Denitrification Conditions Research Articles

The characteristics of a novel heterotrophic nitrification–aerobic denitrification bacterium, Bacillus methylotrophicus strain L7

Bacillus methylotrophicus strain L7, exhibited efficient heterotrophic nitrification–aerobic denitrification ability, with maximum NH4+-N and NO2--N removal rate of 51.58mg/L/d and 5.81mg/L/d, respectively. Strain L7 showed different gaseous emitting patterns from those strains ever described. When 15NH4Cl, or Na15NO2, or K15NO3 was used, results of GC–MS indicated that N2O was emitted as the intermediate of heterotrophic nitrification or aerobic denitrification, while GC–IRMS results showed that N2 was produced as end product when nitrite was used. Single factor experiments suggested that the optimal conditions for heterotrophic nitrification were sodium succinate as carbon source, C/N 6, pH 7–8, 0g/L NaCl, 37°C and a wide range of NH4+-N from 80 to 1000mg/L. Orthogonal tests showed that the optimal conditions for aerobic denitrification were C/N 20, pH 7–8, 10g/L NaCl and DO 4.82mg/L (shaking speed 50r/min) when nitrite was served as substrate.

Denitrification by A Hollow Fiber Membrane Bioreactor Using Phenol As a Carbon Source

The phenol removal was carried out in a hollow fiber membrane bioreactor under denitrification conditions using a mixed microbial culture in this study. In the first part of the study, the effect of C/N ratio (0.5, 1, 1.5, 2, 3, 4) on microbial denitrification using phenol as the carbon source was investigated. The C/N ratio of 1 was identified as the optimum ratio, at which the highest amount of nitrate removal was obtained. The biological removal of phenol in hollow fiber membrane bioreactors under denitrification conditions at the optimum C/N ratio was studied in the second part of the study. Nitrate and phenol loading ratios were investigated. It was determined that denitrification was performed adequately well in hollow fiber membrane bioreactors using phenol as the carbon source.

Denitrification characteristics of a marine origin psychrophilic aerobic denitrifying bacterium

A psychrophilic aerobic denitrifying bacterium, strain S1-1, was isolated from a biological aerated filter conducted for treatment of recirculating water in a marine aquaculture system. Strain S1-1 was preliminarily identified as Psychrobacter sp. based on the analysis of its 16S rRNA gene sequence, which showed 100% sequence similarity to that of Psychrobacter sp. TSBY-70. Strain S1-1 grew well either in high nitrate or high nitrite conditions with a removal of 100% nitrate or 63.50% nitrite, and the total nitrogen removal rates could reach to 46.48% and 31.89%, respectively. The results indicated that nitrate was mainly reduced in its logarithmic growth phase with a very low level accumulation of nitrite, suggesting that the aerobic denitrification process of strain S1-1 occurred mainly in this phase. The GC-MS results showed that N 2O was formed as the major intermediate during the aerobic denitrifying process of strain S1-1. Finally, factors affecting the growth of strain S1-1 and its aerobic denitrifying ability were also investigated. Results showed that the optimum aerobic denitrification conditions for strain S1-1 were sodium succinate as carbon source, C/N ratio 15, salinity 10 g/L NaCl, incubation temperature 20°C and initial pH 6.5.

Behaviour of PAHs during sewage sludge fermentation in the presence of sulphate and nitrate

In this study, the changes of polycyclic aromatic hydrocarbons (PAHs) in sewage sludge and supernatants during anaerobic digestion under nitrate- and sulphate-reducing conditions were investigated. NaNO 3 or Na 2 SO 4 were added to the mixed sludge in order to obtain denitrification and sulphate reduction conditions, respectively. Abiotic loses of PAHs simultaneously were observed. Abiotic samples (with sodium azide) were also prepared. All sludge samples (biotic and abiotic) were incubated at 35°C in the dark. PAHs concentration was determined before incubation, after 13 d and after 20 d of incubation. Quantification of 16 PAHs (EPA) was carried out by GC-MS. PAHs were determined both in sewage sludge and in supernatants. During hydrolysis of organic compounds (first phase of fermentation) in the control samples (in sewage sludge and in superna tants) the increase of PAHs concentration occurred (especially naphthalene and 3-ring PAHs). The concentration of 16 PAHs in sewage sludge was reduced by 30% on average under nitrate-reducing conditions after 20 d and by 32% on average under sulphate-reducing conditions, respectively. The effectiveness of reducing of individual hydrocarbons was different. The concentration of carcino genic PAHs in sewage sludge was reduced by 73% on average under nitrate-reducing conditions and by 79% on average under sulphate-reducing conditions, respectively.

