N2O Accumulation Research Articles

Copper availability governs nitrous oxide accumulation in wetland soils and stream sediments

Denitrification is microbially-mediated through enzymes containing metal cofactors. Laboratory studies of pure cultures have highlighted that the availability of Cu, required for the multicopper enzyme nitrous oxide reductase, can limit N2O reduction. However, in natural aquatic systems, such as wetlands and hyporheic zones in stream beds, the role of Cu in controlling denitrification remains incompletely understood. In this study, we collected soils and sediments from three natural environments -- riparian wetlands, marsh wetlands, and a stream -- to investigate their nitrogen species transformation activity at background Cu levels and different supplemented Cu loadings. All of the systems contained solid-phase associated Cu below or around geological levels (40–280 nmol g−1) and exhibited low dissolved Cu (3–50 nM), which made them appropriate sites for evaluating the effect of limited Cu availability on denitrification. In laboratory incubation experiments, high concentrations of N2O accumulated in all microcosms lacking Cu amendment except for one stream sediment sample. With Cu added to provide dissolved concentrations at trace levels (10–300 nM), the reduction rate of N2O to N2 in the wetland soils and stream sediments was enhanced. A kinetic model could account for the trends in nitrogen species by combining the reactions for microbial reduction of NO3− to NO2−/N2O/N2 and abiotic reduction of NO2− to N2. The model revealed that the rate of N2O to N2 conversion increased significantly in the presence of Cu. For riparian wetland soils and stream sediments, the kinetic model also suggested that overall denitrification is driven by abiotic reduction of NO2− in the presence of inorganic electron donors. This study demonstrated that natural aquatic systems containing Cu at concentrations less than or equal to crustal abundances may display incomplete reduction of N2O to N2 that would cause N2O accumulation and release to the atmosphere.

Micro-analysis of nitrous oxide accumulation in denitrification under acidic conditions: The role of pH and free nitrous acid

Nitrous oxide (N2O), as an energy gas, is of increasing interest for its recovery from wastewater treatment. Acidic pH is one of the important factors affecting N2O accumulation during denitrification; however, its generation mechanism still needs further clarification. In this study, the production efficiency of N2O increased from 1.0 to 65.0% with the decrease in pH from 6.5 to 4.5 during nitrate-based denitrification performed in batch tests, which was accompanied by an increase in the nitrite accumulation. To further explore the mechanism, the net volumetric reaction rates of N2O within denitrified sludge aggregates were quantified by application of a microelectrode. When using N2O as a nitrogen source, the consumption rate of N2O was negatively correlated with both the hydrogen ion and free nitrous acid (FNA) concentrations. The inhibition of nitrous oxide reductase (NOS) activity in the absence of FNA demonstrates that hydrogen ion is the true inhibitor. When using NO as nitrogen source, the net volumetric production rates of N2O increased with decreasing pH, and they were higher in the presence of FNA compared to those in the absence of FNA, revealing that the increase in nitrite substrate promotes N2O production. Overall, the inhibition of NOS activity by hydrogen ions and FNA and the increase of substrate from nitrite accumulation are the main reasons for N2O enrichment under a low-pH condition.

Roots and other residues from leys with or without red clover: Quality and effects on N2O emission factors in a partly frozen soil following autumn ploughing

