Mechanism Of N2O Production Research Articles

Isotope analysis of nitrogen removal pathways and N2O production potential in the SDAD-anammox system under different N/S ratios

This study explored the impact of varying nitrate to sulfide (N/S) ratios on nitrogen removal efficiency (NRE) in the sulfide-driven autotrophic denitrification and anammox (SDAD-anammox) system. Optimal nitrogen removal was observed at N/S ratios between 1.5 and 2.0. Isotope tracing results showed that the contribution of anammox to nitrogen removal was enhanced with increasing N/S ratios, reaching up to 37 % at the N/S ratio of 2.5. Additionally, complex nitrogen pathways were identified, including dissimilatory nitrate reduction to ammonium (DNRA). Furthermore, isotope tracing was innovatively applied to investigate N2O emissions, demonstrating that higher N/S ratios significantly reduced N2O emissions, with the lowest emissions at N/S ratio of 2.5. Gene expression analysis indicated that nitrogen and sulfide transformation genes decreased with increasing N/S ratios, while anammox-related genes first increased and then decreased, reflecting the system's microbial dynamics. These findings offer insights into nitrogen transformation pathways and N2O production mechanisms in the SDAD-anammox process.

Deciphering the functional importance of comammox vs. canonical ammonia oxidisers in nitrification and N2O emissions in acidic agricultural soils

The discovery of comammox Nitrospira has altered our perception of the nitrogen biogeochemical cycle. However, their functional importance compared to canonical ammonia oxidisers (i.e., ammonia-oxidising bacteria (AOB) and archaea (AOA)) in agricultural soils remains elusive, especially in acidic soils. Here, we assessed the functional importance of these functional guilds in nitrification and nitrous oxide (N2O) emissions in three acidic agricultural soils by using a range of nitrification inhibitors (acetylene, 3,4-dimethylpyrazole phosphate (DMPP) and different concentrations of 1-octyne) and monitored their community assemblage and population dynamics. The sensitivity of comammox Nitrospira clade A to 1-octyne varied across soils, highlighting that the inappropriate use of 1-octyne can lead to misestimation of comammox activity. AOA were key NH3 oxidisers in the three soils, while AOB also contributed significantly to nitrification in one soil. In contrast, comammox Nitrospira always played a minor role in ammonia oxidation and N2O emissions, likely due to their low abundances, restricted cellular kinetic properties and N2O production mechanisms. Together, this study demonstrates that comammox Nitrospira play a less important role in ammonia oxidation and N2O production in acidic agricultural soils than AOA and AOB, thereby providing important novel insights into the mitigation of nitrogen fertiliser loss and N2O emissions.

How does microorganism in different zones cooperatively promote N2O emissions from SWIS during freeze-thaw?

Subsurface wastewater infiltration systems (SWIS) are environmentally-friendly technologies for domestic wastewater treatment, where pollutants are removed by physical, chemical and biological reactions. However, SWIS also produce nitrous oxide (N2O), a potent greenhouse gas. Distribution of dissolved oxygen and nitrogen in SWIS determines denitrification process, which affects microbial activity and N2O release degree in different layers of system. Top layer of SWIS substrate is exposed to environmental factors such as freeze-thaw (FT), which changes microbial community structure in different substrates. Exact mechanisms of microbial-mediated N2O emissions in SWIS are still unclear despite extensive research. Therefore, this study simulated FT process using in-situ SWIS, to investigate how FT disturbance affects microbial community structure and N2O release in SWIS profiles. Results showed that after the ninth freeze-thaw cycle, FT stimulated anaerobic bacteria activities such as Euryarchaeota, accounting for 78.4 % of total Euryarchaeota population in middle (60 cm) and 33.97 % in the lower layer. Under low oxygen conditions, NO2−-N accumulation in middle and lower layers provided a sufficient nitrogen source for Euryarchaeota. Canonical correlation analysis (CCA) showed Euryarchaeota was significantly correlated with N2O emissions in middle and lower layers during FT, contributing 31.68 %–32.01 % and 61.78 %–65.15 %, respectively. These results suggested that FT disturbance enhanced denitrification by anaerobic bacteria in middle and lower layers of SWIS, significantly increasing N2O emissions. However, specific pathways and mechanisms of N2O production by Euryarchaeota remain to be elucidated in future studies.

