Nitrous oxide emissions from a large, impounded river: the Ohio River

Urbanization significantly increases greenhouse gas emissions from a subtropical headwater stream in Southeast China

Streams are disproportionately significant contributors to increases in greenhouse gas (GHG) effluxes in river networks. In the context of global urbanization, a growing number of streams are affected by urbanization, which has been suggested to stimulate the water-air GHG emissions from fluvial systems. This study investigated the seasonal and longitudinal profiles of GHG (N2O, CH4, and CO2) concentrations of Jiuxianghe Stream, a headwater stream undergoing urbanization, and estimated its GHG diffusive fluxes and global warming potentials (GWPs) using the boundary layer method. The results showed that N2O, CH4, and CO2 concentrations in Jiuxianghe Stream were 0.45-7.19 μg L-1, 0.31-586.85 μg L-1, and 0.16-11.60 mg L-1, respectively. N2O, CH4, and CO2 concentrations in the stream showed 4.55-, 23.70-, and 7.68-fold increases from headwaters to downstream, respectively, corresponding to the forest-urban transition within the watershed. Multiple linear regression indicated that NO3--N, NH4+-N, and DOC:NO3--N accurately predicted N2O and CO2 concentrations, indicating that N nutrients were the driving factors. The Jiuxianghe Stream was a source of atmospheric GHGs with a daily GWP of 7.31 g CO2-eq m-2 d-1 on average and was significantly positively correlated with the ratio of construction land and forest in the sub-watershed. This study highlights the critical role of urbanization in amplifying GHG emissions from streams, thereby augmenting our understanding of GHG emissions from river networks. With global urbanization on the rise, streams experiencing urbanization are expected to make an unprecedentedly significant contribution to riverine GHG budgets in the future.

Multiple isotopes decipher the nitrogen cycle in the cascade reservoirs and downstream in the middle and lower Yellow River: Insight for reservoir drainage period

Intensive anthropogenic activities, such as excessive nitrogen input and dam construction, have altered the nitrogen cycle in the global river system. However, the focus on the source, transformation and fate of nitrogen in the Yellow River is still scarce. In this study, the multiple isotopes (δ15N-NO3-, δ18O-NO3-, δ15N-NH4+ and δ15N-PN) were deciphered to explore the nitrogen cycling processes and the driving factors in the thermally stratified cascade reservoirs (Sanmenxia Reservoir: SMXR and Xiaolangdi Reservoir: XLDR) and Lower Yellow River (LYR) during the drainage period of the XLDR. In the SMXR, algal bloom triggered the assimilation process in the upper layer before the SMX Dam, followed by remineralization and subsequent nitrification processes in the lower water layers. The nitrification reaction in the XLDR progressively increased along both longitudinal and vertical directions to the lower layer of the XLD Dam, which was linked to the variation in the water residence time of riverine, transition and lentic zones. The robust nitrification rates in the lower layer of the lentic zone coincided with the substantial depletion of nitrate isotopic composition and enrichment of both δ15N-PN and δ15N-NH4+, indicating the longer water residence time not only promoted the growth of the nitrifying population but also facilitated the remineralization to enhance NH4+ availability. In the LYR, the slight nitrate assimilation, as indicated by nitrate isotopic composition and fractionation models, was the predominant nitrogen transformation process. The Bayesian isotope mixing model results showed that manure and sewage was the dominant nitrate source (50 %) in the middle and lower Yellow River. Notably, the in-reservoir nitrification was a significant nitrate source (27 %) in the XLDR and LYR. Our study deepens the understanding of anthropogenic activities impacting the nitrogen cycle in the river-reservoir system, providing valuable insight into water quality management and nitrogen cycle mechanisms in the Yellow River.

Increased nitrous oxide emissions from global lakes and reservoirs since the pre-industrial era

Lentic systems (lakes and reservoirs) are emission hotpots of nitrous oxide (N2O), a potent greenhouse gas; however, this has not been well quantified yet. Here we examine how multiple environmental forcings have affected N2O emissions from global lentic systems since the pre-industrial period. Our results show that global lentic systems emitted 64.6 ± 12.1 Gg N2O-N yr-1 in the 2010s, increased by 126% since the 1850s. The significance of small lentic systems on mitigating N2O emissions is highlighted due to their substantial emission rates and response to terrestrial environmental changes. Incorporated with riverine emissions, this study indicates that N2O emissions from global inland waters in the 2010s was 319.6 ± 58.2 Gg N yr-1. This suggests a global emission factor of 0.051% for inland water N2O emissions relative to agricultural nitrogen applications and provides the country-level emission factors (ranging from 0 to 0.341%) for improving the methodology for national greenhouse gas emission inventories.

