Knowledge of the kinetics of N2 O production and reduction in groundwater is essential for the assessment of potential indirect emissions of the greenhouse gas. In the present study, we investigated ...this kinetics using a laboratory approach. The results were compared to field measurements in order to examine their transferability to the in situ conditions. The study site was the unconfined, predominantly sandy Fuhrberger Feld aquifer in northern Germany. A special characteristic of the aquifer is the occurrence of the vertically separated process zones of heterotrophic denitrification in the near-surface groundwater and of autotrophic denitrification in depths beyond 2-3 m below the groundwater table, respectively. The kinetics of N2 O production and reduction in both process zones was studied during long-term anaerobic laboratory incubations of aquifer slurries using the 15 N tracer technique. We measured N2 O, N2 , NO3- , NO2- , and SO42- concentrations as well as parameters of the aquifer material that were related to the relevant electron donors, i.e. organic carbon and pyrite. The laboratory incubations showed a low denitrification activity of heterotrophic denitrification with initial rates between 0.2 and 13 μg N kg-1 d-1 . The process was carbon limited due to the poor availability of its electron donor. In the autotrophic denitrification zone, initial denitrification rates were considerably higher, ranging between 30 and 148 μg N kg-1 d-1 , and NO3- as well as N2 O were completely removed within 60 to 198 days. N2 O accumulated during heterotrophic and autotrophic denitrification, but maximum concentrations were substantially higher during the autotrophic process. The results revealed a satisfactory transferability of the laboratory incubations to the field scale for autotrophic denitrification, whereas the heterotrophic process less reflected the field conditions due to considerably lower N2 O accumulation during laboratory incubation. Finally, we applied a conventional model using first-order-kinetics to determine the reaction rate constants k1 for N2 O production and k2 for N2 O reduction, respectively. The goodness of fit to the experimental data was partly limited, indicating that a more sophisticated approach is essential to describe the investigated reaction kinetics satisfactorily.
We investigated the dynamics of denitrification and nitrous oxide (N2O) accumulation in 4 nitrate (NO−3) contaminated denitrifying sand and gravel aquifers of northern Germany (Fuhrberg, Sulingen, ...Thülsfelde and Göttingen) to quantify their potential N2O emission and to evaluate existing concepts of N2O emission factors. Excess N2 - N2 produced by denitrification – was determined by using the argon (Ar) concentration in groundwater as a natural inert tracer, assuming that this noble gas functions as a stable component and does not change during denitrification. Furthermore, initial NO−3 concentrations (NO−3 that enters the groundwater) were derived from excess N2 and actual NO−3 concentrations in groundwater in order to determine potential indirect N2O emissions as a function of the N input. Median concentrations of N2O and excess N2 ranged from 3 to 89 μg N L−1 and from 3 to 10 mg N L−1, respectively. Reaction progress (RP) of denitrification was determined as the ratio between products (N2O-N + excess N2) and starting material (initial NO−3 concentration) of the process, characterizing the different stages of denitrification. N2O concentrations were lowest at RP close to 0 and RP close to 1 but relatively high at a RP between 0.2 and 0.6. For the first time, we report groundwater N2O emission factors consisting of the ratio between N2O-N and initial NO−3-N concentrations (EF1). In addition, we determined a groundwater emission factor (EF2) using a previous concept consisting of the ratio between N2O-N and actual NO−3-N concentrations. Depending on RP, EF(1) resulted in smaller values compared to EF(2), demonstrating (i) the relevance of NO−3 consumption and consequently (ii) the need to take initial NO−3-N concentrations into account. In general, both evaluated emission factors were highly variable within and among the aquifers. The site medians ranged between 0.00043–0.00438 for EF(1) and 0.00092–0.01801 for EF(2), respectively. For the aquifers of Fuhrberg and Sulingen, we found EF(1) median values which are close to the 2006 IPCC default value of 0.0025. In contrast, we determined significant lower EF values for the aquifers of Thülsfelde and Göttingen. Summing the results up, our study supports the substantial downward revision of the IPCC default EF5-g from 0.015 (1997) to 0.0025 (2006).
