Dominant N2O Research Articles

Identifying locations of sewage pollution within a Hawaiian watershed for coastal water quality management actions

Study regionPuakō, Hawai‘i Island. Study focusLocations of sewage pollution in the Puakō watershed were identified through measurements of sewage indicators at groundwater wells and within Puakō’s and adjacent resorts’ shoreline waters. Dye tracer tests, water quality, δ15N macroalgal, and δ15N- and δ18O-NO3- measurements, along with stable isotope mixing models, were combined to assess water quality impairment caused by different Onsite Sewage Disposal System (OSDS) types, and used to predict water quality improvements from future management actions. New hydrological insights for the regionSewage indicators were highest within Puakō’s shoreline waters, including: Enterococcus spp., Clostridium perfringens, human-associated Bacteroides, and δ15N-NO3-. Mixing model results using δ15N- and δ18O-NO3- suggest that sewage was a dominant NO3- source, comprising > 40% at 10 of the 16 shoreline stations. δ15N macroalgae measurements confirmed presence of sewage at most stations. In groundwater wells and at adjacent resorts’ shoreline waters, sewage indicators were low, and δ15NO3-was indicative of soils and fertilizers. Puakō dye tracer tests revealed that sewage reached the shoreline within 5 h to 10 d, and that OSDS type did not affect travel time. Water quality was similar in front of homes with different OSDS. In conclusion, sewage is entering the groundwater at Puakō, and the underlying geology, rather than OSDS type, primarily controls the speed at which sewage reaches the shoreline. Our findings highlight the need for improved sewage treatment and collection at Puakō.

Source partitioning using N2O isotopomers and soil WFPS to establish dominant N2O production pathways from different pasture sward compositions.

Nitrous oxide (N2O) is a potent greenhouse gas (GHG) emitted from agricultural soils and is influenced by nitrogen (N) fertiliser management and weather and soil conditions. Source partitioning N2O emissions related to management practices and soil conditions could suggest effective mitigation strategies. Multispecies swards can maintain herbage yields at reduced N fertiliser rates compared to grass monocultures and may reduce N losses to the wider environment. A restricted-simplex centroid experiment was used to measure daily N2O fluxes and associated isotopomers from eight experimental plots (7.8 m2) post a urea-N fertiliser application (40 kg N ha-1). Experimental pastures consisted of differing proportions of grass, legume and forage herb represented by perennial ryegrass (Lolium perenne), white clover (Trifolium repens) and ribwort plantain (Plantago lanceolata), respectively. N2O isotopomers were measured using a cavity ring down spectroscopy (CRDS) instrument adapted with a small sample isotope module (SSIM) for the analysis of gas samples ≤20 mL. Site preference (SP = δ15Nα - δ15Nβ) and δ15Nbulk ((δ15Nα + δ15Nβ) / 2) values were used to attribute N2O production to nitrification, denitrification or a mixture of both nitrification and denitrification over a range of soil WFPS (%). Daily N2O fluxes ranged from 8.26 to 86.86 g N2O-N ha-1 d-1. Overall, 34.2% of daily N2O fluxes were attributed to nitrification, 29.0% to denitrification and 36.8% to a mixture of both. A significant diversity effect of white clover and ribwort plantain on predicted SP and δ15Nbulk indicated that the inclusion of ribwort plantain may decrease N2O emission through biological nitrification inhibition under drier soil conditions (31%-75% WFPS). Likewise, a sharp decline in predicted SP indicates that increased white clover content could increase N2O emissions associated with denitrification under elevated soil moisture conditions (43%-77% WFPS). Biological nitrification inhibition from ribwort plantain inclusion in grassland swards and management of N fertiliser source and application timing to match soil moisture conditions could be useful N2O mitigation strategies.

Contribution of pathogenic fungi to N2O emissions increases temporally in intensively managed strawberry cropping soil.

