Below-ground resource partitioning alone cannot explain the biodiversity–ecosystem function relationship: A field test using multiple tracers

Below-ground resource partitioningis among the most prominent hypotheses for driving the positive biodiversity-ecosystem function relationship. However, experimental tests of this hypothesis in biodiversity experiments are scarce, and the available evidence is not consistent. We tested the hypothesis that resource partitioningin space, in time or in both space and time combined drives the positive effect of diversity on both plant productivity and total community resource uptake. At the community level, we predicted that total community resource uptake and biomass production above- and below-ground will increase with increased species richnessor functional group richness. We predicted that, at the species level, resource partitionbreadth will become narrower, and that overlap between the resource partitionsof different species will become smaller with increasing species richnessor functional group richness. We applied multiple resource tracers (Li and Rb as potassium analogues, the water isotopologues-H2 18O and 2H2O, and 15N) in three seasons at two depths across a species and functional group richness gradient at a grassland biodiversity experiment. We used this multidimensional resource tracer study to test if plant species partitionresources with increasing plant diversity across space, time or both simultaneously. At the community level, total community resource uptake of nitrogen and potassium and above- and below-ground biomass increased significantly with increasing species richnessbut not with increasing functional group richness. However, we found no evidence that resource partitionbreadth or resource partitionoverlap decreased with increasing species richnessfor any resource in space, time or both space and time combined. Synthesis. These findings indicate that below-ground resource partitioningmay not drive the enhanced resource uptake or biomass production found here. Instead, other mechanisms such as facilitation, species-specific biotic feedback or above-ground resource partitioningare likely necessary for enhanced overall ecosystem function.