Revised IPCC guidelines assume that a constant share of N in decomposing crop residues is directly emitted as N2O (emission factor: EFN2O), and calculate the amount of nitrogen (N) in non-removable residues of temporary grasslands proportionally to the average annual herbage yield. However, EFN2O depends on the intrinsic quality of the residues and their interactions with environmental conditions. Only a few field studies on N2O emissions from grassland renewal are available, and none have simultaneously quantified the N amount and quality of non-removable residues (roots and stubble).To gain insight into the effect of non-removable residue quality on EFN2O, we studied the amount and quality of roots and stubble and their effect on EFN2O following the ploughing of three-year-old swards.The measured amount of N in non-removable residues was approximately 20, 25, and 31 kg N per 1 Mg average annual dry matter yield in grass, red clover-grass, and red clover, and 70–83% of it was below ground. However, the EFN2O of non-removable residues measured over 252 days was lower (0.24%, SE = 14% for grass and red clover-grass) than the IPCC default value (0.6%, CV: 50%) for wet regions, although within the uncertainty margin, and was significantly lower than the EFN2O of incorporated herbage, which was related to differences in EFCO2. We advocate for more specific studies that separate the effects of belowground and aboveground residues (AGR), considering the possibility of simplifying the accounting of N2O emissions from belowground residues while improving that of non-removable AGR from temporary grasslands and other green crops.We observed the accumulation of N2O in the frozen soil under snow, which was released during diurnal percolation of meltwater. N2O emissions from frozen soil accounted for 30% or more of the total emissions.

Sensitivity of total column NO2 at a marine site within the Chesapeake Bay during OWLETS-2

In coastal environments like the Chesapeake Bay, the presence of the sea/bay breeze circulation can contribute to poor air quality and makes modeling the meteorological and chemical impacts of the sea/bay breeze in air quality forecast models a challenge. The Ozone Water-Land Environmental Transition Study 2 field campaign aimed to better quantify the mechanisms affecting surface, profile, and columnar trace gas amounts between the land and water. Using HYSPLIT back trajectory modeling, the meteorological variability affecting Pandora NO2 and surface O3 was quantified. Clustered back trajectories showed that westerly and north-northwesterly winds resulted in the highest MDA8 ozone values over the study domain. An analysis of multiday ozone event, demonstrated how TROPOMI can capture the spatial variability of NO2 observed by the Pandora network, including the accumulation of NO2 over the Chesapeake Bay. VOC measurements during multiday ozone event were analyzed and sources of ozone precursors, such as a coal fire power plant, were identified. Further investigation of the surface ozone data at HMI revealed that significant amounts of ozone were maintained over the Chesapeake Bay at night. Using a combination of ozone lidar, Pandora, in situ O3 and NO2, and wind lidar measurements, a lofted plume of NO2 was detected over water. Additionally, the same suite of observations found significant differences in the horizontal and vertical extent of ozone on the highest exceedance day of the event. Surface measurements of trace gases (NO2 and O3) can vary significantly from remote sensing (Pandora, TROPOMI, O3lidar), highlighting the need for sensitive profile, columnar, and in situ measurements in complex urban, marine environments for future geostationary air quality validation.

Responses of nitrogen removal under microplastics versus nanoplastics stress in SBR: Toxicity, microbial community and functional genes

Microplastics (MPs) and nanoplastics (NPs), as emerging pollutants, are frequently detected in wastewater treatment plants. However, studies comparing the effects of MPs versus NPs on nitrogen removal by activated sludge are rarely reported. Here, the responses of nitrogen removal performance, microbial community and functional genes to MPs and NPs in sequencing batch reactors were investigated. Results revealed that MPs (10 and 1000 μg/L) had no effects on nitrogen removal. While upon exposure to NPs, although low concentration (10 μg/L) of NPs showed no remarkable influence on nitrogen removal, high level (1000 μg/L) of NPs decreased NH4+-N removal efficiency by 24.48% and caused accumulation of NO3−-N and NO2−-N. These inhibitory probably due to the acute toxicity of NPs to activated sludge, which was reflected by the increasing reactive oxygen species generation and lactate dehydrogenase release. The toxic effects of NPs further declined the relative abundance of nitrifiers (e.g., Nitrospira) and denitrifiers (e.g., Dechloromonas). These negative effects, accompanied by a decrease in abundance of amoA and nxrA genes related to nitrification (30.01% and 65.24% of control) and narG, nirK and nirS genes associated with denitrification (78.59%, 61.39%, and 86.17% of control), directly illustrated the attenuate phenomenon observed in nitrogen removal.