Intensively managed agriculture land is a significant contributor to nitrous oxide (N2O) emissions, which adds to global warming and the depletion of the ozone layer. Recent studies have suggested that fungal dominant N2O production may be promoted by pathogenic fungi under high nitrogen fertilization and continuous cropping. Here, we measured the contribution of fungal communities to N2O production under intensively managed strawberry fields of three continuous cropping years (1, 5, and 10 years) and compared this adjacent bare soil. Higher N2O emission was observed from the 10-year field, of which fungi and prokaryotes accounted for 79.7% and 21.3%, respectively. Fungal population density in the 10-year field soil (4.25 × 105 colony forming units per g (CFU/g) of air-dried soil) was greater than the other cropping years. Illumina MiSeq sequencing of the nirK gene showed that long-term continuous cropping decreased the diversity of the fungal denitrifier community, but increased the abundance of Fusarium oxysporum. Additionally, F. oxysporum produced large amounts of N2O in culture and in sterile 10-year field soil. A systemic infection displayed by bioassay strawberry plants after inoculation demonstrated that F. oxysporum was a pathogenic fungus. Together, results suggest that long-term intensively managed monocropping significantly influenced the denitrifying fungal community and increased their biomass, which increased fungal contribution to N2O emissions and specifically by pathogenic fungi. KEY POINTS: • Distinguishing the role of fungi in long-term continuous cropping field. • Identifying the abundant fungal species with denitrifying ability.

Comparative research on visibility and light extinction of PM2.5 components during 2014–17 in the North China plain: 2014-2017年华北平原PM2.5组分消光与能见度对比研究

Severe air pollution with visibility deterioration has long been a focus in the North China Plain (NCP). In this study, concentration and light extinction analysis of PM2.5 chemical components were carried out from 2014 to 2017 to study the pollution characteristics in Baoding, a case city of the NCP. The annual average concentration of total PM2.5 components showed a declining trend, decreasing by 11 µg m−3 (water-soluble inorganic ions), 23 µg m−3 (carbonaceous aerosols), and 1796 ng m−3 (inorganic elements). Contributing 82.9% to the concentration of total ions, the dominant components, NH4+, NO3−, and SO42− became the main pollutants in PM2.5 pollution. Based on the IMPROVE algorithm, the average reconstructed PM2.5 mass concentration was 93 ± 69 µg m−3 during the observation period. Meanwhile, the light extinction coefficients were 373.8 ± 233.6 M m−1, 405.3 ± 300.1 M m−1, 554.3 ± 378.2 M m−1 and 1005.2 ± 750.3 M m−1, in spring, summer, autumn, and winter, respectively. Ammonium sulfate, ammonium nitrate, and organic matter were the largest contributors to light extinction, accounting for a total of 55%–77% in the four seasons. The bsca (light scattering by particles and gases) reconstructed from PM2.5 components (Rbsca) and the bsca converted from visibility (Vbsca) were compared to evaluate the performance of the IMPROVE algorithm, revealing a high correlation coefficient of 0.84. The high values of Vbsca were underestimated while the low values were overestimated, as determined through comparison with the one-to-ne line. Especially, when Rbsca > 1123 M m−1 (corresponding to < 2.0 km, approximately), Vbsca was underestimated by 17.6%. PM2.5 mass concentration and relative humidity also had an impact on the estimation.摘要华北平原大气污染与低能见度状况一直是人们关切的问题.本文通过分析2014 - 2017年PM2.5化学成分的浓度和消光效果, 研究了华北平原典型城市保定市的大气污染特征.结果表明, PM2.5组分的年均浓度显示下降趋势, 水溶性无机离子,碳质气溶胶和金属元素分别减少了11 µg m−3, 23 µg m−3和1796 ng m−3.NH4+,NO3−和SO42−是PM2.5污染的主要污染物, 三者之和占总离子浓度的82.9%.基于IMPROVE方程对细颗粒物进行重构, 在观测期间PM2.5质量浓度平均为93 ± 69 µg m−3, 春季,夏季,秋季和冬季的消光系数分别为373.8 ± 233.6 M m−1,405.3 ± 300.1 M m−1,554.3 ± 378.2 M m−1和1005.2 ± 750.3 M m−1.硫酸铵,硝酸铵和有机物对消光的贡献最大, 不同季节下占比达55% ~77%.通过PM2.5组分进行重构, 利用IMPROVE算法计算得到Rbsca, 用能见度测量值转换得到Vbsca, 二者具有较高的相关性 (r2=0.84) ;但存在Vbsca的高值被低估, Vbsca的低值被高估的现象;特别是当Rbsca > 1123 M m−1 (对应能见度约小于2.0 km) 时, Vbsca的值被低估了17.6%.高浓度PM2.5和高湿度对IMPROVE算法结果有显著的影响.

