance biodiversity in grassland.

Interactions between soil microorganisms and plants can play a vital role for plant fitness and therefore also for plant–community composition and biodiversity. However, little is known about how biotic plant–soil interactions influence the local dominance and abundance of plant species and whether specific taxonomic or functional groups of plants are differentially affected by such biotic soil-effects. In two greenhouse experiments, we tested the biotic soil-effects of 33 grassland species differing in individual size and local abundance. We hypothesized that large plants that are not locally dominant (despite their size-related competitive advantage enabling them to potentially outshade competitors) are most strongly limited by negative biotic soil-effects. We sampled soils at the opposite ends of a gradient in land-use intensity in temperate grasslands to account for putative modulating effects of land-use intensity on biotic soil-effects.As hypothesized, large, but non-dominant species (especially grasses) experienced more negative biotic soil-effects compared with small and abundant plant species. Land-use intensity had contrasting effects on grasses and herbs resulting in more negative biotic soil-effects for grasses in less intensively managed grasslands. We conclude that biotic soil-effects contribute to the control of potentially dominant plants and hence enable species coexistence and biodiversity especially in species-rich less intensively managed grasslands.