The Influencing Mechanisms of Reclaimed Water on N2O Production in a Multiyear Maize–Wheat Rotation

Reclaimed water (RW) is widely used in agricultural systems; however, it affects soil properties and the surrounding environment, thus influencing soil nitrogen transformation and increasing N2O and NO emissions. Understanding the influencing mechanism of N2O production in RW-irrigated soil is very important for water resource utilization and environmental protection, but it is rarely studied. This study investigated the impact of three nitrogen ions (NH4+, NO3−, NO2−) on the nitrogen transformation process and non-biological processes affecting NO and N2O emissions from soil under multiyear RW-irrigated conditions. The results showed that RW effectively increased the abundance of nitrifying and denitrifying functional genes, leading to a significant increase (p < 0.05) in soil NO and N2O emissions under ammonium treatment. Furthermore, RW can reduce the cumulative NH3 emission by 19.11% compared to deionized water (DW). In nitrate treatment, RW can significantly increase (p < 0.05) the nitrate conversion rate by increasing the abundance of denitrifying genes, but not significantly enhance N2O and NO emissions. In NO2− oxidation, RW could increase the abundance of nitrifying genes (AOA-amoA, AOB-amoA), thereby promoting the progression of nitrifier denitrification and leading to a substantial increase (p < 0.05) in soil N2O production. In summary, RW irrigation primarily increases N2O emissions from soil by enhancing soil autotrophic nitrification and heterotrophic nitration. To effectively control soil N2O emissions under agricultural irrigation with RW, it is crucial to carefully manage soil nitrification and adjust the ratio of ammonium and nitrate in the soil.

Spatiotemporal variations and dominated environmental parameters of nitrous oxide (N2O) concentrations from cascade reservoirs in southwest China.

Anthropogenic activity has caused rivers and reservoirs to become sources of nitrous oxide (N2O), which is thought to play an important role in global climate change. There are thermal and DO stratification in deep-water reservoirs with long hydraulic retention time, which change N2O production mechanism compared with shallow-water reservoirs. To promote our understanding of the relationship of N2O production in reservoirs at different depths, spatiotemporal variations in water environmental factors and N2O from cascade reservoirs of Chaishitan (CST), Longtan (LT), Yantan (YT) and Dahua (DH) reservoirs in the Zhujiang River were detected, and the LT and YT reservoirs were compared as representatives of deep-water and shallow-water reservoirs in April and July 2019. The average N2O concentrations in the LT and YT reservoirs were 22.82 ± 2.21 and 21.55 ± 1.65nmol L-1, respectively. During spring and summer, the WT (water temperature) and DO (dissolved oxygen) concentrations in the YT reservoir were well mixed. In contrast, the LT reservoir, as a deep-water reservoir, had thermal and DO stratifications in both the shallow and middle water, especially in the summer when the solar radiation intensity was high. During summer stratification, the DO concentration in the LT reservoir showed obvious spatial variation, ranging from 1.23 to 9.84mg L-1, while the DO concentration in the YT reservoir showed very little variation, ranging from 6.45 to 7.09mg L-1. Structural equation modeling results showed that NH4+ was the main determinant of the N2O concentration in the YT reservoir, and DO was the most influential factor of the N2O concentration in the LT reservoir. These results suggest significant variations in the factors influencing N2O concentration among reservoirs. Additionally, the mechanisms of N2O production differ between deep-water and shallow-water reservoirs. This study highlights the spatio-temporal variations and influential factors contributing to N2O concentration. Furthermore, it discusses the production mechanisms of N2O in different types of reservoirs. These findings contribute to our understanding of N2O distribution in hydropower systems and provide valuable data for the management of hydropower facilities and research on greenhouse gas emissions.