Temporal and Vertical Oxygen Gradients Modulate Nitrous Oxide Production in a Seasonally Anoxic Fjord: Saanich Inlet, British Columbia

AbstractNitrous oxide (N2O) is a strong greenhouse gas and an ozone depleting agent. In marine environments, N2O is produced biologically via ammonium oxidation, nitrite, and nitrate reduction. The relative importance of these principle production pathways is strongly influenced by oxygen availability. We conducted 15N tracer experiments of N2O production in parallel with measurements of N2O concentration and natural abundance isotopes/isotopomers in Saanich Inlet, a seasonally anoxic fjord, to investigate how temporal and vertical oxygen gradients regulate N2O production pathways and rates. In April, June, and August 2018, the depth of the oxic‐anoxic interface (dissolved oxygen = 2.5 μmol L−1 isoline) progressively deepened from 110 to 160 m. Within the oxygenated and suboxic water column, N2O supersaturation coincided with peak ammonium oxidation activity. Conditions in the anoxic deep water were potentially favorable to N2O production from nitrate and nitrite reduction, but N2O undersaturation was observed indicating that N2O consumption exceeded rates of production. In October, tidal mixing introduced oxygenated water from outside the inlet, displacing the suboxic and anoxic deep water. This oxygenation event stimulated N2O production from ammonium oxidation and increased water column N2O supersaturation while inhibiting nitrate and nitrite reduction to N2O. Results from 15N tracer incubation experiments and natural abundance isotopomer measurements both implicated ammonium oxidation as the dominant N2O production pathway in Saanich Inlet, fueled by high ammonium fluxes (0.6–3.5 nmol m−2 s−1) from the anoxic depths. Partial denitrification contributed little to water column N2O production because of low availability of nitrate and nitrite.

Nitrification inhibitor 3,4‐dimethylpyrazole phosphate (DMPP) reduces N2O emissions by altering the soil microbial community in a wheat–maize rotation on the North China Plain

AbstractNitrous oxide (N2O) is a potent greenhouse gas that is released from agricultural ecosystems where nitrogen fertilizers are poorly used and contributes to global climate warming. Large nitrogen fertilizer inputs in summer maize fields on the North China Plain (NCP) lead to large N2O emissions. N2O emissions can be mitigated using 3,4‐dimethylpyrazole phosphate (DMPP), but the efficiency and microbial mechanisms of this mitigation at different nitrogen rates remain poorly understood. To evaluate the efficiency of N2O emission mitigation by DMPP at different nitrogen rates and to explore its mechanisms at the microbial level, we monitored the dynamic changes in mineral and N2O fluxes and analysed the abundance and community structure of ammonia oxidizers. We found that DMPP significantly inhibited the oxidation of ammonium (NH4+) to nitrate (NO3−) and thus maintained NH4+ concentrations but decreased the peak value of NO3− concentrations. Total N2O emissions were increased by 6.5‐fold, 13.2‐fold and 26.7‐fold in fields with 112.5 kg N ha−1, 225 kg N ha−1 and 337.5 kg N ha−1, respectively, compared with fields with no nitrogen applied. Nitrogen increased the abundance of ammonia‐oxidizing bacteria (AOB) and decreased that of ammonia‐oxidizing archaea (AOA), whereas DMPP exhibited the opposite effects. Therefore, AOB could be the dominant N2O emission contributors in nitrogen‐treated soils. Sequencing results suggested that all AOB belong to the Nitrosospira and Nitrosomonas nitrosa groups, and all AOA fell within the Nitrososphaera group. In both AOB and AOA communities, changes in the second‐largest cluster after the application of nitrogen fertilizer and DMPP were more pronounced than those in the largest cluster. The potential functional clusters of AOB and AOA were Nitrosospira cluster 3b and Nitrososphaera cluster 4.1, respectively. This study showed that nitrogen and DMPP both affected the abundance and structure of ammonia‐oxidizer communities, which may contribute to the variations in N2O emissions from croplands.Highlights Role of soil microorganisms in the efficiency of DMPP to reduce N2O emissions under different N inputs was explored. N input increased N2O emissions and DMPP reduced N2O emissions N and DMPP affect N2O emission via a changed key cluster of ammonia oxidizers. DMPP was more efficient at higher nitrogen input levels.

