Aims: This study aimed to measure the effect of plant diversity on N uptake in grasslands and to assess the mechanisms contributing to diversity effects. Methods: Annual N uptake into above- and belowground organs and soil nitrate pools were measured in the Jena experiment on a floodplain soil with mixtures of 2-16 species and 1-4 functional groups, and monocultures. In mixtures, the deviation of measured data from data expected from monoculture performance was calculated to assess the contribution of complementarity/facilitation and selection. Results: N uptake varied from <1 to 45 g N m^-2 yr^-1, and was higher in grasslands with than without legumes. On average, N uptake was higher in mixtures (21 ± 1 g N m^-2 yr^-1) than monocultures (13 ± 1 g N m^-2 yr^-1), and increased with species richness in mixtures. However, compared to N uptake expected from biomass proportions of species in mixtures, N uptake of mixtures was only slightly higher and a significant surplus N uptake was confined to mixtures containing legumes and non-legumes. Conclusions: In our study, high N uptake of species rich mixtures was mainly due to dominance of productive species and facilitation by legumes whereas complementarity among non-legumes was of minor relevance.