Characterization of nitrous oxide and nitrite accumulation during iron (Fe(0))- and ferrous iron (Fe(II))-driven autotrophic denitrification: mechanisms, environmental impact factors and molecular microbial characterization

The iron (Fe(0))-/ferrous iron (Fe(II))-driven autotrophic denitrification processes have been alternative methods for nitrogen removal from low organic carbon (OC) wastewater, but the accumulation of nitrous oxide (N2O) and nitrite (NO2−) along with these processes remains unclear. This research aimed to systematically characterize the N2O/NO2− accumulation in Fe(0)-/Fe(II)-ADN processes through investigating the mechanisms, impact factors, and molecular biological characteristics. Results showed that Fe(II)-ADN was effective in NO3− reduction but was less efficient in N2O reduction (k = 0.50 h−1) than Fe(0)-ADN (k = 1.82 h−1). NO2−/N2O accumulation in Fe(II)-ADN (28.6%/30.7%) was much higher than that in Fe(0)-ADN (12.6%/1.5%). Introducing hydrogenotrophic denitrification (H-ADN) into Fe(II)-ADN system significantly (p < 0.05) reduced NO2−/N2O accumulation. Fe(0)-ADN was proved a coupled process of Fe(II)- and H-ADN by in-situ generating Fe(II)/H2, and Fe(II)-ADN and H-ADN mainly contributed to NO3− and NO2−/N2O reduction, respectively. Optimum pH (7.5) and temperature (30–35 °C) were confirmed with controlled NO2–/N2O accumulation and effective denitrification. Dosing inorganic carbon (IC) and OC enhanced denitrification and reduced NO2–/N2O accumulation, where OC was more efficient with an optimum dosage of 0.25 mmol C/mmol N. 16S rRNA high-throughput sequencing and Pearson Correlation Coefficients verified that Thiobacillus was the main contributor to NO3− reduction, whereas Thauera and Acidovorax possessed high NO2−/N2O reduction capability. Real-time quantitative polymerase chain reaction and enzyme activity assay demonstrated that the nitrite reductase encoded by gene nirK and the nitrous oxide reductase encoded by gene nosZ were efficient in catalyzing the further reduction of NO2− and N2O, respectively. This study could provide an in-depth understanding of NO2−/N2O accumulation in Fe(II)-/Fe(0)-ADN processes and contribute to their application, optimization and secondary pollutants control.

Enrichment of a denitratating microbial community through kinetic limitation

Denitratation, or the intentionally engineered accumulation of nitrite (NO2-) from selective reduction of nitrate (NO3-), can be combined with downstream anammox to reduce chemical and energy use associated with conventional nitrification and denitrification. This study aimed to enrich a denitratating microbial community capable of significant NO2- accumulation by applying added kinetic limitation to an already stoichiometrically-limited, glycerol-driven denitratation process. Operation at solids residence time, SRT=3.0 d, resulted in optimal denitratation performance and a microbial community dominated by NO3--respirers, noted by one order of magnitude lower total copy numbers of nirS and nirK gene transcripts compared to longer SRTs. Selective NO3- reduction to NO2- was achieved at all SRTs although longer SRTs (less kinetic limitation) supported microbial communities more capable of full denitrification as described by a lower NO2- accumulation ratio (NAR=42±5%) and higher steady-state nitrous oxide (1.5mg/L N2O-N) accumulation. Shorter SRTs (more kinetic limitation) led to higher observed yields (Y=0.63mg-COD/mg-COD) with more electrons dedicated for cell synthesis (fs=0.56±0.10), which potentially contributed to the accumulation of NO3-. Enrichment of a denitratating-dominant microbial community by optimizing kinetic limitation operating parameters could support significant NO2- accumulation and reduce chemical and energy use for biological nitrogen removal when combined with downstream anammox.