Regulating Strategies of Strengthening Nitrogen Removal for Low-Carbon Wastewater in Reversed A<sup>2</sup>/O Process

According to the ineffective denitrification and the initial controlling technique flaws for low-carbon source in reversed A2/O process, and full scale experimental studies were conducted to strengthen denitrification at normal and low temperature days in 2008~2010, to ensure the best conditions of biological denitrification and promote the stable and efficient operation of the system. The results showed that in normal temperature seasons of 2008, after shortening the HRT of the primary sedimentation tank to 1/3 of design value, improving the MLSS to 4500mg/L, setting the first section of the aerobic zone as the denitrification transition section and improving the reflux ratio, available carbon source was increased by 15%, the effluent NH3-N was 2.5mg / L and the removal rate was 90%, the effluent TN was 17 mg/L and the removal rate was increased to 54% and unit power consumption was reduced by 15% to 0.22Kw•h/m3.At low temperature seasons of 2008~2009,by improving the MLSS to 6000mg/L, extending the sludge age to enrich a large number of nitrification and denitrification bacteria, controlling DO at about 1.2mg / L in the aerobic zone and so on, the effluent NH3-N was 3 mg / L and the removal rate was 88%, the effluent TN was 15.5 mg/L and the removal rate was 62%. In low temperature of 2009 ~2010, besides the same measures as before, adding a blender in the first section of the aerobic zone to provide a better anoxic environment for denitrification and to improve nitrogen removal, the effluent NH3-N was 3 mg / L and the removal rate was 87%, the effluent TN was 13.5 mg/L and the removal rate was 66%.

Introduction to Production, Transport, and Emission of Trace Gases from the Vadose Zone to the Atmosphere

Rising concerns about global warming as a consequence of increased anthropogenic greenhouse gas emissions have markedly strengthened scientific, political, and even public interest in issues surrounding human-induced climate change. This has resulted in fundamental economic and ecologic debates

Biodegradation of naphthalene by enriched marine denitrifying bacteria

Numerous studies have been investigated on the PAHs biodegradation in aerobic and anaerobic environments; however, the biodegradation of PAHs under anoxic conditions, especially denitrifying conditions, has drawn less attention. In this study, four series of batch experiments were conducted to investigate the effect of temperature, pH, naphthalene concentration and nitrate concentration on the naphthalene degradation under denitrification condition. Our results showed that the degradation of naphthalene was most favorable at pH 7 and 25°C. Results also indicated that 30mg/l naphthalene inhibited the biodegradation and the removal efficiency was only 20.2%. Significant degradation (91.7% and 96.3%) of naphthalene occurred when nitrate concentrations were 1.0 and 5.0mM. Moreover, the maximum degradation rates were 0.13 and 0.18mg-NAP/(lh) depending on the concentration of nitrate. Based on 16S rDNA analysis, the denitrifying enriched culture was mainly composed of γ-Proteobacteria (19 clones out of a total of 23 clones) and Actinobacteria (4 clones). Using a primer set specific for naphthalene degrading functional gene nahAc, two operational taxonomy units were obtained in the clone library of nahAc. Both of them were closely related to nahAc genes of known species of Pseudomonas. Quantitative polymerase chain reaction (qPCR) was employed to quantify the change of naphthalene-degrading population during the degradation of naphthalene using nahAc gene as the biomarker. The maximum degradation rate and removal efficiency were strongly correlated with nahAc gene copy number, with R2 of 0.69 and 0.79, respectively.

Integration of nitrification and denitrification by combining anoxic and aerobic conditions in a membrane bioreactor

A membrane bioreactor (MBR) was developed to achieve nitrogen removal by combining nitrification and denitrification conditions in one reactor. The activated sludge was alternated between aerobic and anoxic conditions using peristaltic pump. The biomass concentration and floc morphological properties were observed to be similar in anoxic and aerobic compartments. However, the homogeneous properties of the activated sludge did not lead to the failure of oxygen gradient formation in the reactor. Due to the position of the air diffuser, an anoxic compartment at the bottom and an aerobic compartment in the upper part of the reactor were formed after 40 days. The average total nitrogen (TN) removal efficiency was then observed to increase to 77%. The microbial characterization using polymerase chain reaction-denaturing gradient gel electrophoresis (PCR-DGGE) analysis, as well as the specific nitrogen utilization rate measurements, indicated that the nitrogen removal in the reactor occurred via nitrification and denitrification processes.

