The denitrification in the drainage zone as an important process of the nitrogen cycle

Denitrification, the degradation of nitrate (NO₃^-) into nitrous oxide (N₂O) or dinitrogen (N₂), is an important process of the soil nitrogen cycle. Without denitrification, NO₃^- is leached through the soil and reaches the groundwater. In the groundwater NO₃^- lowers the water quality and leads to eutrophication of water bodies. Therefore, denitrification in soil and groundwater is a well-observed topic. But what happens with the denitrification in the unsaturated zone between soil and groundwater? The drainage zone, also known as deep vadose zone, has been widely overlooked until now. But this zone may play an important role in reducing NO₃^- leaching into the groundwater. Indeed, the frequent lack of available organic carbon as an electron donor and mostly oxic conditions in large parts of the drainage zone typically prevent intense denitrification. But due to the possibly large thickness of the drainage zone and the long travel time of NO₃^-, it is possible that even low denitrification rates could lead to relevant NO₃^- attenuation.

In the project DeniDrain, we focus on the denitrification process in the drainage zone. Using the direct push drilling method, we collect undisturbed samples at depths between 2 and 10 meters at representative locations throughout Germany. In the laboratory, we measure the N₂O and N₂ emissions with the ¹⁵N gas flow method to obtain the current denitrification rates. Our initial findings suggest that denitrification happens in certain sections of our investigated profiles, where the amount of degraded NO₃^- depended on the properties of the drainage zone. Therefore, the denitrification in the drainage zone plays an important role in the nitrogen cycle and should be incorporated in future research about the fate of NO₃^- leached from soil. To minimize the effort of measuring the denitrification in the drainage zone, the application of models is useful. Therefore, using our data we test the existing soil models DENUZ and BODIUM in their ability to predict denitrification in the drainage zone.

We will show the initial results of our measuring and modelling work.