Plant species identity surpasses species richness as a key driver of N 2 O emissions from grassland

Grassland ecosystems worldwide not only provide many important ecosystem services but they also function as a major source of the greenhouse gas nitrous oxide ( N 2 O ), especially in response to nitrogen deposition by grazing animals. To explore the role of plants as mediators of these emissions, we tested whether and how N 2 O emissions are dependent on grass species richness and/or specific grass species composition in the absence and presence of urine deposition. We hypothesized that: (i) N 2 O emissions relate negatively to plant productivity; (ii) four‐species mixtures have lower emissions than monocultures (as they are expected to be more productive); (iii) emissions are lowest in combinations of species with diverging root morphology and high root biomass; and (iv) the identity of the key species that reduce N 2 O emissions is dependent on urine deposition. We established monocultures and two‐ and four‐species mixtures of common grass species with diverging functional traits: Lolium perenne L. (Lp), Festuca arundinacea Schreb. (Fa), Phleum pratense L. (Php) and Poa trivialis L. (Pt), and quantified N 2 O emissions for 42 days. We found no relation between plant species richness and N 2 O emissions. However, N 2 O emissions were significantly reduced in specific plant species combinations. In the absence of urine, plant communities of Fa+Php acted as a sink for N 2 O , whereas the monocultures of these species constituted a N 2 O source. With urine application Lp+Pt plant communities reduced ( P  < 0.001) N 2 O emissions by 44% compared to monocultures of Lp. Reductions in N 2 O emissions by species mixtures could be explained by total biomass productivity and by complementarity in root morphology. This study shows that plant species composition is a key component underlying N 2 O emissions from grassland ecosystems. Selection of specific grass species combinations in the context of the expected nitrogen deposition regimes may therefore provide a key for mitigation of N 2 O emissions.