ABSTRACTPrevious research has shown that plant diversity influences N and P cycles. However, the effect of plant diversity on complete ecosystem N and P budgets has not yet been assessed. For 20 plots of artificially established grassland mixtures differing in plant diversity, we determined N and P inputs by bulk and dry deposition and N and P losses by mowing (and subsequent removal of the biomass) and leaching from April 2003 to March 2004. Total deposition of N and P was 2.3 ± 0.1 and 0.2 ± 0.01 g m−2 yr−1, respectively. Mowing was the main N and P loss. The net N and P budgets were negative (–6.3 ± 1.1 g N and –1.9 ± 0.2 g P m−2 yr−1). For N, this included a conservative estimate of atmospheric N2 fixation. Nitrogen losses as N2O were expected to be small at our study site (<0.05 g m−2 yr−1). Legumes increased the removal of N with the harvest and decreased leaching of NH4–N and dissolved organic nitrogen (DON) from the canopy. Reduced roughness of grass‐containing mixtures decreased dry deposition of N and P. Total dissolved P and NO3–N leaching from the canopy increased in the presence of grasses attributable to the decreased N and P demand of grass‐containing mixtures. Species richness did not have an effect on any of the studied fluxes. Our results demonstrate that the N and P fluxes in managed grassland are modified by the presence or absence of particular functional plant groups and are mainly driven by the management.