Controls of H2S, Fe2 +, and Mn2 + on Microbial NO3 --Reducing Processes in Sediments of an Eutrophic Lake.

Understanding the biogeochemical controls on the partitioning between nitrogen (N) removal through denitrification and anaerobic ammonium oxidation (anammox), and N recycling via dissimilatory nitrate (NO3–) reduction to ammonium (DNRA) is crucial for constraining lacustrine N budgets. Besides organic carbon, inorganic compounds may serve as electron donors for NO3– reduction, yet the significance of lithotrophic NO3– reduction in the environment is still poorly understood. Conducting incubation experiments with additions of 15N-labeled compounds and reduced inorganic substrates (H2S, Fe2+, Mn2+), we assessed the role of alternative electron donors in regulating the partitioning between the different NO3–-reducing processes in ferruginous surface sediments of Lake Lugano, Switzerland. In sediment slurry incubations without added inorganic substrates, denitrification and DNRA were the dominant NO3–-reducing pathways, with DNRA contributing between 31 and 46% to the total NO3– reduction. The contribution of anammox was less than 1%. Denitrification rates were stimulated by low to moderate additions of ferrous iron (Fe2+ ≤ 258 μM) but almost completely suppressed at higher levels (≥1300 μM). Conversely, DNRA was stimulated only at higher Fe2+ concentrations. Dissolved sulfide (H2S, i.e., sum of H2S, HS– and S2−) concentrations up to ∼80 μM, strongly stimulated denitrification, but did not affect DNRA significantly. At higher H2S levels (≥125 μM), both processes were inhibited. We were unable to find clear evidence for Mn2+-supported lithotrophic NO3– reduction. However, at high concentrations (∼500 μM), Mn2+ additions inhibited NO3– reduction, while it did not affect the balance between the two NO3– reduction pathways. Our results provide experimental evidence for chemolithotrophic denitrification or DNRA with Fe2+ and H2S in the Lake Lugano sediments, and demonstrate that all tested potential electron donors, despite the beneficial effect at low concentrations of some of them, can inhibit NO3– reduction at high concentration levels. Our findings thus imply that the concentration of inorganic electron donors in lake sediments can act as an important regulator of both benthic denitrification and DNRA rates, and suggest that they can exert an important control on the relative partitioning between microbial N removal and N retention in lakes.

Crop residue carbon-to-nitrogen ratio regulates denitrifier N2O production post flooding

The response of nitrifier and denitrifier populations and associated N2O emissions to different carbon-to-nitrogen (C/N = 17 or 45) straw amendments was monitored under flooding-drying and non-flooding conditions. A 10-week laboratory mesocosm study was conducted in two soils, (i) a paddy soil with a long history of managed flooding-drying (CN), and (ii) a wheat cropping soil with no previous history of flooding (UK). We measured N2O fluxes and the abundances of ammonia-oxidizing archaea (AOA) and bacteria (AOB), nitrite reductase (nirK and nirS) genes, and nitrous oxide reductase (nosZI and nosZII) genes during flooding (4 weeks) and post-flooding (6 weeks). Straw addition enhanced N2O emissions, with higher fluxes apparent after incorporation of narrow C/N residues. Moreover, the impact of crop amendment on N2O emission was exacerbated when soil was under flooding-drying conditions. The abundances of nirS and nosZI genes in CN soil and AOA gene in UK soil were increased by straw amendment, with highest in narrow C/N straw amendments. Structural equation modeling showed that the impact of denitrifier gene abundance on the N2O flux was stronger than that of nitrifier gene abundance in the two soils, and significant correlations were observed between N2O fluxes and the consumption of DOC and NO3−, indicating that denitrification was the dominant N2O production pathway during the drying phase. The ratio of (nirS + nirK)/(nosZI + nosZII) in the narrow C/N amendment was greater than in the wide C/N treatment after flooding, suggesting that the straw C/N ratio had an effect on the capacity for N2O production via denitrification. We conclude that crop amendments with an appropriate C/N ratio could minimize N2O fluxes through regulating the denitrification process when soils are subjected to regular flooding and drying and also experiencing greater frequencies of flooding.

