Plant species identity surpasses species richness as a key driver of N₂O 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 (N₂O), 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 N₂O 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) N₂O 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 N₂O 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 N₂O emissions for 42 days. We found no relation between plant species richness and N₂O emissions. However, N₂O emissions were significantly reduced in specific plant species combinations. In the absence of urine, plant communities of Fa+Php acted as a sink for N₂O, whereas the monocultures of these species constituted a N₂O source. With urine application Lp+Pt plant communities reduced (P < 0.001) N₂O emissions by 44% compared to monocultures of Lp. Reductions in N₂O 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 N₂O 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 N₂O emissions.