Investigating key drivers of N2O emissions in heterogeneous riparian sediments: Reactive transport modeling and statistical analysis

Nitrous oxide (N2O) is a potent greenhouse gas that also contributes to ozone depletion. Recent studies have identified river corridors as significant sources of N2O emissions. Surface water-groundwater (hyporheic) interactions along river corridors induce flow and reactive nitrogen transport through riparian sediments, thereby generating N2O. Despite the prevalence of these processes, the controlling influence of physical and geochemical parameters on N2O emissions from coupled aerobic and anaerobic reactive transport processes in heterogeneous riparian sediments is not yet fully understood. This study presents an integrated framework that combines a flow and multi-component reactive transport model (RTM) with an uncertainty quantification and sensitivity analysis tool to determine which physical and geochemical parameters have the greatest impact on N2O emissions from riparian sediments. The framework involves the development of thousands of RTMs, followed by global sensitivity and responsive surface analyses. Results indicate that characterizing the denitrification reaction rate constant and permeability of intermediate-permeability sediments (e.g., sandy gravel) are crucial in describing coupled nitrification-denitrification reactions and the magnitude of N2O emissions. This study provides valuable insights into the factors that influence N2O emissions from riparian sediments and can help in developing strategies to control N2O emissions from river corridors.

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 ± 1.65 nmol 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 9.84 mg L-1, while the DO concentration in the YT reservoir showed very little variation, ranging from 6.45 to 7.09 mg 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.

Inland Waters Increasingly Produce and Emit Nitrous Oxide

Nitrous oxide (N2O) is a long-lived greenhouse gas and currently contributes ∼10% to global greenhouse warming. Studies have suggested that inland waters are a large and growing global N2O source, but whether, how, where, when, and why inland-water N2O emissions changed in the Anthropocene remains unclear. Here, we quantify global N2O formation, transport, and emission along the aquatic continuum and their changes using a spatially explicit, mechanistic, coupled biogeochemistry-hydrology model. The global inland-water N2O emission increased from 0.4 to 1.3 Tg N yr-1 during 1900-2010 due to (1) growing N2O inputs mainly from groundwater and (2) increased inland-water N2O production, largely in reservoirs. Inland waters currently contribute 7 (5-10)% to global total N2O emissions. The highest inland-water N2O emissions are typically in and downstream of reservoirs and areas with high population density and intensive agricultural activities in eastern and southern Asia, southeastern North America, and Europe. The expected continuing excessive use of nutrients, dam construction, and development of suboxic conditions in aging reservoirs imply persisting high inland-water N2O emissions.

Pollution abatement reducing the river N2O emissions although it is partially offset by a warming climate: Insights from an urbanized watershed study

Global nitrogen (N) pollution has resulted in increased river nitrous oxide (N₂O) emissions, which contribute to climate change. However, little is known about how pollution abatement conversely reduces river N₂O production in a warming climate. Here, field observations and microcosmic experiments were conducted in a coastal urbanized watershed (S.E. China) to explore the interactive effect of changing nitrate and temperature on river sediment denitrification (DNF) and N₂O production. The results showed that urban river reaches (UR) with higher organic carbon content and denitrifying gene abundance in sediments have a greater DNF rate, nitrate removal efficiency (NRE), and N₂O concentration than agricultural river reaches (AR). Microcosmic incubation suggested that the DNF rate and associated N₂O production decreased under low nitrate addition, wherein the NRE increased. The scenario simulation illustrated a nonlinear response of N₂O production to nitrate removal (i.e., ΔN₂O/ΔNO₃-N) from both UR and AR sediments at a given temperature, and the DNF rate and N₂O production increased with increasing temperature. An increase in temperature by 1 degree Celsius would offset 18.75% of the N₂O reduction by nitrate removal via DNF. These findings implied that watershed pollution abatement undoubtedly contributes to the reduction in global river N₂O emissions although it is partially offset by extra N₂O production caused by global warming.