Biodegradation of 2,4‐dinitrophenol under denitrification conditions

AbstractNitrophenols are often found in industrial wastewaters for a wide range of processes; they are highly toxic and potentially carcinogenic. Anaerobic biodegradation of nitrophenols has advantages of relatively low cost, less sludge produced, and high efficiency, when compared with aerobic biodegradation and physical/chemical treatments, although available published results are often contradictory. This paper reports on 2,4‐dinitrophenol biodegradation in an anaerobic bioreactor operated in a continuous mode in order to determine optimal operating conditions, and the effects of the main operating parameters such as hydraulic retention time and chemical oxygen demand (COD) loading on the process efficiency.In this study, an 80‐day gradual enrichment of an anaerobic culture has been carried out at 25 °C in an anaerobic bioreactor for continuously treating a synthetic wastewater containing 210 mg/l 2,4‐dinitrophenol and 100 mg/l nitrate. The results showed that the enriched culture could utilize 2,4‐dinitrophenol as a sole electron donor and nitrate as an electron acceptor at the end of the enrichment (on Day 80). Almost all nitrate and 95.5% 2,4‐dinitrophenol could be simultaneously removed at a hydraulic retention time of 12 h in the anaerobic bioreactor. The removal of 1 g of the added nitrate required about 4.60 g COD (as provided by the 2,4‐dinitrophenol) as the electron donor, and the removal of 2,4‐dinitrophenol by this enriched culture was strongly dependent on the presence of nitrate. Copyright © 2009 Curtin University of Technology and John Wiley & Sons, Ltd.

A Eukaryotic Copper-Containing Nitrite Reductase Derived from a NirK Homolog Gene ofAspergillus oryzae

We cloned a bacterial copper-containing nitrite reductase (NirK) homolog gene of Aspergillus oryzae (AonirK). Alignment showed that amino acid residues crucial for copper binding are conserved in the deduced sequence of the fungal protein. The recombinant protein exhibited distinct nitrite reductase activity, and its absorption and EPR spectra showed the presence of type 1 and type 2 copper atoms in the molecule. AonirK transcriptionally responded to denitrification conditions. Although the denitrifying activity of A. oryzae was weak under the conditions employed, high expression of the gene in the fungal cells enhanced the denitrifying activity 6-fold, accompanied by distinct cell growth. Furthermore, the highly expressed AoNirK was subcellularly localized to the mitochondria. The results demonstrated that AoNirK is responsible for fungal denitrification. Discussion is added on the novel insight concerning the origin and evolution of the mitochondrion provided by the findings for eukaryotic NirKs.

Land-use type and temperature affect gross nitrogen transformation rates in Chinese and Canadian soils

Land-use type affects gross nitrogen transformation and this information is particularly lacking under varied low temperature conditions. In this study, the effects of land-use type (forest vs. grassland) and temperature (10 vs. 15°C) on gross N transformation rates under aerobic conditions were investigated using the 15N isotope pool dilution technique in the laboratory. Soils were collected from forest and grassland sites in China and Canada. The results showed that gross N mineralization and immobilization rates were significantly higher in forest soils than in grassland soils, while the reverse was true for gross nitrification rates. The higher TC and lower SOCw concentrations in the Chinese soils relative to the Canadian soils were related to the greater gross N mineralization rates and lower gross N immobilization rates in Chinese soils. The greater gross N mineralization rates and lower gross N immobilization rates resulted in much higher inorganic N accumulation and that may increase the risk of NO3− leaching in the Chinese soils. Increasing temperature significantly increased gross nitrification rates in grassland soils and gross N immobilization rates in forest soils, suggesting that grassland soils maybe more vulnerable to N loss through NO3− leaching or denitrification (when conditions for denitrification exist) and that conversion of grassland to forest soils may exert less negative effects on the environment by promoting the retention of N and decreasing the production of NO3− and subsequently the risk of NO3− leaching under increasing temperature by global warming.