Sedimentary processes dominate nitrous oxide production and emission in the hypoxic zone off the Changjiang River estuary

Coastal oceans, known as the major nitrous oxide (N2O) source to the atmosphere, are increasingly subject to eutrophication and concurrent near-bottom hypoxia. The natural nitrogen cycle is likely to be altered markedly in hypoxic coastal oceans. However, the processes responsible for N2O production and emission remain elusive because of lacking field rate measurements simultaneously conducted in the water column and sediment. Here, we quantified N2O production rates using a 15N-labeled technique in the water-column and surface sediments off the Changjiang (Yangtze) River estuary, the largest hypoxic zone in the Pacific margins. Our results showed that the estuarine surface sediments were the major source for N2O production, accounting for approximately 90% of the total water-column accumulation and consequent efflux of N2O in the hypoxic zone, whereas the water-column nitrification and denitrification combined only contributed <10%. More importantly, the coupling of nitrification and denitrification at the presence of abundant supply and remineralization of labile organic matter was the main driver of the N2O release from the sediment-water interface in this region. This study highlights the dominant role of benthic processes occurring at the sediment-water interface controlling the coastal N2O budget, as the anthropogenic eutrophication and hypoxia are expanding in coastal oceans.

Isotopic signals in an agricultural watershed suggest denitrification is locally intensive in riparian areas but extensive in upland soils

Nitrogen loss from cultivated soils threatens the economic and environmental sustainability of agriculture. Nitrate (NO3−) derived from nitrification of nitrogen fertilizer and ammonified soil organic nitrogen may be lost from soils via denitrification, producing dinitrogen gas (N2) or the greenhouse gas nitrous oxide (N2O). Nitrate that accumulates in soils is also subject to leaching loss, which can degrade water quality and make NO3− available for downstream denitrification. Here we use patterns in the isotopic composition of NO3− observed from 2012 to 2017 to characterize N loss to denitrification within soils, groundwater, and stream riparian corridors of a non-irrigated agroecosystem in the northern Great Plains (Judith River Watershed, Montana, USA). We find evidence for denitrification across these domains, expressed as a positive linear relationship between δ15N and δ18O values of NO3−, as well as increasing δ15N values with decreasing NO3− concentration. In soils, isotopic evidence of denitrification was present during fallow periods (no crop growing), despite net accumulation of NO3− from the nitrification of ammonified soil organic nitrogen. We combine previous results for soil NO3− mass balance with δ15N mass balance to estimate denitrification rates in soil relative to groundwater and streams. Substantial denitrification from soils during fallow periods may be masked by nitrification of ammonified soil organic nitrogen, representing a hidden loss of soil organic nitrogen and an under-quantified flux of N to the atmosphere. Globally, cultivated land spends ca. 50% of time in a fallow condition; denitrification in fallow soils may be an overlooked but globally significant source of agricultural N2O emissions, which must be reduced along-side other emissions to meet Paris Agreement goals for slowing global temperature increase.

The accumulation characteristics of NO and N2O in A/O-SBR for treating ammonium-rich organic wastewater

The emissions of NO and N2O during biological nitrogen removal harm the environment. In this study, a sequencing batch reactor (SBR) was used to treat ammonium-rich organic wastewater. The nitrogen transformation and characteristics of NO and N2O accumulation in typical cycles under different nitrification conditions were monitored, and the mechanisms of NO and N2O production in A/O-SBR were analyzed. The results showed that NO accumulated in the anoxic phase under a fixed concentration of organic carbon source during denitrification with nitrite as electron acceptor or nitrate as electron acceptor. NO production in the aerobic phase could be related to hydroxylamine oxidation and nitrite reduction caused by high ammonia nitrogen. The accumulation of N2O mainly occurred in the aerobic phase, and the cumulative concentration of N2O at the end of the four typical cycles was above 4.02 mg/L. N2O accumulation attributed to the inhibition of free nitrous acid (FNA) and NO, and electronic competition, which made N2O reductase (N2OR) activity insufficient, failing N2O reduction.