Urban landscapes and legacy industry provide hotspots for riverine greenhouse gases: A source-to-sea study of the River Clyde

There is growing global concern that greenhouse gas (GHG) emissions from water bodies are increasing because of interactions between nutrient levels and climate warming. This paper investigates key land-cover, seasonal and hydrological controls of GHGs by comparison of the semi-natural, agricultural and urban environments in a detailed source-to-sea study of the River Clyde, Scotland. Riverine GHG concentrations were consistently oversaturated with respect to the atmosphere. High riverine concentrations of methane (CH₄) were primarily associated with point source inflows from urban wastewater treatment, abandoned coal mines and lakes, with CH₄-C concentrations between 0.1 - 44 µg l^-1. Concentrations of carbon dioxide (CO₂) and nitrous oxide (N₂O) were mainly driven by nitrogen concentrations, dominated by diffuse agricultural inputs in the upper catchment and supplemented by point source inputs from urban wastewater in the lower urban catchment, with CO₂-C concentrations between 0.1 - 2.6 mg l^-1 and N₂O-N concentrations between 0.3 - 3.4 µg l^-1. A significant and disproportionate increase in all GHGs occurred in the lower urban riverine environment in the summer, compared to the semi-natural environment, where GHG concentrations were higher in winter. This increase and change in GHG seasonal patterns points to anthropogenic impacts on microbial communities. The loss of total dissolved carbon, to the estuary is approximately 48.4 ± 3.6 Gg C yr^-1, with the annual inorganic carbon export approximately double that of organic carbon and four times that of CO_2, with CH₄ accounting for 0.03%, with the anthropogenic impact of disused coal mines accelerating DIC loss. The annual loss of total dissolved nitrogen to the estuary is approximately 4.03 ± 0.38 Gg N yr^-1 of which N₂O represents 0.06%. This study improves our understanding of riverine GHG generation and dynamics which can contribute to our knowledge of their release to the atmosphere. It identifies where action could support reductions in aquatic GHG generation and emission.

Development of N2O emission factors derived from nitrification of treated sewage water in rivers

Because treated sewage water contains nitrogen compounds such as NH₄-N and NO₂₊₃-N, nitrous oxide (N₂O) which is one of the greenhouse gases is generated by nitrification and denitrification reactions in rivers. This study aimed to develop a method for estimating the amount of N₂O generated from the nitrification of residual N compounds in treated sewage water by subtracting the amount of residual N₂O dissolved in the treated sewage water. A year-round field survey of dissolved N₂O (D-N₂O) in the treated sewage water and river water showed that nitrification proceeded in the downstream direction in the rivers following the discharge of the treated sewage water, thereby generating N₂O. The deaeration rate of N₂O dissolved in the river water was obtained from a laboratory experiment. The N₂O emission factor that can be applied to in-river nitrification of NH₄-N contained in the treated sewage water was calculated to be 0.0025 kgN₂O-N kgNH₄-N-¹.

Deoxygenation turns the coastal Red Sea lagoons into sources of nitrous oxide

Direct measurements of dissolved N₂O concentrations, fluxes and saturation percentages undertaken for the first time in two coastal lagoons - Al-Shabab and Al-Arbaeen, along the east coast of the Red Sea, revealed the region as a significant source of N₂O to the atmosphere. The exacerbated dissolved inorganic nitrogen (DIN) from various anthropogenic sources led to substantial oxygen depletion in both the lagoons, which turned to bottom anoxia at Al-Arbaeen lagoon during the spring season. We assume that the accumulation of N₂O is caused by nitrifier-denitrification in the hypoxic/anoxic boundaries. In fact, the results indicated that oxygen-depleted bottom waters favoured denitrification when the oxygenated surface waters recorded nitrification signals. Overall, the N₂O concentration ranged from 109.4 to 788.6 nM (40.6-325.6 nM) in spring and 58.7 to 209.8 nM (35.8-89.9 nM) in winter in the Al-Arbaeen (Al-Shabab) lagoon. The N₂O flux ranged from 647.1 to 1763.2 μmol m^-2 day^-1 (85.9 to 160.2 μmol m^-2 day^-1) and 112.5 to 150.8 μmol m^-2 day^-1 (76.1 to 88.7 μmol m^-2 day^-1) in the spring and winter respectively, in the Al-Arbaeen (Al-Shabab) lagoons. The ongoing developmental activities may worsen the current situation of hypoxia and associated biogeochemical feedbacks; therefore, the present results underline the need for continuous monitoring of both lagoons to restrict more severe oxygen depletion in future.

Shifts in composition and function of bacterial communities reveal the effect of small barriers on nitrous oxide and methane accumulation in fragmented rivers