Organic substitution stimulates ammonia oxidation-driven N2O emissions by distinctively enriching keystone species of ammonia-oxidizing archaea and bacteria in tropical arable soils

Partial organic substitution (POS) is pivotal in enhancing soil productivity and changing nitrous oxide (N2O) emissions by profoundly altering soil nitrogen (N) cycling, where ammonia oxidation is a fundamental core process. However, the regulatory mechanisms of N2O production by ammonia oxidizers at the microbial community level under POS regimes remain unclear. This study explored soil ammonia oxidation and related N2O production, further building an understanding of the correlations between ammonia-oxidizing archaea (AOA) and ammonia-oxidizing bacteria (AOB) activity and community structure in tropical arable soils under four-year field management regimes (CK, without fertilizer N; N, with only inorganic N; M1N1, with 1/2 organic N + 1/2 inorganic N; M1N2, with 1/3 organic N + 2/3 inorganic N). AOA contributed more to potential ammonia oxidation (PAO) than AOB across all treatments. In comparison with CK, N treatment had no obvious effects on PAO and lowered related N2O emissions by decreasing soil pH and downregulating the abundance of AOA- and AOB-amoA. POS regimes significantly enhanced PAO and N2O emissions relative to N treatment by promoting the abundances and contributions of AOA and AOB. The stimulated AOA-dominated N2O production under M1N1 was correlated with promoted development of Nitrososphaera. By contrast, the increased AOB-dominated N2O production under M1N2 was linked to the enhanced development of Nitrosospira multiformis. Our study suggests organic substitutions with different proportions of inorganic and organic N distinctively regulate the development of specific species of ammonia oxidizers to increase associated N2O emissions. Accordingly, appropriate options should be adopted to reduce environmental risks under POS regimes in tropical croplands.

Increased Nitrogen Loading Facilitates Nitrous Oxide Production through Fungal and Chemodenitrification in Estuarine and Coastal Sediments.

Estuarine and coastal environments are assumed to contribute to nitrous oxide (N2O) emissions under increasing nitrogen loading. However, isotopic and molecular mechanisms underlying N2O production pathways under elevated nitrogen concentration remain poorly understood. Here we used microbial inhibition, isotope mass balance, and molecular approaches to investigate N2O production mechanisms in estuarine and coastal sediments through a series of anoxic incubations. Site preference of the N2O molecule increased due to increasing nitrate concentration, suggesting the changes in N2O production pathways. Enhanced N2O production under high nitrate concentration was not mediated by bacterial denitrification, but instead was mainly regulated by fungal denitrification. Elevated nitrate concentration increased the contribution of fungal denitrification to N2O production by 11-25%, whereas it decreased bacterial N2O production by 16-33%. Chemodenitrification was also enhanced by high nitrate concentration, contributing to 13-28% of N2O production. Elevated nitrate concentration significantly mediated nirK-type denitrifiers structure and abundance, which are the keystone taxa driving N2O production. Collectively, these results suggest that increasing nitrate concentration can shift N2O production pathways from bacterial to fungal and chemodenitrification, which are mainly responsible for the enhanced N2O production and have widespread implications for N2O projections under ongoing nitrogen pollution in estuarine and coastal ecosystems.

Contribution of Ammonium-Induced Nitrifier Denitrification to N2O in Paddy Fields.

