Unveiling below-ground species abundance in a biodiversity experiment: a test of vertical niche differentiation among grassland species

P>1. Plant diversity has profound effects on primary production. Plant diversity has been shown to correlate with increased primary production in nutrient-limited grassland ecosystems. This overyielding has been attributed to vertical niche differentiation among species below-ground, allowing for complementarity in resource capture. However, a rigorous test of this longstanding hypothesis is lacking because roots of different species could not be distinguished in diverse communities. 2. Here, we present the first application of a DNA-based technique that quantifies species abundances in multispecies root samples. We were thus able to compare root distributions in monocultures of two grasses and two forbs with root distributions in four-species mixtures. In order to investigate if vertical niche differentiation is driven by soil nutrient depletion, the topsoil layer of the communities were either nutrient-rich or -poor. 3. Immediately in the first year, 40% more root biomass was produced in mixtures than expected from the monocultures, together with significant below-ground complementarity effects, probably preceding above-ground overyielding. This below-ground overyielding appeared not to be the result of vertical niche differentiation, as rooting depth of the community tended to decrease, rather than increase in mixtures compared to monocultures. Roots thus tended to clump in the very dense topsoil layer rather than segregate over the whole profile in mixtures. The below-ground overyielding was mainly driven by enhanced root investments of one species, Anthoxanthum odoratum, in the densely rooted topsoil layer without retarding the growth of the other species. 4. Synthesis. Conventional ecological mechanisms, such as competition for nutrients, do not seem to be able to explain the increased root investments of A. odoratum in mixtures compared to monocultures, with apparently little effect on the root growth of the other species. Instead, the observed root responses are consistent with species-specific root recognition responses. From a community perspective, the observed early below-ground overyielding may initiate the recently reported increased soil organic matter, mineralization and N availability and thus may ultimately be responsible for the higher productivity at high plant species diversity.