Indirect N2O emissions from shallow groundwater in an agricultural catchment (Seine Basin, France)

Production and accumulation of nitrous oxide (N2O), a major greenhouse gas, in shallow groundwater might contribute to indirect N2O emissions to the atmosphere (e.g., when groundwater flows into a stream or a river). The Intergovernmental Panel on Climate Change (IPCC) has attributed an emission factor (EF5g) for N2O, associated with nitrate leaching in groundwater and drainage ditches—0.0025 (corresponding to 0.25% of N leached which is emitted as N2O)—although this is the subject of considerable uncertainty. We investigated and quantified the transport and fate of nitrate (NO3−) and dissolved nitrous oxide from crop fields to groundwater and surface water over a 2-year period (monitoring from April 2008 to April 2010) in a transect from a plateau to the river with three piezometers. In groundwater, nitrate concentrations ranged from 1.0 to 22.7 mg NO3−–N l−1 (from 2.8 to 37.5 mg NO3−–N l−1 in the river) and dissolved N2O from 0.2 to 101.0 μg N2O–N l−1 (and from 0.2 to 2.9 μg N2O–N l−1 in the river). From these measurements, we estimated an emission factor of EF5g = 0.0026 (similar to the value currently used by the IPCC) and an annual indirect N2O flux from groundwater of 0.035 kg N2O–N ha−1 year−1, i.e., 1.8% of the previously measured direct N2O flux from agricultural soils.