Paddy fields are one of the most important sources of nitrous oxide (N2O), but biogeochemical N2O production mechanisms in the soil profile remain unclear. Our study used incubation, dual-isotope (15N-18O) labeling methods, and molecular techniques to elucidate N2O production characteristics and mechanisms in the soil profile (0-60 cm) during summer fallow, rice cropping, and winter fallow periods. The results pointed out that biotic processes dominated N2O production (72.2-100%) and N2O from the tillage layer accounted for 91.0-98.5% of total N2O in the soil profile. Heterotrophic denitrification (HD) was the main process generating N2O, contributing between 53.4 and 96.6%, the remainder being due to ammonia oxidation pathways, which was further confirmed by metagenomics and quantitative polymerase chain reaction (qPCR) assays. Nitrifier denitrification (ND) was an important N2O production source, contributing 0-46.6% of total N2O production, which showed similar trends with N2O emissions. Among physicochemical and biological factors, ammonium content and the ratio of total organic matter to nitrate were the main driving factors affecting the contribution ratios of the ammonia oxidation pathways and HD pathway, respectively. Moisture content and pH affect norC-carrying Spirochetes and thus the N2O production rate. These findings confirm the importance of ND to N2O production and help to elucidate the impact of anthropogenic activities, including tillage, fertilization, and irrigation, on N2O production.

Distribution and Production of N2O in the Subtropical Western North Pacific Ocean During the Spring of 2020

Nitrous oxide (N2O) is an important greenhouse gas emitted in significant volumes by the Pacific Ocean. However, the relationship between N2O dynamics and environmental drivers in the subtropical western North Pacific Ocean (STWNPO) remains poorly understood. We investigated the distribution of N2O and its production as well as the related mechanisms at the surface (0–200 m), intermediate (200–1500 m), and deep (1500–5774 m) layers of the STWNPO, which were divided according to the distribution of water masses. We applied the transit time distribution (TTD) method to determine the ventilation times, and to estimate the N2O equilibrium concentration of water parcels last in contact with the atmosphere prior to being ventilated. In the surface layer, biologically derived N2O (ΔN2O) was positively correlated with the apparent oxygen utilization (AOU) (R2 = 0.48), suggesting that surface N2O may be produced by nitrification. In the intermediate layer, ΔN2O was positively correlated with AOU and NO3− (R2 = 0.92 and R2 = 0.91, respectively) and negatively correlated with nitrogen sinks (N*) (R2 = 0.60). Hence, the highest ΔN2O value in the oxygen minimum layer suggested N2O production through nitrification and potential denitrification (up to 51% and 25% of measured N2O, respectively). In contrast, the deep layer exhibited a positive correlation between ΔN2O and AOU (R2 = 0.92), suggesting that the N2O accumulation in this layer may be caused by nitrification. Our results demonstrate that the STWNPO serves as an apparent source of atmospheric N2O (mean air−sea flux 2.0 ± 0.3 μmol m-2 d-1), and that nitrification and potential denitrification may be the primary mechanisms of N2O production in the STWNPO. We predict that ongoing ocean warming, deoxygenation, acidification, and anthropogenic nitrogen deposition in the STWNPO may elevate N2O emissions in the future. Therefore, the results obtained here are important for elucidating the relationships between N2O dynamics and environmental changes in the STWNPO and the global ocean.

Identifying the Sources and Drivers of Nitrous Oxide Accumulation in the Eddy‐Influenced Eastern Tropical North Pacific Oxygen‐Deficient Zone

AbstractNitrous oxide (N2O) is a powerful greenhouse gas, and oceanic sources account for up to one third of the total natural flux to the atmosphere. In oxygen‐deficient zones (ODZs) like the Eastern Tropical North Pacific (ETNP), N2O can be produced and consumed by several biological processes. In this study, the concentration and isotopocule ratios of N2O from a 2016 cruise in the ETNP were analyzed to examine sources of and controls on N2O cycling across this region. Along the north‐south transect, three distinct biogeochemical regimes were identified: background, core‐ODZ, and high‐N2O stations. Background stations were characterized by smaller variations in N2O concentration and isotopic profiles relative to the other regimes. Core‐ODZ stations were characterized by co‐occurring N2O production and consumption at anoxic depths, indicated by high δ18O‐N2O (>90‰) and low δ15N2Oβ (<−10‰) values, and confirmed by a time‐dependent model, which indicated that N2O production via denitrification was significant and may occur with a nonzero site preference. High‐N2O stations, located at the periphery of a mesoscale eddy, were defined by N2O reaching 126.07 ± 12.6 nM and low oxygen concentrations expanding into near‐surface isopycnals. At these stations, model results indicated significant N2O production from ammonia‐oxidizing archaea and denitrification from nitrate at the N2O maximum within the oxycline, while bacterial nitrification and denitrification from nitrite were insignificant. This study also represents the first in the ETNP to link N2O production mechanisms to a mesoscale eddy through isotopocule measurements, suggesting the importance of eddies to spatiotemporal variability in N2O cycling and emissions from this region.