Nitrate source apportionment in groundwater using Bayesian isotope mixing model based on nitrogen isotope fractionation

Accurate identification of nitrate (NO3−) sources is critical to address the issue of groundwater pollution. The nitrogen (N) isotopic enrichment factor (ɛp/s) is an important parameter to explain the N cycle and determine the proportional contribution of NO3− sources. Considering the isotopic fractionation effects in N transformation processes, this study quantitatively analyzed the NO3− sources in groundwater using stable isotopes (δ15N-NO3− and δ18O-NO3−) and the Bayesian isotope mixing model (SIAR). For the first time, the ɛp/s values (0.0‰, −8.7‰, −8.7‰, and 14.7‰) of atmospheric deposition (AD), soil nitrogen (SN), chemical fertilizers (CF), and manure and sewage (M&S) were calculated to determine the NO3− source apportionment in groundwater. It was proved that the isotopic fractionation effect could produce a more accurate NO3− source apportionment. We also found that the NO3− source contributions were closely related to the cropping system. In the vegetable cultivation area, CF (54.32%) and SN (37.75%) were the dominant NO3− source, while in the grain cultivation area, NO3− pollution was largely influenced by SN (33.67%), CF (33.27%), and M&S (30.16%). According to this study, the isotope fractionation is strongly recommended for NO3− source apportionment in groundwater system.

Quantitative identification of nitrate sources in a coastal peri-urban watershed using hydrogeochemical indicators and dual isotopes together with the statistical approaches.

Surplus nutrient load and complex migration and transformation processes are the challenges for water quality management in the peri-urban coastal watershed, leading to increasing concerns worldwide. We investigated the spatio-temporal variation of hydrogeochemical parameters in surface water of Jimei Lake watershed, and distinguished the sources and transformation of nitrate-N (NO3--N) using dual isotopes of nitrate (δ15N and δ18O in NO3-) with hydrogeochemical indicators. Principal component analysis (PCA) on hydrogeochemical parameters demonstrated that surface water was seriously polluted by nutrients, especially in the southeast of the downstream. There were signs of seawater intrusion and increased wastewater discharge in the mid-lower reaches with high ammonium concentrations. Nitrification occurred throughout the monitoring period with lower δ15N and δ18O values and NO3- derived from mixed pollution sources. Results of Bayesian model showed that dominant NO3- input originated from manure and sewage (M&S, 71% and 76% in the wet and dry season, respectively) and atmospheric deposition (22% and 16%, respectively). This result implied that the controls and treatment of M&S discharges are essential to alleviate of NO3- pollution. The proposed method is helpful to understand the origins of NO3- and may be suitable to develop measures for the reducing of nitrogen loadings in the peri-urban watershed.

Differential responses of soil N2O to biochar depend on the predominant microbial pathway

Biochar amendment has been proposed as a potential strategy to reduce soil nitrous oxide (N2O) emissions, although experimental studies have generated inconsistent results on N2O emissions following biochar amendment. Differential responses of soil N2O to biochar amendment may depend on soil microbial functional genes abundance and abiotic properties. Here we sampled three types of soil from fields under long-term cultivation of green tea (TG), film greenhouse vegetable cabbage (GV) and Jerusalem artichoke (JA), respectively. We conducted a microcosm experiment to examine N2O emissions from the different soils following biochar amendment. Results showed that biochar amendment increased N2O emissions from the GV soil while decreasing N2O emissions from the TG and JA soils in the presence of nitrogen fertilizer. Biochar amendment increased soil pH and C/N ratio across the three soils. Quantitative PCR (qPCR) analysis showed that biochar amendment also consistently increased the abundances of AOB and nosZ genes but decreased the AOA abundances for all the soils, while the effects of biochar on the abundances of nirK and nirS genes differed between the soils. Our results suggest that biochar amendment can affect the processes of both ammonia oxidation and reduction of N2O to N2 for all the soils and the net effect of biochar on N2O emissions depended on the predominant process in a specific soil. Biochar-induced increase in N2O emissions in the GV soil was largely attributed to the stimulated nitrification rate, which was primarily driven by AOB. Biochar-induced decreases in N2O emissions in the TG and JA soils were linked to the increased nosZ gene abundances. Overall, the effectiveness of biochar for mitigating N2O emissions is linked to its dominant N2O production pathway in soils.

