Hot spots of N2O emission move with the seasonally mobile oxic-anoxic interface in drained organic soils

•Extremely high N2O emission from drained organic soil, 73 kg N/ha/year.•Emission peaks in an interface, i.e. narrow band on a toposequence.•The interface with length about 20 m moves up- and downhill with season.•Position of 15N in the N2O molecule tell that denitrification produced the N2O.

N2O emission from soil is regulated by the more or less sporadic occurrence of anoxic voids within the otherwise oxic soil matrix. Differences in size and density of these voids makes the process highly variable at the microscale. In a search for situations where variation in N2O emission is at the mesoscale rather than at the microscale we selected a drained and nitrogen rich organic soil incline. N2O emission rate gradients were observed at the mesoscale (10 m range). Massive N2O emissions occurred at an interface that may be defined by upslope oxic conditions inducing mineralizing organic nitrogen nitrate production and downslope anoxic conditions inducing the conversion of nitrate into N2O. This N2O-producing interface probably contained many small anoxic volumes distributed in the aerated soil matrix. The interface moved uphill in the wet season and downhill in the dry season so the mesoscale pattern also had a clear temporal component. When scaled on a hectare basis, the site with such a hot spot emitted 73 kg N annually as N2O, indicating the magnitude of this activity.