Low nitrous oxide concentration and spatial microbial community transition across an urban river affected by treated sewage

Urban rivers receive used water derived from anthropogenic activities and are a crucial source of the potent greenhouse gas nitrous oxide (N2O). However, considerable uncertainties still exist regarding the variation and mechanisms of N2O production in response to the discharge of treated sewage from municipal wastewater treatment plants (WWTPs). This study investigated N2O concentrations and microbial processes responsible for nitrogen conversion upstream and downstream of WWTPs along the Tama River flowing through Tokyo, Japan. We evaluated the effect of treated sewage on dissolved N2O concentrations and inherent N2O consumption activities in the river sediments. In summer and winter, the mean dissolved N2O concentrations were 0.67 µg-N L−1 and 0.82 µg-N L−1, respectively. Although the dissolved N2O was supersaturated (mean 288.7% in summer, mean 240.7% in winter) in the river, the N2O emission factors (EF5r, 0.013%–0.025%) were significantly lower than those in other urban rivers and the Intergovernmental Panel on Climate Change default value (0.25%). The nitrate (NO3−) concentration in the Tama River increased downstream of the WWTPs discharge sites, and it was the main nitrogen constituent. An increasing trend of NO3− concentration was observed from upstream to downstream, along with an increase in the N2O consumption potential of the river sediment. A multiple regression model showed that NO3− is the crucial factor influencing N2O saturation. The diversity in the upstream microbial communities was greater than that in the downstream ones, indicating the involvement of treated sewage discharge in shaping the microbial communities. Functional gene quantification for N2O production and consumption suggested that nirK-type denitrifiers likely contributed to N2O production. Structural equation models (SEMs) revealed that treated sewage discharged from WWTPs increased the NO3− loading from upstream to downstream in the river, inducing changes in the microbial communities and enhancing the N2O consumption activities. Collectively, aerobic conditions limited denitrification and in turn facilitated nitrification, leading to low N2O emissions even despite high NO3− loadings in the Tama River. Our findings unravel an overestimation of the N2O emission potential in an urban oxygen-rich river affected by treated sewage discharge.

Mechanism of nitrous oxide (N2O) production during thermal stratification of a karst, deep-water reservoir in southwestern China

Reservoirs are hotspots of greenhouse gas (GHG) emission, and the global warming potential of nitrous oxide (N2O) is higher than that of CO2 and CH4. To promote our understanding of the relationship of biotic and abiotic environments with the mechanism of N2O production in a quintessential karst reservoir, water samples were collected in a weak thermal stratification period (WTSP) and a strong thermal stratification period (STSP) in 2019 in the Longtan (LT) Reservoir. In the LT Reservoir, higher N2O concentrations and functional gene abundances (nitrification and denitrification genes) were found in the STSP than in the WTSP. In detail, the N2O concentrations were 10.45 ± 6.99 nmol L−1 and 23.77 ± 7.06 nmol L−1 in the WTSP and STSP, respectively; the functional gene abundances were higher in the STSP than in the WTSP in each water layer. During the STSP, the nitrification gene abundances were higher at a 30 m depth than at a 100 m depth, while denitrification gene abundances were higher at 100 m than at 30 m. Along the river, N2O flux was significantly higher in the LT Reservoir’s released water than in upstream water in the STSP, while it exhibited no significant difference in whole river-reservoir system in the WTSP. In conclusion, stratification and nitrogen (N) loading facilitated higher N2O production in STSP than in WTSP; different N transformation processes and sources of N2O were present between the water layers above and below the thermocline (in deep water); finally, the river-reservoir system here acted as a source of N2O release to the atmosphere in the STSP, while it exhibited different emission patterns in the WTSP. This work highlights the mechanism of N2O production and functional gene abundances in water layers during different thermal stratification periods, which will help to elucidate the biotic and abiotic environment in a karst deep-water reservoir and will help to inform N2O production mechanisms in other reservoirs worldwide. More sampling campaigns and the denitrification potential of sediment were worthy of conducting to comprehend the mechanisms of N2O production from stratification to turnover period in the reservoir.

