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

Abstract

International audienceProduction 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 mu g N2O-N l(-1) (and from 0.2 to 2.9 mu g 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

Budget of N<sub>2</sub>O emissions at the watershed scale: role of land cover and topography (the Orgeval basin, France)

“…4). In order to determine the main mechanism responsible for the nitrous oxide concentrations in the groundwater, it is interesting to note that the ratio of N 2 O produced by nitrification of 0.28 % is close to the mean ratio found in the plateau piezometer (0.26 %; see Vilain et al, 2011). On the other hand, regarding the seasonal peaks observed either after fertilization or heavy autumn rainfalls, they can be much higher and closer to the 45 % ratio found by denitrification in laboratory.…”

“…This method, fully described in Vilain et al (2010), consisted in measuring the gas fluxes from series of five aluminum non-vented and hermetically closed chambers (open bases of 50 cm × 50 cm × 30 cm). Four gas samples were taken from each chamber headspace with a 30 mL Terumo syringe and transferred to a 12.5 mL pre-evacuated glass vial (Labco Exetainer) for transport to the laboratory.…”

“…To extend the analysis even further, we developed an index to differentiate topographical landscape positions on cropland, as this was shown to largely influence the N 2 O emissions (Pennock et al, 1992;van Kessel et al, 1993;Izaurralde et al, 2004;Vilain et al, 2010). The topographical index was first suggested as an indicator for surface runoff contributing areas by Kirkby (1975) and was the basis for the rainfallrunoff model called TOPMODEL (Beven and Kirkby, 1979).…”

“…Three piezometers were installed along a transect over an elevation gradient (mean slope, 2.2 %) from agricultural fields toward the stream including three slope positions (see Vilain et al, 2011) seamless polyester filter sock to prevent coarse sand particles from entering the well. Groundwater was sampled using an immerged pump from April 2008 to April 2010, with the piezometer emptied by flushing out water prior to collecting the sample in order to remove the standing water.…”

“…Its formation is influenced by several factors: climate (rainfall, temperature), soils (physical and chemical composition), substrate availability (nitrogen and carbon) as well as land management practices (Vilain et al, 2010;Skiba et al, 1998;Smith et al, 1998).…”