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TECHNICAL REPORTS

Estimation of Indirect Nitrous Oxide Emissions from a Shallow Aquifer in Northern Germany

^a Institute of Soil Science, Univ. of Hannover, Herrenhäuser Str. 2, 30419 Hannover, Germany
^b Plant & Food Research Palmerston North Private Bag 11030, Manawatu Mail Centre, Palmerston North, 4442, New Zealand
^c Federal Institute for Geosciences and Natural Resources, Stilleweg 2, 30655 Hannover, Germany
^d Institute of Soil Science and Forest Nutrition, Univ. of Göttingen, Büsgenweg 2, 37077 Göttingen, Germany
^e Johann Heinrich von Thünen-Institut, Federal Research Institute for Rural Areas, Forestry and Fisheries, Institute of Agricultural Climate Research, Bundesallee 50, 38116 Braunschweig, Germany

^* Corresponding author (cvdh{at}ifbk.uni-hannover.de).

Received for publication July 14, 2008. Ground water is considered to be an important source for indirect N₂O emissions. We investigated indirect N₂O emissions from a shallow aquifer in Germany over a 1-yr period. Because N₂O accumulated in considerable amounts in the surface ground water (mean, 52.86 µg N₂O–N L^–1) and corresponding fluxes were high (up to 34 µg N₂O–N m^–2 h^–1), it was hypothesized that significant indirect N₂O emissions would occur via the vertical and the lateral emission pathway. Vertical N₂O emissions were investigated by measuring N₂O concentrations and calculating fluxes from the surface ground water to the unsaturated zone and at the soil surface. Lateral N₂O fluxes were investigated by measuring ground water N₂O and NO₃^– concentrations at five multilevel wells and at a waterworks well. Negligible amounts of N₂O were emitted vertically into the unsaturated zone; most of it was convectively transported into the deeper autotrophic denitrification zone. Only a ground water level fall and rise triggered the emission of N₂O (up to 3 µg N₂O–N m^–2 h^–1) into the unsaturated zone. Ground water–derived N₂O was probably reduced during the upward diffusion, and soil surface emissions were governed by topsoil processes. Along the lateral pathway, N₂O and NO₃^– concentrations decreased with increasing depth in the aquifer. Discharging ground water was almost free of N₂O and NO₃^–, and indirect N₂O emissions were small.

Abbreviations: DOC, dissolved organic carbon • EF5-g, Emission Factor 5 for ground water • FFA, Fuhrberger Feld Aquifer • IPCC, Intergovernmental Panel on Climate Change