The contrasting effects of biochar and straw on N2O emissions in the maize season in intensively farmed soil.

This study evaluated the combined effects of biochar and straw on N2O flux and the community compositions of nitrifiers and denitrifiers in the maize season in an intensively farmed area in northern China. The experiment consisted of four treatments: (1) CK (only chemical fertilizer application); (2) C (biochar application); (3) SR (straw application to the field); and (4) C+SR (the application of both biochar and straw). The results indicated that during the maize growing season, N2O flux decreased by 30.3% in the C treatment and increased by 13.2% and 37.0% in the SR and C+SR treatments compared with CK, respectively. NO3--N, NH4+-N, and microbial biomass carbon (MBC) were the main soil factors affecting N2O flux, and they were positively correlated with NO3--N and negatively correlated with MBC in the C treatment and positively correlated with NH4+-N in the SR and C+SR treatments. Both biochar addition and straw return shifted the community compositions of nitrifiers and denitrifiers. N2O production was mainly reduced by promoting the ammonia-oxidizing bacteria (AOB) gene abundance and inhibiting the nirK gene abundance in the C treatment but promoted by inhibiting the AOB and nosZ gene abundances in the SR and C+SR treatments. Nitrosospira (AOB) and Rhizobium (nirK) were the main contributors among the treatments. NO3--N, NH4+-N, and MBC were the main soil factors affecting the denitrifier communities. The predominant species associated with the nirK, nirS, and nosZ genes were positively correlated with NO3--N and MBC and negatively correlated with NH4+-N. These results provide valuable information on the mechanism of N2O production and reduction in biochar- and straw-amended soil under field conditions.

Potential Pathway of Nitrous Oxide Formation in Plants.

Plants can produce and emit nitrous oxide (N2O), a potent greenhouse gas, into the atmosphere, and several field-based studies have concluded that this gas is emitted at substantial amounts. However, the exact mechanisms of N2O production in plant cells are unknown. Several studies have hypothesised that plants might act as a medium to transport N2O produced by soil-inhabiting microorganisms. Contrarily, aseptically grown plants and axenic algal cells supplied with nitrate (NO3) are reported to emit N2O, indicating that it is produced inside plant cells by some unknown physiological phenomena. In this study, the possible sites, mechanisms, and enzymes involved in N2O production in plant cells are discussed. Based on the experimental evidence from various studies, we determined that N2O can be produced from nitric oxide (NO) in the mitochondria of plants. NO, a signaling molecule, is produced through oxidative and reductive pathways in eukaryotic cells. During hypoxia and anoxia, NO3 in the cytosol is metabolised to produce nitrite (NO2), which is reduced to form NO via the reductive pathway in the mitochondria. Under low oxygen condition, NO formed in the mitochondria is further reduced to N2O by the reduced form of cytochrome c oxidase (CcO). This pathway is active only when cells experience hypoxia or anoxia, and it may be involved in N2O formation in plants and soil-dwelling animals, as reported previously by several studies. NO can be toxic at a high concentration. Therefore, the reduction of NO to N2O in the mitochondria might protect the integrity of the mitochondria, and thus, protect the cell from the toxicity of NO accumulation under hypoxia and anoxia. As NO3 is a major source of nitrogen for plants and all plants may experience hypoxic and anoxic conditions owing to soil environmental factors, a significant global biogenic source of N2O may be its formation in plants via the proposed pathway.