Rivers are often blocked by barriers to form different habitats, but it is not clear whether this change will affect the accumulation of N₂O and CH₄ in rivers. Here, low barriers (less than 2 m, LB) increased N₂O concentration by 1.13 times and CH₄ decreased by 0.118 times, while high barriers (higher than 2 m, less than 5 m high, HB) increased N₂O concentration by 1.19 times and CH₄ by 2.76 times. Co-occurrence network analysis indicated LB and HB can promote the enrichment of Cyanobium and Chloroflexi, further limiting complete denitrification and increasing N₂O accumulation. The LB promotes methanotrophs (Methylocystis, Methylophilus, and Methylotenera) to compete with denitrifiers (Pseudomonas) in water, and reduce CH₄ accumulation. While the HB can promote the methanotrophs to compete with nitrifiers (Nitrosospira) in sediment, thus reducing the consumption of CH₄. LB and HB reduce river velocity, increase water depth, and reduce dissolved oxygen (DO), leading to enrichment of nirS-type denitrifiers and the increase of N₂O concentration in water. Moreover, the HB reduces DO concentration and pmoA gene abundance in water, which can increase the accumulation of CH₄. In light of the changes in the microbial community and variation in N₂O and CH₄ accumulation, the impact of fragmented rivers on global greenhouse gas emissions merits further study.

Shifts in the spatiotemporal distribution and sources of nitrous oxide in sediment cores from the Bohai Sea and South Yellow Sea

N₂O is among the most potent greenhouse gases. In this study, we investigated one of the important N₂O production hotspots, the continental margins. We looked at N₂O spatiotemporal distributions in situ as well as the potential contributions of nitrification and denitrification to N₂O production in sediment cores from the Bohai and South Yellow Seas. Real-time PCR and shotgun metagenomics sequencing were used to analyze the microbial communities related to N₂O production. The results showed that N₂O concentrations roughly decreased with depth-a trend that was consistent throughout the year and showed no significant seasonal variations. When all the research stations along the continental margin were considered, the estuary exhibited the lowest average N₂O concentration. Moreover, nitrification was identified as the main process responsible for N₂O production in estuary areas. This study demonstrates that spatial, as opposed to temporal, heterogeneity is the primary factor influencing N₂O concentration differences in sediments.

Methane and nitrous oxide concentrations and fluxes from heavily polluted urban streams: Comprehensive influence of pollution and restoration

Streams draining urban areas are usually regarded as hotspots of methane (CH₄) and nitrous oxide (N₂O) emissions. However, little is known about the coupling effects of watershed pollution and restoration on CH₄ and N₂O emission dynamics in heavily polluted urban streams. This study investigated the CH₄ and N₂O concentrations and fluxes in six streams that used to be heavily polluted but have undergone different watershed restorations in Southwest China, to explore the comprehensive influences of pollution and restoration. CH₄ and N₂O concentrations in the six urban streams ranged from 0.12 to 21.32 μmol L^-1 and from 0.03 to 2.27 μmol L^-1, respectively. The calculated diffusive fluxes of CH₄ and N₂O were averaged of 7.65 ± 9.20 mmol m^-2 d^-1 and 0.73 ± 0.83 mmol m^-2 d^-1, much higher than those in most previous reports. The heavily polluted streams with non-restoration had 7.2 and 7.8 times CH₄ and N₂O concentrations higher than those in the fully restored streams, respectively. Particularly, CH₄ and N₂O fluxes in the fully restored streams were 90% less likely than those found in the unrestored ones. This result highlighted that heavily polluted urban streams with high pollution loadings were indeed hotspots of CH₄ and N₂O emissions throughout the year, while comprehensive restoration can effectively weaken their emission intensity. Sewage interception and nutrient removal, especially N loadings reduction, were effective measures for regulating the dynamics of CH₄ and N₂O emissions from the heavily polluted streams. Based on global and regional integration, it further elucidated that increasing environment investments could significantly improve water quality and mitigate CH₄ and N₂O emissions in polluted urban streams. Overall, our study emphasized that although urbanization could inevitably strengthen riverine CH₄ and N₂O emissions, effective eco-restoration can mitigate the crisis of riverine greenhouse gas emissions.

