Estimation of indirect nitrous oxide emissions from a shallow aquifer in Northern Germany

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 fl uxes were high (up to 34 µg N2O–N m–2 h–1), it was hypothesized that signifi cant indirect N2O emissions would occur via the vertical and the lateral emission pathway. Vertical N2O emissions were investigated by measuring N2O concentrations and calculating fl uxes from the surface ground water to the unsaturated zone and at the soil surface. Lateral N2O fl uxes were investigated by measuring ground water N2O and NO3 – concentrations at fi ve 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 denitrifi cation 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 diff usion, 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.