Combination of drip irrigation and organic fertilizer for mitigating emissions of nitrogen oxides in semiarid climate

Fertilizer and irrigation management are two of the most important factors which directly influence nitrification and denitrification processes from agricultural soils and, therefore, emissions of nitrous oxide (N 2O) and nitric oxide (NO). To date, the effect of combining organic fertilizer (applied before planting) with a drip irrigation system (DI) on the emissions of these nitrogen (N) oxides has not been evaluated. The different distribution and amount of water applied with this irrigation system should produce less favourable conditions for denitrification than other irrigation systems, such as furrow irrigation (FI), and this could be used to mitigate emissions of N 2O, especially following organic fertilizer application. In order to test this hypothesis, a field experiment was carried out to compare the influence of FI and DI on N 2O and NO emissions using digested pig slurry (DPS) as organic fertilizer during a melon crop ( Cucumis melo L. cv. Sancho) season. Plots without any N fertilizer were used as a control (control). The irrigation system influenced the water distribution pattern, producing significantly higher ammonium-N (NH 4 +-N) and nitrate-N (NO 3 −-N) concentrations in the soil under DI than under FI. Moreover, the differences observed in water filled pore space (WFPS) and mineral N also favoured a different pattern of emissions of N oxides. Only one large pulse of N 2O (106.9 mg N m −2 d −1) and NO (0.81 mg N m −2 d −1) were observed for FI after the first irrigation event while several pulses were observed for DI during the entire experimental period. As a consequence, nitrous oxide emissions were 75% and 28% lower from control and DPS, respectively under DI than under FI. In contrast, NO emissions were higher for DI on some sampling dates in both treatments, but without significant differences in the cumulative NO emissions between both irrigation systems. In conclusion, this work shows that combining drip irrigation with organic fertilizer for horticultural crops in semiarid climates could be a good management strategy for three reasons: it saves water; it re-uses organic residues and it mitigates N 2O emissions.

Membrane fouling propensity of denitrifying organisms

The impact of denitrification on membrane fouling propensity was investigated using three different sludges, i.e. conventional activated sludge (CAS), ordinary heterotrophic organisms (OHO) and denitrifying phosphorous accumulating organisms (DPAO) taken from a lab-scale biological nutrient removal (BNR) system. Fouling propensity was evaluated with the modified filtration index (MFI), soluble microbial products (SMP), bound extracellular polymeric substances (bEPS), and relative hydrophobicity (RH). The tests indicated that DPAO denitrification decreased cake layer resistance, as indicated by 45% reduction in MFI as compared to an increase of 123 and 52% for CAS and OHO denitrification. Pore blocking fouling was greater in CAS and OHO than DPAO. However, SMP increased after CAS and OHO denitrification but decreased in DPAO while bEPS increased in all cases. DPAO denitrification also increased RH of bEPS but decreased carbohydrate/protein ratio, possibly due to carbohydrate utilization in EPS by OHO under substrate limiting conditions for denitrification, substantiating the reduction in cake layer resistance in DPAO denitrification. CAS fouling during denitrification was more severe than BNR sludges as reflected by higher MFI, SMP and bEPS production.

Study of operational conditions of simultaneous nitrification and denitrification in a Carrousel oxidation ditch for domestic wastewater treatment

The study on the operational conditions of simultaneous nitrification and denitrification (SND) in the channel of oxidation ditch (OD) without the need for a special anoxic tank was carried out based on lab-scale and pilot-scale experiments using real domestic wastewater. The influence of sludge loading and component proportion in influent, temperature, hydraulic retention time (HRT), dissolved oxygen (DO) and operational mode on SND was investigated. The result indicated that the optimal DO (ODO) of SND occurrence was confirmed majorly by the sludge loading of influent and temperature, the high TCOD/NH 3–N and short HRT can enhance the occurrence of SND. A new operational mode was proposed that achieved a higher removal efficiency of 60–70% for total nitrogen by SND with HRT of 4–6 h, and the concentrations of NH 3–N and TN in effluent are less than 5 and 15 mg/L, respectively.

