Increasing plant species richness decreases soil NO₃^- concentrations in experimental plant mixtures, but the role of particular plant functional groups has remained unclear. Most analyses have focused on particular times of the year or were restricted to NO₃^-. We tested whether plant species richness or particular plant functional groups affect the size of plant-available N pools in soil (KCl-extractable NO ₃^-, dissolved inorganic N and organic N [DON] and total dissolved N [TDN] in soil solution) and N concentrations and pools in aboveground biomass. Furthermore, we assessed seasonal variations in the effects of plant species richness and plant functional groups. The experimental grassland site had 86 plots with different combinations of numbers of species (1, 2, 4, 8, 16, and 60) and numbers of functional groups (1, 2, 3, and 4, being grasses, small nonlegume herbs, tall nonlegume herbs, and legumes). In the second year after establishment, increasing species richness reduced soil NO₃^- concentrations (ANOVA, 11% of sum of squares [SS]). The presence of legumes correlated positively with soil NO₃ ^- concentrations (17% of SS). The presence of grasses significantly decreased soil NO₃^- concentrations (11% of SS). Seasonality had no influence on the relationships between NO₃ ^- concentrations and species richness. Volume-weighted mean DON and TDN concentrations in soil solution correlated negatively with species richness. Nitrogen pools in plant mixture biomass correlated positively with species diversity (14% of SS), indicating that total N uptake increased with increasing diversity. We conclude mat both diversity (either in species or functional groups) and functional composition of grassland mixtures are significant controls of soil and plant N pools. Plant communities with more diverse mixtures are liable to use limiting resources such as N more effectively.