Beneficial effects of multi-species mixtures on NO emissions from intensively managed grassland swards.

In a field experiment, annual nitrous oxide (NO) emissions and grassland yield were measured across different plant communities, comprising systematically varying combinations of monocultures and mixtures of three functional groups (FG): grasses (Lolium perenne, Phleum pratense), legumes (Trifolium pratense, Trifolium repens) and herbs (Cichorium intybus, Plantago lanceolata). Plots received 150 kg ha year nitrogen (N) (150 N), except L. perenne monocultures which received two N levels: 150 N and 300 N. The effect of plant diversity on NO emissions was derived from linear combinations of species performances' in monoculture (species identity) and not from strong interactions between species in mixtures. Increasing from 150 N to 300 N in L. perenne resulted in a highly significant increase in cumulative NO emissions from 1.39 to 3.18 kg NO-N ha year. Higher NO emissions were also associated with the legume FG. Emissions intensities (yield-scaled NO emissions) from multi-species mixture communities around the equi-proportional mixture were lowered due to interactions among species. For NO emissions scaled by nitrogen yield in forage, the 6-species mixture was significantly lower than L. perenne at both 300 N and 150 N. In comparison to 300 N L. perenne, the same N yield or DM yield could have been produced with the equi-proportional 6-species mixture (150 N) while reducing NO losses by 63% and 58% respectively. Compared to 150 N L. perenne, the same N yield or DM yield could have been produced with the 6-species mixture while reducing NO losses by 41% and 24% respectively. Overall, this study found that multi-species grasslands can potentially reduce both NO emissions and emissions intensities, contributing to the sustainability of grassland production.