Bi-directional soil/atmosphere N₂ O exchange over two mown grassland systems with contrasting management practices

Nitrous oxide (N₂ O) fluxes from soil under mown grassland were monitored using static chambers over three growing seasons in intensively and extensively managed systems in Central Switzerland. Emissions were largest following the application of mineral (NH₄ NO₃ ) fertilizer, but there were also substantial emissions following cattle slurry application, after grass cuts and during the thawing of frozen soil. Continuous flux sampling, using automatic chambers, showed marked diurnal patterns in N₂ O fluxes during emission peaks, with highest values in the afternoon. Net uptake fluxes of N₂ O and subambient N₂ O concentrations in soil open pore space were frequently measured on both fields. Flux integration over 2.5 years yields a cumulated emission of +4.7 kgN₂ O-N ha^-1 for the intensively managed field, equivalent to an average emission factor of 1.1%, and a small net sink activity of -0.4 kg N₂ O-N ha^-1 for the unfertilized system. The data suggest the existence of a consumption mechanism for N₂ O in dry, areated soil conditions, which cannot be explained by conventional anaerobic denitrification. The effect of fertilization on greenhouse gas budgets of grassland at the ecosystem level is discussed.