N₂O concentrations and denitrification-related factors (NO₃, SO₄, dissolved organic carbon (DOC) and CO₂) were investigated in the surface groundwater of a catchment in northern Germany, the ...Fuhrberger Feld Aquifer (FFA). We sampled 79 plots that were selected according to the three criteria of land use, historical land use conversion (1954-1995) and groundwater level. We sampled three sites within each plot. The sampling depth was 0.5m below the groundwater surface. We found no indication for the occurrence of autotrophic denitrification in the surface groundwater. Heterotrophic denitrification was identified as the main process for N₂O accumulation. The variability of N₂O concentrations on the plot-scale was extremely high and was poorly explained by the three sampling criteria. Other denitrification-related variables such as NO₃, SO₄ and DOC were less variable. The selection criteria land use and groundwater level clearly influenced the order of magnitude of N₂O concentrations in the surface groundwater. Under arable land, high NO₃ concentrations resulted in high N₂O concentrations. The surface groundwater under forest and pasture was almost NO₃-free and had also very small N₂O concentrations. Plots where the distance from the soil surface to the groundwater surface was large (>1m up to 3.4m) showed higher N₂O concentrations in the surface groundwater than plots where the distance was small (<1m). A larger distance from the soil surface to the groundwater leads to a longer residence time and more decomposition of DOC in the soil. Consequently the less bioavailable DOC could inhibit the efficiency of the heterotrophic denitrification in the groundwater, yielding more N₂O. Elevated organic carbon levels in plots with historic land use conversion (pasture to arable) were very stable and did not influence N₂O concentrations. The high within plot variability showed that an upscaling of N₂O from the plot-scale to the catchment-scale is possible as long as the groundwater level regime and the land use do not change.
The knowledge of the spatial and temporal variability of N₂O concentrations in surface groundwater is the first step towards upscaling of potential indirect N₂O emissions from the scale of localized ...samples to aquifers. This study aimed to investigate the spatial and the temporal variability of N₂O concentrations at different scales in the surface groundwater of a denitrifying aquifer in northern Germany. The spatial variability of N₂O concentrations in the surface groundwater was analysed at the plot (200 × 200 m) and at the transect scale (12 m). Twenty plots that were distributed across an area of 11 km² and 6 transects were sampled. Sixty per cent of the spatial variance of N₂O was located at the plot scale and 68-79% was located at the transect scale. This indicates that small-scale processes governed the spatial variability of N₂O in the surface groundwater. A spatial upscaling of N₂O from the transect to the aquifer scale might be possible with an adequate number of samples that represent important boundary conditions for N₂O accumulation in the catchment (topography, groundwater level, land use). For the investigation of the temporal variability, 4 multilevel wells were sampled monthly over a period of 13 months. In two periods, a multilevel well was additionally sampled in 2-day intervals over 8 days. At the annual scale, N₂O concentrations in the surface groundwater were higher during the vegetation period (median 87 μg N₂O-N l⁻¹) and could change rapidly on the day scale whereas the concentrations were smaller in winter (median 21 μg N₂O-N l⁻¹). Groundwater recharge events seemed to be crucial for the day scale variability. Capture of the temporal variations for upscaling might be achieved with a process-based sampling strategy with weekly sampling intervals during the vegetation period, the additional sampling after groundwater recharge events and monthly sampling intervals in winter.
Indirect emissions of the major greenhouse gas nitrous oxide (N2O) occurring from aquatic ecosystems are considered to be a highly uncertain component in the global N2O budget. In this study, we ...investigated the fate of N2O produced by denitrification in a sandy shallow aquifer in northern Germany. The experimental data from a previous 15N field study and site‐specific diffusion coefficients were used to simulate upward fluxes of groundwater‐derived (15N‐)N2O in the soil as well as its ultimate emission into the atmosphere. The one‐dimensional simulation model considered gas diffusion and gas retardation by dissolution in the water phase. The modelled concentration gradients and emissions were in good agreement with the experimental data, indicating that diffusion was the dominant transport process in the soil, and that our model approach was thus suitable for simulating N2O fluxes from the unsaturated zone to the atmosphere. Furthermore, the results revealed that there was no evidence for consumption of 15N‐N2O during upward diffusion from the surface groundwater to the atmosphere. Simulated concentrations and emissions of groundwater‐derived N2O were found to be very small and a negligible component of total N2O.
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