From meso- to macro-scale dynamic water quality modelling for the assessment of land use change scenarios

The implementation of the European Water Framework Directive requires reliable tools to predict the water quality situations in streams caused by planned land use changes at the scale of large regional river basins. This paper presents the results of modelling the in-stream nitrogen load and concentration within the macro-scale basin of the Saale river (24,167 km 2) using a semi-distributed process-based ecohydrological dynamic model SWIM (Soil and Water Integrated Model). The simulated load and concentration at the last gauge of the basin show that SWIM is capable to provide a satisfactory result for a large basin. The uncertainty analysis indicates the importance of realistic input data for agricultural management, and that the calibration of parameters can compensate the uncertainty in the input data to a certain extent. A hypothesis about the distributed nutrient retention parameters for macro-scale basins was tested aimed in improvement of the simulation results at the intermediate gauges and the outlet. To verify the hypothesis, the retention parameters were firstly proved to have a reasonable representation of the denitrification conditions in six meso-scale catchments. The area of the Saale region was classified depending on denitrification conditions in soil and groundwater into three classes (poor, neutral and good), and the distributed parameters were applied. However, the hypothesis about the usefulness of distributed retention parameters for macro-scale basins was not confirmed. Since the agricultural management is different in the sub-regions of the Saale basin, land use change scenarios were evaluated for two meso-scale subbasins of the Saale. The scenario results show that the optimal agricultural land use and management are essential for the reduction in nutrient load and improvement of water quality to meet the objectives of the European Water Framework Directive and in view of the regional development plans for future.

Compared microbiology of granular sludge under autotrophic, mixotrophic and heterotrophic denitrification conditions

Water contamination by nitrate is a wideworld extended phenomena. Biological autotrophic denitrification has a real potential to face this problem and presents less drawbacks than the most extended heterotrophic denitrification. Three bench-scale UASB reactors were operated under autotrophic (R1, H2S as electron donor), mixotrophic (R2, H2S plus p-cresol as electron donors) and heterotrophic (R3, p-cresol as electron donor) conditions using nitrate as terminal electron acceptor. 16S rDNA genetic libraries were built up to compare their microbial biodiversity. Six different bacteria phyla and three archaeal classes were observed. Proteobacteria was the main phyla in all reactors standing out the presence of denitrifiers. Microorganisms similar to Thiobacillus denitrificans and Acidovorax sp. performed the autotrophic denitification. These OTUs were displaced by chemoheterotrophic denitrifiers, especially by Limnobacter-like and Ottowia-like OTUs. Other phyla were Bacteroidetes, Chloroflexi, Firmicutes and Actinobacteria that--as well as Archaea members--were implicated in the degradation of organic matter, as substrate added as coming from endogenous sludge decay under autotrophic conditions. Archaea diversity remained low in all the reactors being Methanosaeta concilii the most abundant one.

Planning and implementation of nitrogen reduction measures in catchment areas based on a determination and ranking of target areas

The implementation schedule for the EU water framework directive requires the creation of monitoring programmes for water bodies according to the intensity of the pollution risk by the end of 2006. Until 2009 river basin district management programmes have to be established followed by the implementation of measures from the management plans (until 2012). The WAgriCo-project (Water Resources Management in Cooperation with Agriculture), is a collaborative project funded by the European Commission Life Fund within the period October 2005 to September 2008, involving six British and four German institutions. The WAgriCo-project is based around the catchment management process, of identifying diffuse pollution issues and potential management solutions, involving land managers/advisors to implement methods and to assess their effectiveness in terms of pollution reduction and costs. For a basic understanding of the interactions between these issues an integrated hydrological/hydrogeological conceptual model has been set up. The model consists of three main features: natural characteristic assessment, anthropogenic impact assessment using monitoring data and model tools providing a linkage between the first two parts. In this paper the application of the integrated hydrological/hydrogeological conceptual model for geographical prioritization and determination of environmental target areas in the case study area Große Aue in the Federal State of Lower Saxony, Germany is described and discussed. It is demonstrated how area differentiated parameters like the denitrification condition in soils, the runoff ratio, nitrate concentrations in the leachate and groundwater monitoring data are determined and combined in order to determine target areas. With the given example of the case study area Grosse Aue, the importance of defining the receptor of nitrate outputs (groundwater or surface water) and the environmental target prior to the delineation of environmental target areas could be shown.