The effect of S/N ratio on NO and N2O accumulation during sulfide autotrophic denitrification

Nitro oxide (NO) and Nitrous oxide (N2O) are inevitable intermediates of sulfide autotrophic denitrification (SAD) process, and have huge environmental hazard. This study investigated the produce mechanism and accumulation characteristic of NO and N2O in SAD process under different S/N ratios. The results showed that the lower S/N ratio, the higher NO accumulation. Which was due to the strong reducing effect of S2- and the electronic competition between nitrogen reductase. And under S/N ratio = 2, N2O level reached its peak, which was caused by electronic competition. When the S/N ratio = 1, although the electronic competition was more intense, the total amount of electrons was less, thus the N2O produced is lower than that produced when S/N=2. Microbial community analysis showed that Thiobacillus sp. (62.44%) was the predominant genus, and conducted the suldide-oxiding autotrophic denitrification process.

Impact of soluble organic matter and particulate organic matter on anammox system: Performance, microbial community and N2O production

In this study, the effects of soluble readily biodegradable COD (sCOD) and particulate slowly biodegradable COD (pCOD) on anammox process were investigated. The results of the long-term experiment indicated that a low sCOD/N ratio of 0.5 could accelerate the anammox and denitrification activity, to reach as high as 84.9%±2.8% TN removal efficiency. Partial denitrification-anammox (PDN/anammox) and denitrification were proposed as the major pathways for nitrogen removal, accounting for 91.3% and 8.7% of the TN removal, respectively. Anammox bacteria could remain active with high abundance of anammox genes to maintain its dominance. Candidatus Kuenenia and Thauera were the predominant genera in the presence of organic matter. Compared with sCOD, batch experiments showed that the introduction of pCOD had a negative effect on nitrogen removal. The contribution of denitrification to nitrogen removal decreased from approximately 14% to 3% with increasing percentage of pCOD. In addition, the analysis result of the process data using an optimized ASM1 model indicated that high percentage of pCOD resulted in serious N2O emission (the peak value up to 0.25 mg N/L), which was likely due to limited mass diffusion and insufficient available carbon sources for denitrification. However, a high sCOD/N ratio was beneficial for alleviating N2O accumulation.

Enhanced simultaneous partial nitrification and denitrification performance of aerobic granular sludge via tapered aeration in sequencing batch reactor for treating low strength and low COD/TN ratio municipal wastewater

The aerobic granular sludge simultaneous partial nitrification, denitrification and phosphorus removal (AGS-SPNDPR) process was carried out via tapered aeration in sequencing batch reactor (SBR) for treating low strength and low COD/TN ratio municipal wastewater. The results showed that aerobic granular sludge was successfully cultivated with good sedimentation performance when treating the municipal wastewater. Meanwhile, the median granule size increased to 270 (R1) and 257 (R2) μm on day 80. The excellent removal performance of COD (92%) and NH4+-N (95%) were achieved under different aeration modes, while the higher TN removal efficiency (76%) was achieved by tapered aeration. The accumulation of NO2−-N in R2 indicated that the tapered aeration was beneficial to achieve simultaneously partial nitrification and denitrification. Meanwhile, the high-efficiency phosphorus (95%) removal was realized via additional carbon source, and SPNDPR process was formed under tapered aeration. The bacterial community analysis indicated denitrifying glycogen-accumulating organisms (DGAOs) Candidatus_Competibacter and ammonia-oxidizing bacteria (AOB) Nitrosomonas were more effectively enriched via tapered aeration, while phosphorus-accumulating organisms (PAOs) Candidatus_Accumulibacter were effectively enriched under additional organic carbon. AOB, denitrifying bacteria and PAOs were simultaneously enriched by tapered aeration and additional carbon source, which was beneficial to nutrients removal. This study might be conducive to the application of AGS-SPNDPR system for treating low strength and low COD/TN ratio municipal wastewater under tapered aeration.

