Nitrous Oxide in denitrifying Aquifers: Reaction Kinetics, Significance of Groundwater-derived Emission and an improved Concept for the Groundwater Emission Factor

Abstract

Beside carbon dioxide and methane, the atmospheric trace gas nitrous oxide (N2O) is a major greenhouse gas. It is predominantly produced in soils and aquatic systems during microbiological processes. Global N2O emissions have been substantially increased due to the intensification of agricultural practices and the related inputs of nitrogen compounds. High N2O concentrations were found in the groundwater of agricultural ecosystems. Thus, agricultural groundwater is assumed to be a potential source of N2O emissions into the atmosphere. The significance of N2O emissions from agricultural groundwater is the key question of this thesis. First, this key question is introduced in a preliminary chapter. In the following three chapters, different methods and approaches are described and discussed in order to provide knowledge of different aspects of the topic. Finally, these findings are assessed within the scope of a final synthesis and general conclusions are drawn. Research activities were conducted within four denitrifying aquifers in Lower Saxony, but the Fuhrberger Feld aquifer situated close to the city of Hannover was the main study site. In all investigated aquifers, the input of nitrate-contaminated agricultural seepage water causes elevated nitrate concentrations at the groundwater table. This nitrate is reduced during denitrification, yielding N2O as an intermediate and finally dinitrogen. The kinetics of N2O production and reduction in the Fuhrberger Feld aquifer was investigated during long-term anaerobic incubations. The results were compared with concentration profiles obtained from multilevel well measurements (chapter 2). It was confirmed that two vertically separated denitrification zones exist within the aquifer, heterotrophic denitrification in the surface groundwater and autotrophic denitrification in the deeper aquifer and both reactions were identified to be a significant source for N2O. The time courses of the N-species obtained from the laboratory incubations showed that heterotrophic denitrification is kinetically much slower than the autotrophic process. This was quantitatively proven by derived reaction rate constants following first order kinetics and attributed to the different microbial bioavailability of the associated electron donors, i.e. organic carbon and reduced sulfur compounds. The field measurements revealed considerable N2O accumulation in both denitrification zones, e.g. the mean N2O concentration close to the water table at one of the investigated wells was 1.84 mg N2O-N L-1. The N2O concentration profiles enabled a further refinement of the existing process model of denitrification in the Fuhrberger Feld aquifer. Within the scope of a 15N field experiment it was investigated to what extent groundwater-derived N2O emissions occurring via the vertical emission pathway contribute to total N2O emissions at the soil surface. This approach was based on stable labeling of the groundwater surface during the entire measuring period with K15NO3 tracer solution. 15N-labeled N2O was produced during denitrification and could be measured within the system groundwater / unsaturated zone / soil surface. Fluxes of groundwater-derived N2O were very low and found to be between 0.0002 und 0.0018 kg N2O-N ha-1 year-1. Only 0.13 % of the total positive N2O fluxes at the soil surface originated from groundwater-derived N2O. This showed that groundwater N2O emissions occurring via the vertical pathway are negligible in the Fuhrberger Feld aquifer. Determination and assessment of emission factors for indirect N2O emissions from agricultural groundwater was a further main objective of this thesis. A new emission factor basing on reconstructed initial nitrate concentrations was introduced. Thus, the concept relates potential N2O emission to the input of nitrogen to the groundwater surface. The application of this concept yielded emission factors that were considerably lower than conventional emission factors derived from the ratio between N2O concentrations and measured nitrate concentrations. This showed the necessity to take initial nitrate concentrations for calculating the groundwater N2O emission factor into account. The reaction kinetics as well as the evaluated rate constants (chapter 2) could be a basis for modeling the reactive transport of N2O and may contribute to further improve the emission factor for indirect N2O emissions from agricultural groundwater. Summarizing the results, it can be underlined for the investigated aquifers that N2O produced in groundwater is hardly reaching the atmosphere and thus contributes to a very low extent to total emissions of the greenhouse gas.