Transformation and source identification of N in the upper reaches of the Han River basin, China: evaluated by a stable isotope approach

Given the spatial and temporal variability in hydrological conditions and nitrogen (N) processes, it is of great uncertainty to identify the N sources and evaluate N transformation processes in the upper Han River. Investigations were conducted in November 2015 and January, April, and July 2016, using an isotopic method and water quality monitoring. The significant and positive correlation between NO3- concentrations and Cl- (p < 0.01) in most sampling months suggested that the great influence of human activities and sewage or manure was the dominant NO3- source. The δ15NO3- values and NO3-/Cl- variations indicated that riverine N mainly came from soil organic N and sewage in November. Fertilizer and sewage were the major N sources in January and April, respectively. In July, water was influenced by various N inputs. The nitrification process played an important role in the low δ15NO3- values in January, while both nitrification and plant uptake resulted in the increase in δ15NH4+ values in April. The simultaneous effect of N fixation and plant uptake maintained the stabilization of δ15NH4+ concentrations. Our study provides theoretical basis on N sources and transformations for controlling N pollution and improving water quality in the upper Han River in the near future.

N2O emission in partial nitritation-anammox process

Nitrous oxide (N2O) is one of the significant greenhouse gases, and partial nitritation-anammox (PNA) process emits higher N2O than traditional nitrogen removal processes. N2O production in PNA mainly occurs in three different pathways, i.e., the ammonia oxidizing bacteria (AOB) denitrification, the hydroxylamine (NH2OH) oxidation and heterotrophic denitrifiers denitrification. N2O emission data vary significantly because of the different operational conditions, bioreactor configurations, monitoring systems and quantitative methods. Under the common operational parameter scopes of PNA, N2O emission via NH2OH oxidation dominates at relatively low dissolved oxygen (DO), low inorganic carbon (IC), high pH or low NO2− concentration, while N2O emission via AOB denitrification dominates at relative higher DO, higher IC, lower pH or higher NO2− concentration. AOB are highly enriched while nitrite-oxidizing bacteria (NOB) are rarely found in partial nitritation process, and the order Nitrosomonadales of AOB is the dominant group and N2O producer. Anammox bacteria, AOB and certain amount of heterotrophic denitrifying bacteria are observed in the anammox process, the genus Denitratisoma and the heterotrophic denitrifying bacteria in the deep layer of anammox granules are the dominant N2O generation bacteria. In one-stage PNA reactors, anammox bacteria account for a large fraction of the biomass, AOB account for small portion, and NOB account for even less. The microbial community, diversity and N2O producers in one-stage PNA reactors are similar with those in two-stage PNA reactors. The dominant anammox bacteria, AOB and NOB in PNA are the species Candidatus Brocadia, the genera of Nitrotoga, Nitrospira and Nitrobacter, and the genus Nitrosomonas, respectively. The relations between N2O emission pathways and microbial communities need further study in the future.

The isotopomer ratios of N2O in the Shaying River, the upper Huai River network, Eastern China: The significances of mechanisms and productions of N2O in the heavy ammonia polluted rivers

In order to figure out the effects of nitrogen pollution and dams on N2O production paths in the river systems, the isotopomer ratios in N2O, NH4+ and NO3−, as well as N2 concentrations and physico-chemical characteristics of water and sediments from the Shaying River system, which is the biggest tributary and major NH4+ contributor of the Huai River, were analyzed in the years 2015 and 2016. The results showed that the net productions of N2O (△N2O) in the river were pretty high, ranging from 12.9 to 440 nmol/L. N2O exhibited a narrow range in δ15Nbulk (−0.04 to 13.51‰), nevertheless a wide range in δ18O (22.54 to 59.90‰). Isotopocule diagram and Pearson correlation analysis indicated that isotopomer ratios of N2O were significantly affected by the mixing of N2O from difference production paths, not by N2O reduction. Relative contributions of nitrification and denitrification to N2O in the Shaying river system were deduced from the two end-members model. The contribution of nitrification to gross N2O was 58.5% on average, almost equal to the contribution of denitrification in summer, although denitrification was the dominant N2O source with average contribution of 75.6% in winter. No significant relationship was found either between △N2O and NH4+ or between △N2O and NO3− in the Shaying River. Heavy NO3− and COD loading reduced nitrification and increased the relative contribution of denitrification to N2O in winter. Heavy ammonia pollution caused pH values to decrease apparently, from 7.5 ± 0.3 in July to 6.3 ± 0.1 in December, resulting in denitrification being the dominant source to N2O in winter. Assimilation enhanced by the construction of dams had weakened the contribution of nitrification to N2O in summer in the Shaying River.

