Kinetics of N2O production and reduction in a nitrate-contaminated aquifer inferred from laboratory incubation experiments

Knowledge of the kinetics of N2O 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 N2O production and reduction in both process zones was studied during long-term anaerobic laboratory incubations of aquifer slurries using the 15N tracer technique. We measured N2O, 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 N2O were completely removed within 60 to 198 days. N2O 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 N2O accumulation during laboratory incubation. Finally, we applied a conventional model using first-order-kinetics to determine the reaction rate constants k1 for N2O production and k2 for N2O 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.DF

Groundwater N2O emission factors of nitrate-contaminated aquifers as derived from denitrification progress and N2O accumulation

We investigated the dynamics of denitrification and nitrous oxide (N2O) accumulation in 4 nitrate (NO3-) contaminated denitrifying sand and gravel aquifers of northern Germany (Fuhrberg, Sulingen, Thulsfelde and Gottingen) to quantify their potential N2O emission and to evaluate existing concepts of N2O emission factors. Excess N-2 - N-2 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 NO3- concentrations (NO3- that enters the groundwater) were derived from excess N-2 and actual NO3- concentrations in groundwater in order to determine potential indirect N2O emissions as a function of the N input. Median concentrations of N2O and excess N-2 ranged from 3 to 89 mu g 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 N-2) and starting material (initial NO3- 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 NO3--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 NO3--N concentrations. Depending on RP, EF(1) resulted in smaller values compared to EF(2), demonstrating (i) the relevance of NO3- consumption and consequently (ii) the need to take initial NO3--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 Thulsfelde and Gottingen. 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).DF

Emissionen von grundwasserbürtigem N2O in die Atmosphäre: Modellrechnungen zu einem 15N-Tracerversuch unter Feldbedingungen

In einem Tracerversuch mit isotopisch mar-kiertem Nitrat wurde unter Feldbedingungen (Gley-Podsol aus Talsand, darunter ein Lockergesteinsaquifer) die Denitrifikation im oberflächennahen Grundwasser und die daraus resultierende Emission von N2O in die Atmosphäre über einen Zeitraum von Juli bis September 2007 verfolgt. Die Messergebnisse dienten als Anfangs- und Randbedingungen und Vergleichswerte für Simulationsrechnungen zum Transport von N2O durch den Boden zur Atmosphäre mit einem numerischen Gasdiffusionsmodell. Die Ergebnisse der Simulationsrechnungen bestätigen, dass Diffusion der wesentliche Prozess für den N2O-Transport in dem untersuchten Boden war. Für die geringen grundwasserbürtigen N2O-Emissionen spielte N2O-Abbau im Boden keine Rolle. Wir schlussfolgern aus den Ergebnissen, dass die Simulationsrechnungen eine wertvolle Ergänzung zur experimentellen Datenbasis darstellen. Sie verdeutlichen u.a., dass die N2O-Emission am Versuchsstandort hauptsächlich durch N2O-Umsetzungen im Oberboden gesteuert wurde

Infestation levels of Apis mellifera scutellata swarms by socially parasitic Cape honeybee workers (Apis mellifera capensis)

A single clonal lineage of socially parasitic Cape honeybee workers, Apis mellifera capensis, has caused dramatic losses in managed populations of A. m. scutellata, raising concerns that wild populations might also be affected. We surveyed A. m. scutellata swarms at 27 localities in beekeeping areas (N = 87) and in nature reserves (N = 79). While eleven swarms were infested in beekeeping areas, we found no infestations in nature reserves. Eight swarms had no symptoms except workers with black tergites. However, DNA data revealed that these workers are not parasitic, showing that diagnoses based on tergite colour alone yield false positive results. Nevertheless, it is practical because we had no false negative diagnoses either. Nature reserves may be important refuges to protect wild A. m. scutellata populations against imported honeybees