Do belowground vertical niche differences between deep- and shallow-rooted species enhance resource uptake and drought resistance in grassland mixtures?

We investigated whether spatial differences in belowground niches between shallow- and deep-rooted species result in increased resource uptake and drought resistance in agricultural grassland communities. We injected the tracers N-15 and rubidium (Rb) into the soil and measured tracer uptake from 5 and 35 cm soil depth in monocultures and mixtures containing shallow-rooted (Lolium perenne, non-legume; Trifolium repens, legume) and deep-rooted (Cichorium intybus, non-legume; Trifolium pratense, legume) species, grown under drought or control conditions. Deep-rooted species had a higher proportional uptake of both tracers from 35 cm soil depth than shallow-rooted species (p < 0.001). Under drought conditions, all species enhanced their proportional tracer uptake from the deeper soil layer (p < 0.004) and species with a high proportional N-15 uptake from the deeper soil layer were more drought resistant. Total community uptake of Rb was significantly higher in mixed communities than monocultures (p < 0.007); however, this diversity effect was the result of mixing legumes with non-legumes and not the result of mixing deep- with shallow-rooted species. Clear evidence for spatial niche differences in resource uptake between shallow- and deep-rooted species did not translate into increased resource uptake in mixtures. Resource uptake from deeper soil layers was found to contribute to adaptation and resistance to drought stress.