Determination sources of nitrates into the Three Gorges Reservoir using nitrogen and oxygen isotopes

Identification of nitrate sources and its transformations are important for the management of large lakes and reservoirs. The Three Gorges Reservoir (TGR) in China is one of the largest reservoirs around the world. In this study, stable isotopes of nitrogen (δ15N–NO3−) and oxygen (δ18O–NO3−) of nitrate in water were used to gain insights into nitrate sources and transformations in the tail area of the TGR. Bayesian mixing model has been conducted to estimate the proportional contribute of nitrate sources. The mixing modelling results indicated that NH4+ fertilizer (range 7–54%) and soil organic nitrogen (range 2–45%) were the dominant NO3−–N sources in the tail area of the TGR during the three season study period. Nitrification contributed a part of NO3−–N in the river water during the dry season. The nitrate from soil solution in the riparian zone with denitrified NO3− might be another major reason for the enrichment of δ15N–NO3− and δ18O–NO3− during the normal season. Reducing the use of chemical nitrogen fertilizers, especially NH4+ fertilizers, and protecting soil from erosion may be effective measures to improve water quality in the TGR.

The effect of pH on N2O production in intermittently-fed nitritation reactors

The effect of pH on nitrous oxide (N2O) production rates was quantified in an intermittently-fed lab-scale sequencing batch reactor performing high-rate nitritation. N2O and other nitrogen (N) species (e.g. ammonium (NH4+), nitrite, hydroxylamine and nitric oxide) were monitored to identify in-cycle dynamics and determine N conversion rates at controlled pH set-points (6.5, 7, 7.5, 8 and 8.5). Operational conditions and microbial compositions remained similar during long-term reactor-scale pH campaigns. The specific ammonium removal rates and nitrite accumulation rates varied little with varying pH levels (p > 0.05). The specific net N2O production rates and net N2O yield of NH4+ removed (ΔN2O/ΔNH4+) increased up to seven-fold from pH 6.5 to 8, and decreased slightly with further pH increase to 8.5 (p < 0.05). Best-fit model simulations predicted nitrifier denitrification as the dominant N2O production pathway (≥87% of total net N2O production) at all examined pH. Our study highlights the effect of pH on biologically mediated N2O emissions in nitrogen removal systems and its importance in the design of N2O mitigation strategies.

Hydroxylamine as a Potential Indicator of Nitrification in the Open Ocean

AbstractHydroxylamine (NH2OH), a short‐lived intermediate in the nitrogen cycle, is a potential precursor of nitrous oxide (N2O) in the ocean. However, measurements of NH2OH in the ocean are sparse. Here we present a data set of depth profiles of NH2OH from the equatorial Atlantic Ocean and the eastern tropical South Pacific and compare it to N2O, nitrate, and nitrite profiles under varying oxygen conditions. The presence of NH2OH in surface waters points toward surface nitrification in the upper 100 m. Overall, we found a ratio of 1:3 between NH2OH and N2O in open ocean areas when oxygen concentrations were &gt;50 μmol/L. In the equatorial Atlantic Ocean and the open ocean eastern tropical South Pacific, where nitrification is the dominant N2O production pathway, stepwise multiple regressions demonstrated that N2O, NH2OH, and nitrate concentrations were highly correlated, suggesting that NH2OH is a potential indicator for nitrification.

Biochar amendment and Calamagrostis angustifolia planting affect sources and production pathways of N2O in agricultural ditch systems.

