Tuesday, 11 July 2006

Transport and Transformation Processes of the Greenhouse Gas N2O in the System Groundwater/Vadose Zone of a Catchment in Germany.

Markus Deurer1, Carolin von der Heide2, Jürgen Böttcher2, Wilhelmus Duijnisveld3, and Klaus Schäfer4. (1) HortResearch, P.O. Box 11030, Palmerston North, New Zealand, (2) Institute for Soil Science, University of Hannover, Herrenhäuserstr. 2, Hannover, Germany, (3) Federal Institute for Geosciences and Natural Resources, Stilleweg 2, Hannover, Germany, (4) Institute for Meteorology and Climate Research, Forschungszentrum Karlsruhe, Garmisch-Partenkirchen, Germany

The contribution of N2O to global warming has increased in recent decades. Nitrogen from agriculture is one of the main sources for both direct and indirect N2O emissions. So far, most research focused on the direct emissions from agricultural fields as a consequence of the nitrification and denitrification in soils. Little is known about the processes and magnitude of the indirect emissions of N2O from groundwater. In agricultural areas with shallow aquifers considerable amounts of nitrate might enter the groundwater and be denitrified. The intermediate product of this reaction, N2O, can diffuse through the vadose zone to the atmosphere. In the “Fuhrberger Feld” catchment in northern Germany we analyzed the dynamics of the indirect emissions of N2O from groundwater. Previous studies showed the occurrence of an intensive denitrification within this sandy unconfined aquifer. Our objective was to identify the physicochemical conditions that favour production of N2O during denitrification in the upper part of the aquifer. Also we sought to quantify the transport/emission of N2O through the vadose zone to the atmosphere. We will give an overview of the entire project, and we discuss our initial results: •In March 2005 we sampled near the surface of the groundwater at 79 sites with 3 replications per site across the catchment. The sites were representative for the different soil and landuse types. We found highly variable N2O concentrations even within a site. Other parameters such as NO3, SO4, DOC or pH were much less variable. They showed very weak or no correlation with N2O. •We installed 10 wells along a groundwater flowline. The wells were sampled monthly every 0.2 m in the upper 2 m, and then every third month at depth intervals of 1 m for the next 10 m. We found that the N2O depth profiles of individual wells in the upper 2 m significantly depended on just a few other parameters (e.g. NO3, SO4). However, the relationship between N2O and other variables was well and time specific. •At one of the wells we installed a suite of probes, including tdr's, tensiometers, suction cups and mini-gas lysimeters. These were at different depths in the vadose zone to monitor the water, gas and solute dynamics at a small scale. From these results we describe the process of N2O diffusion and the factors influencing transfer from groundwater to the atmosphere. •In order to have a realistic estimate of the large-scale N2O emissions we installed a large removable tent of dimensions 100 m x 5 m x 0.5 m. Monthly over a period of approximately 36 hours we measured N2O emissions from the soil surface with spectrometric methods. We measured the background concentrations and the concentrations that result from the injection of N2O near the surface of the groundwater.

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