Global methane and nitrous oxide emissions from inland waters and estuaries

Inland waters (rivers, reservoirs, lakes, ponds, streams) and estuaries are significant emitters of methane (CH₄ ) and nitrous oxide (N₂ O) to the atmosphere, while global estimates of these emissions have been hampered due to the lack of a worldwide comprehensive data set of CH₄ and N₂ O flux components. Here, we synthesize 2997 in-situ flux or concentration measurements of CH₄ and N₂ O from 277 peer-reviewed publications to estimate global CH₄ and N₂ O emissions from inland waters and estuaries. Inland waters including rivers, reservoirs, lakes, and streams together release 95.18 Tg CH₄  year^-1 (ebullition plus diffusion) and 1.48 Tg N₂ O year^-1 (diffusion) to the atmosphere, yielding an overall CO₂ -equivalent emission total of 3.06 Pg CO₂  year^-1 . The estimate of CH₄ and N₂ O emissions represents roughly 60% of CO₂ emissions (5.13 Pg CO₂  year^-1 ) from these four inland aquatic systems, among which lakes act as the largest emitter for both CH₄ and N₂ O. Ebullition showed as a dominant flux component of CH₄ , contributing up to 62%-84% of total CH₄ fluxes across all inland waters. Chamber-derived CH₄ emission rates are significantly greater than those determined by diffusion model-based methods for commonly capturing of both diffusive and ebullitive fluxes. Water dissolved oxygen (DO) showed as a dominant factor among all variables to influence both CH₄ (diffusive and ebullitive) and N₂ O fluxes from inland waters. Our study reveals a major oversight in regional and global CH₄ budgets from inland waters, caused by neglecting the dominant role of ebullition pathways in those emissions. The estimated indirect N₂ O EF₅ values suggest that a downward refinement is required in current IPCC default EF₅ values for inland waters and estuaries. Our findings further indicate that a comprehensive understanding of the magnitude and patterns of CH₄ and N₂ O emissions from inland waters and estuaries is essential in defining the way of how these aquatic systems will shape our climate.

Three Gorges Dam: Friend or Foe of Riverine Greenhouse Gases?

Dams are often regarded as greenhouse gas (GHG) emitters. However, our study indicated that the world's largest dam, the Three Gorges Dam (TGD), has caused significant drops in annual average emissions of CO₂, CH₄ and N₂O over 4300 km along the Yangtze River, accompanied by remarkable reductions in the annual export of CO₂ (79%), CH₄ (50%) and N₂O (9%) to the sea. Since the commencement of its operation in 2003, the TGD has altered the carbonate equilibrium in the reservoir area, enhanced methanogenesis in the upstream, and restrained methanogenesis and denitrification via modifying anoxic habitats through long-distance scouring in the downstream. These findings suggest that ‘large-dam effects’ are far beyond our previous understanding spatiotemporally, which highlights the fundamental importance of whole-system budgeting of GHGs under the profound impacts of huge dams.

Environmental heterogeneity determines the response patterns of microbially mediated N-reduction processes to sulfamethoxazole in river sediments

The widespread occurrence of antibiotics in aquatic ecosystems leads to potential ecological risks to organisms, in turn affecting microbially mediated processes. Here, we investigated the response of dominant N-reduction processes to the frequently detected antibiotic sulfamethoxazole (SMX) along the Chaobai River with regional environmental heterogeneity, including denitrification, anaerobic ammonium oxidation (anammox), dissimilatory nitrate reduction to ammonium (DNRA), and nitrous oxide (N₂O) release. We found two divergent SMX response patterns for denitrification in contrasting scenarios of geochemical properties. In the context of low nitrate and carbon, SMX weakened denitrification with a slightly stimulation first. Whereas SMX directly inhibited denitrification when nitrate and carbon were sufficient. High SMX concentration suppressed anammox (26-72%) and DNRA activities (48-84%) via restraining the activities of anammox and DNRA bacteria. Notably, SMX increased the contribution of denitrification to N-reduction at the expense of DNRA to N-reduction, leading to a shift in nitrogen conversion towards denitrification. Additionally, SMX stimulated N₂O emission (up to 91%) due to superior restraint on process of N₂O reduction to N₂ and an incline for N-reduction towards denitrification, thereby exacerbating greenhouse effect. Our results advance the understanding of how nitrogen cycling is affected by SMX in aquatic ecosystems with environmental heterogeneity.

Mitigating NO and N2O emissions from a pilot-scale oxidation ditch using bioaugmentation of immobilized aerobic denitrifying bacteria

Nitrous oxide (N₂O) emission from wastewater treatment plants (WWTPs) requires urgent mitigation because of its significant contribution to the greenhouse effect. In this study, bioaugmentation was applied in a pilot-scale oxidation ditch with the aerobic denitrifying bacteria strain PCN-1 immobilized on polyurethane biocarriers, which demonstrated effective N₂O mitigation. Microbial community analysis suggested that the bioaugmentation facilitated a symbiotic relationship of the bacterial populations between the activated sludge and the biocarriers. The denitrifying bacteria with well-known N₂O reducing capabilities predominated on the biocarriers. Correspondingly, the increases of denitrifying genes and NO and N₂O reductase provided evidence for the enhanced genetic potential for NO and N₂O reduction. Besides, the enriched comammox Nitrospira on the biocarriers is proposed as another significant driver for N₂O mitigation by avoiding nitrite accumulation. In addition, the bioaugmentation enhanced the stability and recovery capability of the system in the ammonia overload and aeration failure shock tests.

