Minimizing nitrogen (N) losses and increasing plant N uptake in agroecosystems is a major global challenge. Ecological concepts from (semi)natural grasslands suggest that manipulating plant community composition using plant species with different traits may represent a promising opportunity to face this challenge. Here, we translate these trait-based concepts to agricultural systems in a field experiment, aiming to reveal the main determinants of how plant community composition regulates N-cycling in intensively managed grasslands. We focused on key N pools (plant N from soil and from biological N-fixation, soil mineral N and N2O emissions) as well as on biological drivers of N-cycling in soil (abundance of N-cycling microbial communities, earthworm populations and arbuscular mycorrhizal fungi), using three common grass and one legume species in monoculture, two- and four-species mixtures. We hypothesized that: (a) plant species mixtures increase plant N uptake, reduce soil mineral N concentrations and N2O emissions and promote the abundance of biological N-cyclers; (b) legume presence stimulates N pools, fluxes and biological N-cycling activity, (c) but in combination with a grass with acquisitive traits, more N is retained in the plant community, while N2O emissions are reduced. We found that mixtures increased plant N and lowered the soil mineral N pool compared to monocultures. However, plant species identity played an overarching role: Legume presence increased N2O emissions, plant N pools, soil mineral N and the abundance of N-cycling microbes and earthworms. Combining the legume with a grass with low leaf dry matter content and high root length density (and with high root biomass) reduced the higher soil mineral N and N2O emissions induced by the legume, while harnessing positive effects on plant N pools and biological N-fixation. Synthesis and applications. Our results show the potential of plant community composition to steer N-cycling in fertilized agroecosystems, paving the way for a more biologically based agriculture. Legumes will play a crucial role, but selecting an optimum companion species is key for the sustainability of the agroecosystem.