Nitrous oxide (N2O) from agricultural ditches is a non-negligible source of anthropogenic greenhouse gas (GHG) emissions, but few studies have addressed this topic in depth. On the other hand, although there are numerous reports that biochar application can affect N2O emissions from soil, the understanding of the process and source of changes is still incomplete. To examine the effect of biochar and Calamagrostis angustifolia on N2O emissions, we conducted experiments with constructed ditches where corn stalk biochar (pyrolysis temperature of 450 °C) was applied at a rate of 16.77 Mg ha-1 and C. angustifolia was planted. The sources (native sediment versus exogenous inorganic N) and production pathways (nitrification versus denitrification) of N2O emissions were discriminated using the 15N isotope tracer method. We observed that biochar application reduced the cumulative total N-N2O emissions from the native sediment by 10.8-18.7% and reduced the cumulative 15N-N2O emissions from the exogenous 15N-labelled inorganic N by 25.7-68.6%; C. angustifolia planting reduced these cumulative N2O emissions by 48.8-53.3% and 93.3-92.4%, respectively. The results showed that biochar stimulated nitrification and nitrification-derived 15N-N2O emissions, but reduced denitrification-derived 15N-N2O emissions in bare sediment microcosms; C. angustifolia effectively reduced both nitrification-derived and denitrification-derived 15N-N2O emissions. Therefore, we concluded that the effect of biochar application on N2O emissions may depend on its dominant N2O production pathway and biochar application plus C. angustifolia planting could be beneficial for the mitigation of N2O emissions in agricultural ditch systems.

Coupling stable isotopes and water chemistry to assess the role of hydrological and biogeochemical processes on riverine nitrogen sources

Accurate source identification is critical for optimizing water pollution control strategies. Although the dual stable isotope (15N-NO3-/18O-NO3-) approach has been widely applied for differentiating riverine nitrogen (N) sources, the relatively short-term (<1 yr) 15N-NO3-/18O-NO3- records typically used in previous studies often hinders rigorous assessment due to high temporal variability associated with watershed N dynamics. Estimated contributions of legacy N sources in soils and groundwater to riverine N export by modeling approaches in many previous studies also lack validation from complementary information, such as multiple stable isotopes. This study integrated three years of multiple stable isotope (15N-NO3-/18O-NO3- and 2H-H2O/18O-H2O) and hydrochemistry measurements for river water, groundwater and rainfall to elucidate N dynamics and sources in the Yongan watershed (2474 km2) of eastern China. Nonpoint source N pollution dominated and displayed considerable seasonal and spatial variability in N forms and concentrations. Information from δ15N-NO3- and δ18O-NO3- indicated that riverine N dynamics were regulated by contributing sources, nitrification and denitrification, as well as hydrological processes. For the three examined catchments and entire watershed, slow subsurface and groundwater flows accounted for >75% of river discharge and were likely the major hydrological pathways for N delivery to the river. Riverine NO3− sources varied with dominant land use (p < 0.001), with the highest contributions of groundwater (60%), wastewater (35%), and soil (50%) occurring in agricultural, residential and forest catchments, respectively. For the entire watershed, groundwater (∼50%) and soil N (>30%) were the dominant riverine NO3− sources, implying considerable potential for N pollution legacy effects. Results were consistent with observed nitrous oxide dynamics and N sources identified in previous modeling studies. As the first attempt to apply multiple isotope tracers for exploring and quantifying N transformation and transport pathways, this study provides an integrated approach for verifying and understanding the N pollution legacy effects observed in many watersheds worldwide. This study highlights that river N pollution control in many watersheds requires particular attention to groundwater restoration and soil N management in addition to N input control strategies.

Global Nitrous Oxide Production Determined by Oxygen Sensitivity of Nitrification and Denitrification

AbstractThe ocean is estimated to contribute up to ~20% of global fluxes of atmospheric nitrous oxide (N2O), an important greenhouse gas and ozone depletion agent. Marine oxygen minimum zones contribute disproportionately to this flux. To further understand the partition of nitrification and denitrification and their environmental controls on marine N2O fluxes, we report new relationships between oxygen concentration and rates of N2O production from nitrification and denitrification directly measured with 15N tracers in the Eastern Tropical Pacific. Highest N2O production rates occurred near the oxic‐anoxic interface, where there is strong potential for N2O efflux to the atmosphere. The dominant N2O source in oxygen minimum zones was nitrate reduction, the rates of which were 1 to 2 orders of magnitude higher than those of ammonium oxidation. The presence of oxygen significantly inhibited the production of N2O from both nitrification and denitrification. These experimental data provide new constraints to a multicomponent global ocean biogeochemical model, which yielded annual oceanic N2O efflux of 1.7–4.4 Tg‐N (median 2.8 Tg‐N, 1 Tg = 1012 g), with denitrification contributing 20% to the oceanic flux. Thus, denitrification should be viewed as a net N2O production pathway in the marine environment.