Modeling Riverine N2O Sources, Fates, and Emission Factors in a Typical River Network of Eastern China

Estimates of riverine N₂O emission contain great uncertainty because of the lack of quantitative knowledge concerning riverine N₂O sources and fates. Using a 3.5-year record of monthly N₂O measurements from the Yongan River network of eastern China, we developed a mass-balance model to address the riverine N₂O source and sink processes. We achieved reasonable model efficacies (R² = 0.44-0.84, Nash-Sutcliffe coefficients = 0.40-0.80) across three tributaries and the entire river system. Estimated riverine N₂O loads originated from groundwater (38-88%), surface runoff (3-26%), and in-stream production (4-48%). Estimated in-stream losses via atmospheric release + complete denitrification accounted for 76, 95, 25, and 89% of riverine N₂O fate for the agricultural, residential, forest, and entire river system, respectively. Considering limited complete denitrification, the model estimated an upper-bound riverine N₂O emission rate of 2.65 ton N₂O-N km^-2 year^-1 for the entire river system. Riverine N₂O emission estimates were of comparable magnitude to those estimated with a power-law scaling model. Riverine N₂O emissions using the IPCC default emission factor (0.26%) overestimated emissions by 3-15 times, whereas the dissolved N₂O concentration-based emission factor overestimated or underestimated emissions. This study highlights the importance of combining comprehensive information on N₂O sources and fates to achieve accurate riverine N₂O emission estimates.

Hydrochemistry and nutrients determined the distribution of greenhouse gases in saline groundwater

The geography patterns and generation mechanisms of greenhouse gases (GHGs) in groundwater, especially in saline groundwater, are critical but rarely studied. Herein, we investigated the GHGs distribution in an aquifer, located upstream of Baiyangdian Lake, China, with a distinctive salinity gradient. A total of 132 groundwater samples were collected from 44 new-constructed wells along the lateral dimensions, and analyzed for dissolved GHGs concentrations, physiochemical parameters, and isotopes. The results showed that the dissolved CO₂, CH₄ and N₂O concentrations ranged from 9.47 to 79.3 mg/L, 1.05-56.9 μg/L, and 0.84-7.03 μg/L, respectively. The groundwater was supersaturated with GHGs with respect to atmospheric equilibrium, suggesting groundwater discharge as a potential source of GHGs emission. CO₂ significantly decreased while CH₄ and N₂O distinctively increased with the decline of total dissolved solids (TDS) concentration, illustrating an obvious spatial pattern in the GHGs distribution. The CO₂ distributions mainly depended on the bicarbonate radical and TDS, indicating carbonate equilibrium as the main process involving in the CO₂ generation. CH₄ and N₂O was primarily generated through the methanogenesis and denitrification processes, respectively. Nutrients including SO₄^2- and total organic carbon predominately shaped the CH₄ distributions, while nitrate mainly governed the N₂O distributions. Our study highlights the important roles of hydrochemistry and nutrients in the GHGs generation and distributions, which provides a significant insight on managing the GHGs emissions from the saline groundwater.

High denitrification potential but low nitrous oxide emission in a constructed wetland treating nitrate-polluted agricultural run-off

Constructed wetlands (CW) can efficiently remove nitrogen from polluted agricultural run-off, however, a potential caveat is nitrous oxide (N₂O), a harmful greenhouse gas and stratospheric ozone depleter. During five sampling campaigns, we measured N₂O fluxes from a 0.53 ha off-stream CW treating nitrate-rich water from the intensively fertilized watershed in Rampillon, France, using automated chambers with a quantum cascade laser system, and manual chambers. Sediment samples were analysed for potential N₂ flux using the HeO₂ incubation method. Both inlet nitrate (NO₃^-) concentrations and N₂O emission varied significantly between the seasons. In the Autumn and Winter inlet concentrations were about 11 mg NO₃^--N L^-1, and < 6.5 mg NO₃^--N L^-1 in the Spring and Summer. N₂O emission was highest in the Autumn (mean ± standard error: 9.7 ± 0.2 μg N m^-2 h^-1) and lowest in the Summer (wet period: 0.2 ± 0.3 μg N m^-2 h^-1). The CW was a very weak source of N₂O emitting 0.32 kg N₂O-N ha^-1 yr^-1 and removing around 938 kg NO₃^--N ha^-1 yr^-1, the ratio of N₂O-N emitted to NO₃^--N removed was 0.033%. The automated and manual chambers gave similar results. From the potential N₂O formation in the sediment, only 9% was emitted to the atmosphere, the average N₂ N ₂O ratio was high: 89:1 for N₂-N_potential: N₂O-N_potential and 1353:1 for N₂-N_potential: N₂O-N_emitted. These results indicate complete denitrification. The focused principal component analysis showed strong positive correlation between the gaseous N₂O fluxes and the following environmental factors: NO₃^--N concentrations in inlet water, streamflow, and nitrate reduction rate. Water temperature, TOC and DOC in the water and hydraulic residence time showed negative correlations with N₂O emissions. Shallow off-stream CWs such as Rampillon may have good nitrate removal capacity with low N₂O emissions.