Evaluation of 2- and 3-stage combinations of vertical and horizontal flow constructed wetlands for treating greenhouse wastewater

The adsorption characteristics of various filter media and treatment efficiency of small pilot-scale constructed wetlands (CWs) were investigated in order to design optimum CWs for treating greenhouse wastewater. Calcite was the best filter medium for the adsorption of ammonium nitrogen and phosphorus under various temperature and pH conditions. However, removal efficiency of calcite for total nitrogen (T-N) removal was low due primarily to high nitrate levels. Thus, several hybrid CWs (containing calcite as filter media) consisting of combinations of vertical flow (VF) and horizontal flow (HF) beds were evaluated for improving efficiency for T-N removal. Both 2- and 3-stage combinations of the VF and HF beds were tested. The optimum hybrid CWs was demonstrated to be a 3-stage combination of horizontal flow (HF)–vertical flow (VF)–horizontal flow (HF), which provided suitable conditions for both nitrification and denitrification, which improved removal of T-N in wastewater containing nitrate. In the HF–VF–HF 3-stage hybrid CWs, the reduction in chemical oxygen demand (COD), T-N, and total phosphorus (T-P) in the effluent were 95.1, 68.4 and 94.3%, respectively. The removal of COD, T-N and T-P in 3-stage HF–VF–HF CWs was rapid in order of VF (second stage) ≫ HF (first stage) ≫ HF (third stage), VF (second stage) ≫ HF (third stage) > HF (first stage) and VF (second stage) ≫ HF (first stage) ≫ HF (third stage), respectively.

Biological nutrient removal using an alternating of anoxic and anaerobic membrane bioreactor (AAAM) process

An innovative alternating of anoxic and anaerobic membrane bioreactor (AAAM) process was developed to enhance nitrogen and phosphorus removal simultaneously. The process was composed of a continuous aerated MBR and an alternating anaerobic and anoxic zone containing two separated bioreactors (reactor A and reactor B). By switching on and off the control valves, the mixed liquor in the aerobic zone was recycled to the reactor A or reactor B alternately, thus, the anoxic conditions for denitrification and anaerobic conditions for phosphorus release were implemented in two single tanks alternately. Additionally, by this operation mode, the denitrifying bacteria and phosphate-accumulating organisms (PAOs) could make full use of the organic substrate present in the wastewater alternately. A laboratory-scale experiment treating synthetic wastewater was applied to investigate the performance of the AAAM process. The results showed that COD removal efficiency was high and stable (over 93%) throughout the experiment. Nitrogen and phosphorus removal efficiency also attained high treatment levels (67.4% and 94.1% respectively). Comparing to DO and pH, oxidation–reduction potential (ORP) could provide much better information about the processes going on in reactors A and B during the 120-min cycle. Analysis of membrane fouling showed that small particles seemed to play an important role on the membrane fouling.

Nitrous oxide generation, denitrification, and nitrate removal in a seepage wetland intercepting surface and subsurface flows from a grazed dairy catchment

Little is known about seepage wetlands, located within agricultural landscapes, with respect to removing nitrate (NO3−) from agricultural catchments, mainly through gaseous emissions of nitrous oxide (N2O) and dinitrogen (N2) via denitrification. These variables were quantified using a push–pull technique where we introduced a subsurface water plume spiked with 15N-enriched NO3− and 2 conservative tracers [bromide (Br−) and sulfur hexafluoride (SF6)] into each of 4 piezometers and extracted the plume from the same piezometers throughout a 48-h period. To minimise advective and dispersive flux, we placed each of these push–pull piezometers within a confined lysimeter (0.5 m diameter) installed around undisturbed wetland soil and vegetation. Although minimal dilution of the subsurface water plumes occurred, NO3−-N concentration dropped sharply in the first 4 h following dosing, such that NO3−-limiting conditions (<2 mg/L of NO3-N) for denitrification prevailed over the final 44 h of the experiment. Mean subsurface water NO3− removal rates during non-limiting conditions were 15.7 mg/L.day. Denitrification (based on the generation of isotopically enriched N2O plus N2) accounted for only 7% (1.1 mg/L.day) of the observed groundwater NO3− removal, suggesting that other transformation processes, such as plant uptake, were responsible for most of the NO3− removal. Although considerable increases in 15N-enriched N2O levels were initially observed following NO3− dosing, no net emissions were generated over the 48-h study. Our results suggest that this wetland may be a source of N2O emissions when NO3− concentrations are elevated (non-limited), but can readily remove N2O (function as a N2O sink) when NO3− levels are low. These results argue for the use of engineered bypass flow designs to regulate NO3− loading to wetland denitrification buffers during high flow events and thus enhance retention time and the potential for NO3−-limiting conditions and N2O removal. Although this type of management may reduce the full potential for wetland NO3− removal, it provides a balance between water quality goals and greenhouse gas emissions.