Greenhouse gas dynamics in an urbanized river system: influence of water quality and land use.

Rivers act as a natural source of greenhouse gases (GHGs). However, anthropogenic activities can largely alter the chemical composition and microbial communities of rivers, consequently affecting their GHG production. To investigate these impacts, we assessed the accumulation of CO2, CH4, and N2O in an urban river system (Cuenca, Ecuador). High variation of dissolved GHG concentrations was found among river tributaries that mainly depended on water quality and land use. By using Prati and Oregon water quality indices, we observed a clear pattern between water quality and the dissolved GHG concentration: the more polluted the sites were, the higher were their dissolved GHG concentrations. When river water quality deteriorated from acceptable to very heavily polluted, the mean value of pCO2 and dissolved CH4 increased by up to ten times while N2O concentrations boosted by 15 times. Furthermore, surrounding land-use types, i.e., urban, roads, and agriculture, could considerably affect the GHG production in the rivers. Particularly, the average pCO2 and dissolved N2O of the sites close to urban areas were almost four times higher than those of the natural sites while this ratio was 25 times in case of CH4, reflecting the finding that urban areas had the worst water quality with almost 70% of their sites being polluted while this proportion of nature areas was only 12.5%. Lastly, we identified dissolved oxygen, ammonium, and flow characteristics as the main important factors to the GHG production by applying statistical analysis and random forests. These results highlighted the impacts of land-use types on the production of GHGs in rivers contaminated by sewage discharges and surface runoff.

Physiological and kinetic evaluation of ampicillin oxidation as unique electron source by a denitrifying sludge

AbstractBACKGROUNDDenitrification allows the simultaneous oxidation of recalcitrant compounds and the reduction of nitrate. Scarce information on consumption and oxidation of ampicillin (AMP) as sole electron source is found in the literature. The physiological and kinetic ability of a denitrifying sludge without previous exposure to antibiotics for oxidizing several AMP concentrations was evaluated.RESULTSComplete denitrification was accomplished in batch reference assays with acetate (20–100 mg C L−1) and nitrate (C/N 1.1) within 4 h of culture, resulting in yields of HCO3− (YHCO3−) and N2 (YN2) of 0.89–1.0 mg C or N produced mg−1 C or N consumed, respectively. Assays amended with different concentrations of AMP (5–100 mg C L−1) as unique electron source at stoichiometric C/N ratio (0.86) showed that within 8–12 h of culture the antibiotic was totally consumed and oxidized to HCO3− and CO2 (YHCO3− between 0.27 and 0.67 and YCO2 between 0.35 and 0.54 mg C produced mg−1 C consumed), while nitrate was mostly reduced to N2 (YN2 = 0.77–1.0 mg N produced mg−1 N consumed). At AMP concentrations higher than 20 mg C L−1, N2O and carbon intermediates production was detected. Decreases of 20–90% in denitrifying specific rates were determined regarding the denitrifying reference. After exposure to the antibiotic, the sludge was able to recover its denitrifying activity in the absence of AMP with no N2O accumulation, whereas only 39% of the denitrification rates were recuperated.CONCLUSIONIt was clearly evidenced that AMP was used as electron donor for conducting denitrification; however, the denitrifying process occurred at lower specific rates. © 2021 Society of Chemical Industry (SCI).

Sustainable treatment of nitrate-containing wastewater by an autotrophic hydrogen-oxidizing bacterium