Impact of nitrogen compound variability of sewage treated water on N2O production in riverbeds

Nitrous oxide (N₂O), a strong greenhouse and ozone depleting gas, is known to be generated in the river environment. However, the impact of sewage treated water on the production mechanism has not been clarified. In this study, N₂O production in the upper reach of a river was evaluated by field survey and activity test. The results demonstrated that the N₂O production activity of the river pebbles increased with the inflow of the sewage treated water, which was supported by field survey results, such as the dissolved N₂O concentrations and water quality. The emission factors of N₂O were determined to be 0.02-0.05% in nitrification and 0.01-0.025% in denitrification. Our study shows that combining a field survey and an activity test improves the reliability of the results and leads to the appropriate quantitative evaluation. From a perspective of controlling the N₂O emissions from the sewage treatment plant, N₂O generation inside the plant is critical. However, appropriate nitrogen removal in the treatment plant is connected to the reduction of N₂O generation in the river environment.

The role of freshwater eutrophication in greenhouse gas emissions: A review

Greenhouse gases (GHGs) have long received public attention because they affect the Earth's climate by producing the greenhouse effect. Freshwaters are an important source of GHGs, and the enhancement in their eutrophic status affects GHG emissions. Along with the increasing eutrophication of water bodies, the relevant quantitative and qualitative studies of the effects of freshwater eutrophication on GHG emissions have made substantial progress, particularly in the past 5 years. However, to our knowledge, this is the first critical review to focus on the role of freshwater eutrophication in GHG emissions. In this review, the emissions of common GHGs from freshwater are quantitatively described. Importantly, direct (i.e., dissolved oxygen, organic carbon, and nutrients) and indirect factors (i.e., dominant primary producer and algal blooms) affecting GHG emissions from eutrophic freshwater are systematically analyzed. In particular, the existence and significance of feedback loops between freshwater eutrophication and GHG emissions are emphasized considering the difficulties managing freshwater ecosystems and the Earth's climate. Finally, several future research directions as well as mitigation measures are described to provide useful insight into the dynamics and control of GHG emissions.

Effects of untreated or insufficiently treated wastewater discharges on the spatial and temporal variability of nitrous oxide (N2O) emissions from different streams in southeastern Brazil

Increasing atmospheric N₂O concentrations is of great environmental concern due to the role of this gas in climate change and stratospheric ozone destruction. Nitrogen-enriched lotic water bodies are significant sources of N₂O. However, N₂O emissions from rivers and streams, particularly those that receive untreated or insufficiently treated wastewater discharge, are poorly understood, especially in Brazil. The present study investigated the effects of the discharge of untreated or insufficiently treated wastewater on the spatial-temporal variability of N₂O emissions from different streams in Ilha Grande, located within the Abraão hydrographic system, in southeastern Brazil. Estimated N₂O fluxes determined in Abraão streams and upstream of the urbanized stretch ranged from 18.4 and 96.5 μg N m^-2 h^-1. Inside the urbanized stretch, estimated N₂O fluxes ranged from 110 to 561 μg N m^-2 h^-1 under non-limited dissolved oxygen (DO) conditions and 133 to 2,229 μg N m^-2 h^-1 under hypoxic conditions (DO < 2 mg O₂ L^-1). High spatial and temporal variability in N₂O emissions were noted, with the highest emissions in Abraão urban areas. Therefore, the differences observed between N₂O fluxes from the studied streams at Abraão seem to be associated with different lotic water body conditions, such as availability of reactive N and DO.

Nitrogen loss from a turbid river network based on N2 and N2O fluxes: Importance of suspended sediment