Algal photosynthesis converts nitric oxide into nitrous oxide

Nitrous oxide (N2O), a potent greenhouse gas in the atmosphere, is produced mostly from aquatic ecosystems, to which algae substantially contribute. However, mechanisms of N2O production by photosynthetic organisms are poorly described. Here we show that the green microalga Chlamydomonas reinhardtii reduces NO into N2O using the photosynthetic electron transport. Through the study of C. reinhardtii mutants deficient in flavodiiron proteins (FLVs) or in a cytochrome p450 (CYP55), we show that FLVs contribute to NO reduction in the light, while CYP55 operates in the dark. Both pathways are active when NO is produced in vivo during the reduction of nitrites and participate in NO homeostasis. Furthermore, NO reduction by both pathways is restricted to chlorophytes, organisms particularly abundant in ocean N2O-producing hot spots. Our results provide a mechanistic understanding of N2O production in eukaryotic phototrophs and represent an important step toward a comprehensive assessment of greenhouse gas emission by aquatic ecosystems.

Impact of nitrogen compounds on fungal and bacterial contributions to codenitrification in a pasture soil

Ruminant urine patches on grazed grassland are a significant source of agricultural nitrous oxide (N2O) emissions. Of the many biotic and abiotic N2O production mechanisms initiated following urine-urea deposition, codenitrification resulting in the formation of hybrid N2O, is one of the least understood. Codenitrification forms hybrid N2O via biotic N-nitrosation, co-metabolising organic and inorganic N compounds (N substrates) to produce N2O. The objective of this study was to assess the relative significance of different N substrates on codenitrification and to determine the contributions of fungi and bacteria to codenitrification. 15N-labelled ammonium, hydroxylamine (NH2OH) and two amino acids (phenylalanine or glycine) were applied, separately, to sieved soil mesocosms eight days after a simulated urine event, in the absence or presence of bacterial and fungal inhibitors. Soil chemical variables and N2O fluxes were monitored and the codenitrified N2O fluxes determined. Fungal inhibition decreased N2O fluxes by ca. 40% for both amino acid treatments, while bacterial inhibition only decreased the N2O flux of the glycine treatment, by 14%. Hydroxylamine (NH2OH) generated the highest N2O fluxes which declined with either fungal or bacterial inhibition alone, while combined inhibition resulted in a 60% decrease in the N2O flux. All the N substrates examined participated to some extent in codenitrification. Trends for codenitrification under the NH2OH substrate treatment followed those of total N2O fluxes (85.7% of total N2O flux). Codenitrification fluxes under non-NH2OH substrate treatments (0.7–1.2% of total N2O flux) were two orders of magnitude lower, and significant decreases in these treatments only occurred with fungal inhibition in the amino acid substrate treatments. These results demonstrate that in situ studies are required to better understand the dynamics of codenitrification substrates in grazed pasture soils and the associated role that fungi have with respect to codenitrification.

Dominance of nitrous oxide production by nitrification and denitrification in the shallow Chaohu Lake, Eastern China: Insight from isotopic characteristics of dissolved nitrous oxide