Bacteria are key denitrifiers in the reduction of nitrate (NO3−-N), which is a contaminant in wastewater treatment plants (WWTPs). They can also produce carbon dioxide (CO2) and nitrous oxide (N2O). In this study, the autotrophic hydrogen-oxidizing bacterium Rhodoblastus sp. TH20 was isolated for sustainable treatment of NO3−-N in wastewater. Efficient removal of NO3−-N and recovery of biomass nitrogen were achieved. Up to 99% of NO3−-N was removed without accumulation of nitrite and N2O, consuming CO2 of 3.25 mol for each mole of NO3−-N removed. The overall removal rate of NO3−-N reached 1.1 mg L−1 h−1 with a biomass content of approximately 0.71 g L−1 within 72 h. TH20 participated in NO3−-N assimilation and aerobic denitrification. Results from 15N-labeled-nitrate test indicated that removed NO3−-N was assimilated into organic nitrogen, showing an assimilation efficiency of 58%. Seventeen amino acids were detected, accounting for 43% of the biomass. Nitrogen loss through aerobic denitrification was only approximately 42% of total nitrogen. This study suggests that TH20 can be applied in WWTP facilities for water purification and production of valuable biomass to mitigate CO2 and N2O emissions.

Dissolved Nitrous Oxide and Hydroxylamine in the South Yellow Sea and the East China Sea During Early Spring: Distribution, Production, and Emissions

Coastal marine systems are active regions for the production and emission of nitrous oxide (N2O), a potent greenhouse gas. Due to the inherently high variability in different coastal biogeochemical cycles, the factors and mechanisms regulating coastal N2O cycling remain poorly understood. Hydroxylamine (NH2OH), a potential precursor of N2O, has received less attention than other compounds in the coastal areas. Here, we present the spatial distribution of N2O and the first reported NH2OH distribution in the South Yellow Sea (SYS) and the East China Sea (ECS) between March and April 2017. The surface N2O concentrations in the SYS and the ECS varied from 5.9 to 11.3 nmol L–1 (average of 8.4 ± 1.4 nmol L–1) and were characterized by offshore and north–south decreasing gradients. NH2OH showed patchy characteristics and was highly variable, fluctuating between undetectable to 16.4 nmol L–1. We found no apparent covariation between N2O and NH2OH, suggesting the NH2OH pathway, i.e., nitrification (ammonium oxidation), was not the only process affecting N2O production here. The high NH2OH values co-occurred with the greatest chlorophyll-a and oxygen levels in the nearshore region, along with the relationships between NO2–, NO3–, and NH2OH, indicating that a “fresh” nitrifying system, favoring the production and accumulation of NH2OH, was established during the phytoplankton bloom. The high N2O concentrations were not observed in the nearshore. Based on the correlations of the excess N2O (ΔN2O) and apparent oxygen utilization, as well as ΔN2O vs. NO3–, we concluded that the N2O on the continental shelf was mainly derived from nitrification and nitrifier denitrification. Sea-to-air fluxes of N2O varied from −12.4 to 6.6 μmol m–2 d–1 (−3.8 ± 3.7 μmol m–2 d–1) using the Nightingale et al. (2000) formula and −13.3 to 6.9 μmol m–2 d–1 (−3.9 ± 3.9 μmol m–2 d–1) using the Wanninkhof (2014) formula, which corresponds to 75–112% in saturation, suggesting that the SYS and the ECS acted overall as a sink of atmospheric N2O in early spring, with the strength weakening. Our results reveal the factors and potential mechanisms controlling the production and accumulation of NH2OH and N2O in the SYS and the ECS during early spring.

Sulfide-driven nitrous oxide recovery during the mixotrophic denitrification process

We propose a novel sulfide-driven process to recover N2O during the traditional denitrification process. The optimum initial sulfide concentration was 120 mg/L, and the N2O percentage in the gaseous products (N2O+N2) was up to 82.9%. Moreover, sulfide involved in denitrification processes could substitute for organic carbon as an electron donor, e.g., 1 g sulfide was equivalent to 0.5-2 g COD when sulfide was oxidized to sulfur and sulfate. The accumulation of N2O was mainly due to the inhibiting effect of sulfide on nitrous oxide reductase (N2OR), which was induced by the supply insufficiency of electrons from cytochrome c (cyt c) to N2OR. When the initial sulfide concentration was 120 mg/L, the N2OR activity was only 36.8% of its original level. According to the results of cyclic voltammetry, circular dichroism spectra and fluorescence spectra, significant changes in the conformations and protein structures of cyt c were caused by sulfide, and cyt c completely lost its electron transport capacity. This study provides a new concept for N2O recovery driven by sulfide in the denitrification process. In addition, the findings regarding the mechanism of the inhibition of N2OR activity have important implications both for reducing emissions of N2O and recovering N2O in the sulfide-driven denitrification process.

