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

Abstract

Ground water is considered to be an important source for indirect N(2)O emissions. We investigated indirect N(2)O emissions from a shallow aquifer in Germany over a 1-yr period-Because N(2)O accumulated in considerable amounts in the surface ground water (mean. 52.86 mu g N(2)O-N L(-1)) and corresponding fluxes were high (up to 34 mu g N(2)O-N m(-2) h(-1)), it was hypothesized that significant indirect N(2)O emissions would occur via the vertical and the lateral emission pathway. Vertical N(2)O emissions were investigated by measuring N(2)O concentrations and calculating fluxes from the surface ground water to the unsaturated zone and at the soil surface ground water to the unsaturated zone and the soil surface. Lateral N(2)O fluxes were investigated by measuring ground water N(2)O and NO(3)(-) concentrations at five multilevel wells and at a waterworks well, Negligible amounts of N(2)O 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 N(2)O (up to 3 mu g N(2)O-N m(-2) h(-1)) into the unsaturated zone. Ground water-derived N(2)O was probably reduced during the upward diffusion, and soil surface emissions were governed by topsoil processes. Along the lateral pathway, N(2)O and NO(3)(-) concentrations decreased with increasing depth in the aquifer. Discharging ground water was almost free of N(2)O and NO(3)(-), and indirect N(2)O emissions were small.