In recent decades, most lakes in Eastern China have suffered unprecedented nitrogen pollution, making them potential “hotspots” for N2O production and emission. Understanding the mechanisms of N2O production and quantifying emissions in these lakes is essential for assessing regional and global N2O budgets and for mitigating N2O emissions. Here, we measure isotopic compositions (δ15N-N2O and δ18O-N2O) and site preference (SP) of dissolved N2O in an attempt to differentiate the relative contribution of N2O production processes in the shallow, eutrophic Chaohu Lake, Eastern China. Our results show that the bulk isotope ratios for δ15N-N2O, δ18O-N2O, and SP were 5.8 ± 3.9‰, 29.3 ± 13.4‰, and 18.6 ± 3.2‰, respectively. More than 76.8% of the dissolved N2O was produced via microbial processes. Findings suggest that dissolved N2O is primarily produced via nitrification (between 27.3% and 48.0%) and denitrification (between 31.9% and 49.5%). In addition, isotopic data exhibit significant N2O consumption during denitrification. We estimate the average N2O emission rate (27.5 ± 26.0 μg N m−2 h−1), which is higher than that from rivers in the Changjiang River network (CRN). We scaled-up the regional N2O emission (from 1.98 Gg N yr−1 to 4.58 Gg N yr−1) using a N2O emission factor (0.51 ± 0.63%) for shallow lakes in the middle and lower region of the CRN. We suggest that beneficial circumstances for promoting complete denitrification may be helpful for reducing N2O production and emissions in fresh surface waters.

Effect of Nitrification on N2O Emissions and Their Environmental Factors in Saline-alkali Wetlands

Wetlands are important sources and sinks for N2O. Exploring the role of N2O emissions in saline-alkali wetlands has great significance in understanding the nitrification mechanism of N2O production and assessing the role of saline-alkali wetlands in the greenhouse effect. The present study examined the N2O fluxes and environmental factors of a typical Zhalong reed wetland during the growing season. The results suggested that the N2O fluxes tended to decrease in volatility, with the highest value in mid-July. The mean flux of N2O was (37.49±15.75) μg·(m2·h)-1, indicating that the typical Zhalong reed wetland was a source of N2O. The N2O fluxes exhibited a significantly positive correlation with soil temperature at different depths (P<0.05), and the impact of the upper soil temperature on N2O flux was higher than that of deep soil. In the flooding period, the relationship between N2O fluxes and water table depth was negatively correlated (P<0.05). Meanwhile, the TOC and TN contents were lower, and the N2O flux was significantly positively correlated with the NH4+-N content in the 0-40 cm soil layer (P<0.05), but it was not related to NO3--N content. Nitrification was stronger than denitrification. There was a significant positive correlation between ammonia-oxidizing bacterial activity and soil temperature in 0-20 cm layer (P<0.01). Additionally, the activity of ammonia-oxidizing bacteria also presented significantly positive linear correlation with the N2O fluxes (P<0.001), which indicated that the release of N2O in saline-alkali wetlands was greatly affected by nitrification.

N2O emission and control strategy in different wastewater treatment processes

N2O (Nitrous oxide) is a strong greenhouse gas, and wastewater treatment process is one of the important artificial sources of N2O. Controlling N2O production and discharge in wastewater treatment process is beneficial to alleviate the greenhouse effect. Based on the mechanism of N2O production in biological nitrogen removal process, emission characteristics and influencing factors of N2O in three traditional biological nitrogen removal processes, A/A/O, SBR and oxidation ditch, and two new biological nitrogen removal processes, SBBR and CANON, have been reviewed. Furthermore, a control strategy for slowing the generation and release of N2O is proposed, which provides a reference for researchers in related field.

Evidences of N2O Emissions in Chloropicrin-Fumigated Soil.

The mechanism of N2O production following chloropicrin (CP) fumigation was investigated in this study. Our results showed that CP fumigation increased N2O production from 23 to 25 times in comparison with the control and significantly decreased the abundance of 16S rRNA and N-cycling functional genes. CP also decreased the soil bacterial diversity and caused a shift in the community composition. The N2O emissions in fumigated soil were significantly correlated with soil environmental factors (NH4+, dissolved amino acid, microbial biomass nitrogen, and NO3-) but were not correlated with the abundance of functional genes. Metatranscriptomes and dual-label 15N-18O isotope analysis revealed that CP fumigation inhibited the expression of gene families involved in N2O production and sink processes and shifted the main pathway of N2O production from nitrification to denitrification. These results provided useful information for environmental safety assessments of CP in China, to improve our understanding of the N-cycling pathways in fumigated soils.