Nitrite accumulation and impairment of N2O reduction explains contrasting soil denitrification phenotypes

Soil denitrification produces the potent greenhouse gas and ozone depleter nitrous oxide (N2O). We previously linked soil N2O emission potential (N2O/N2O + N2) to a continuum of denitrification phenotypes in 20 pasture soils but were unable to determine their proximal cause. At one end of the continuum, soils carried out completely concurrent production and reduction of N2O (concurrent phenotype) while at the other end soils delayed N2O reduction until almost all added nitrate (NO3−) was accumulated as N2O (sequential phenotype). In an unsealed environment, the later phenotype is predicted to emit most added N as N2O. Here we tested the role of delayed N2O reductase synthesis and nitrite (NO2−) based impairment of N2O reduction as determinants of delayed N2O reduction in soils with sequential phenotypes. Nitric oxide (NO), N2O and N2 accumulation were measured in response to added NO3−, NO2− and N2O in anoxic batch incubations of fresh or pre-incubated soil using automated gas chromatography. Successive NO3− additions drove 6 of 7 soils towards increasingly concurrent N2O production/reduction, suggesting delayed production of N2O reductase may be the cause of delayed N2O reduction in initially sequentially denitrifying soils. NO2− addition (2 mM NO3− + 1 mM NO2− vs. 3 mM NO3− control) to sequentially and concurrently denitrifying soil demonstrated that NO2− impairs N2O reduction, even when N2O reductase was fully induced (pre-incubated soils, 1 mM NO2− + 42 μmol N2O vs. 42 μmol N2O control). Further, 2 pieces of evidence suggest that differences in NO2− accumulation are a probable cause of the previously observed denitrification phenotypes. 1) 48 fold higher NO2− accumulation in the sequential vs. concurrent soil in response to 3 mM NO3− addition. 2) the concurrent timing of NO2− depletion/N2O reduction in the sequential soil. However, these observations should be confirmed in a greater number of soils. Inhibition of N2O reductase by NO is proposed as a potential proximal cause of impaired early N2O reduction which could link separate effectors (NO2−, carbon, successive NO3− additions) to a common inhibitory mechanism.

Short- and long-term effects of decabromodiphenyl ether (BDE-209) on sediment denitrification using a semi-continuous microcosm

The widespread use of decabromodiphenyl ether (BDE-209) resulted in its deposition in environmental media and biological matrices. However, to date, few studies focused on the effect of BDE-209 on microorganisms, and those available were investigated via an enclosed system completely cutting off the communication between testing system and its native environment. Herein, 4.0 mg/g BDE-209 acute exposure induced a 20% decline of NOX-N (the sum of NO3−–N and NO2−–N) removal efficiency and a significant accumulation of NO2−–N and N2O. These inhibitory effects presented in a BDE-209 concentration-dependent manner. Using a semi-continuous microcosm, the inhibitory effects of BDE-209 on denitrification were observed to be significantly enhanced with the extending of exposure duration. Denitrifying genes assay illustrated that BDE-209 has an insignificant effect on the global abundance of denitrifying bacteria because of microbial exchange with its overlying water. But the utilization of electron donor (carbon substrate), the activity of electron transport system and denitrifying enzymes were significantly inhibited by BDE-209 exposure in a exposure-duration-dependent manner. Finally, insufficient electron donor and lower efficiency of electron transport and utilization on denitrifying enzymes deteriorated the denitrification performance. These results provided a new insight into BDE-209 influence on denitrification in the natural environment.