Riverine nitrogen loss makes a large contribution to the global nitrogen budget. However, little research has focused on nitrogen loss from large turbid rivers with high suspended sediment (SPS) concentrations. In this work, nitrogen loss amounts and related drivers were studied across fluvial networks of the Yellow River, the largest turbid river in the world, based on in situ measurement of nitrogen gas (N₂) and nitrous oxide (N₂O) fluxes at the water-air interface via the diffusion model and floating chamber methods, respectively. The results showed that N₂ and N₂O fluxes from the Yellow River ranged from -2.93 to 48.54 mmol m^-2 d^-1 and from 2.42 to 712.23 μmol m^-2 d^-1, respectively, with the nitrogen loss amount estimated to be 5.56 × 10⁷ kg N yr^-1 for the Yellow River, including the mainstem and main tributaries. Other than nitrogen compounds and water temperature, nitrogen loss from the Yellow River was also affected by SPS. Both N₂ flux: DIN and N₂O flux: DIN ratios increased remarkably in the middle reaches, probably due to a sharp increase of SPS concentration in this section. Furthermore, greater SPS concentrations were a main cause for the higher N₂O flux in the middle reaches than those in the other reaches of the Yellow River, and the possible effect of SPS was stronger on N₂O flux than on N₂ flux. This study demonstrates the importance of SPS in nitrogen loss from large turbid rivers, and more research is demanded to further clarify the role of SPS in riverine nitrogen cycle.

Density currents reduce nitrous oxide emissions in a tributary bay of Three Gorges Reservoir

Reservoirs are a significant source of the potent greenhouse gas nitrous oxide (N₂O), but there are few data on N₂O in the world's largest reservoirs and limited understanding of the factors controlling their emission rates. Here we analyzed high-resolution measurements of dissolved N₂O concentrations and fluxes in a typical tributary bay of Three Gorges Reservoir. The surface water was oversaturated in N₂O during both low and high water level (8.6 -16.4 nmol/L, 107% - 180% saturation) and N₂O fluxes varied nearly tenfold (0.2 and 1.6 μmol/(m² h)). Dissolved N₂O concentrations were characterized by pronounced vertical gradients, which were controlled by bidirectional density currents. The river water with high concentrations entered the bay as an underflow along the riverbed, the upper part of the water column was formed by intrusive backwater of Three Gorges Reservoir having significantly lower N₂O concentrations. In consequence, the N₂O emission potential of the impoundment was reduced compared to pre-impoundment conditions. These results reveal the importance of hydraulic conditions on N₂O emission from large reservoirs and suggest that flow regulation can be a potential tool for mitigating greenhouse gas emissions from manmade impoundments.

Urban rivers are hotspots of riverine greenhouse gas (N2O, CH4, CO2) emissions in the mixed-landscape chaohu lake basin

Growing evidence shows that riverine networks surrounding urban landscapes may be hotspots of riverine greenhouse gas (GHG) emissions. This study strengthens the evidence by investigating the spatial variability of diffusive GHG (N₂O, CH₄, CO₂) emissions from river reaches that drain from different types of landscapes (i.e., urban, agricultural, mixed, and forest landscapes), in the Chaohu Lake basin of eastern China. Our results showed that almost all the rivers were oversaturated with dissolved GHGs. Urban rivers were identified as emission hotspots, with mean fluxes of 470 μmol m^-2d^-1 for N₂O, 7 mmol m^-2d^-1 for CH₄, and 900 mmol m^-2d^-1 for CO₂, corresponding to ~14, seven, and two times of those from the non-urban rivers in the Chaohu Lake basin, respectively. Factors related to the high N₂O and CH₄ emissions in urban rivers included large nutrient supply and hypoxic environments. The factors affecting CO₂ were similar in all the rivers, which were temperature-dependent with suitable environments that allowed rapid decomposition of organic matter. Overall, this study highlights that better recognition of the influence that river networks have on global warming is required-particularly when it comes to urban rivers, as urban land cover and populations will continue to expand in the future. Management measures should incorporate regional hotspots to more efficiently mitigate GHG emissions.

Minimization of N2O Emission through Intermittent Aeration in a Sequencing Batch Reactor (SBR): Main Behavior and Mechanism

To explore the main behavior and mechanism of minimizing nitrous oxide (N2O) emission through intermittent aeration during wastewater treatment, two lab-scale sequencing batch reactors operated at intermittently aerated mode (SBR1), and continuously aerated mode (SBR2) were established. Compared with SBR2, the intermittently aerated SBR1 reached not only a higher total nitrogen removal efficiency (averaged 93.5%) but also a lower N2O-emission factor (0.01–0.53% of influent ammonia), in which short-cut nitrification and denitrification were promoted. Moreover, less accumulation and consumption of polyhydroxyalkanoates, a potential endogenous carbon source promoting N2O emission, were observed in SBR1. Batch experiments revealed that nitrifier denitrification was the major pathway generating N2O while heterotrophic denitrification played as a sink of N2O, and SBR1 embraced a larger N2O-mitigating capability. Finally, quantitative polymerase chain reaction results suggested that the abundant complete ammonia oxidizer (comammox) elevated in the intermittently aerated environment played a potential role in avoiding N2O generation during wastewater treatment. This work provides an in-depth insight into the utilization of proper management of intermittent aeration to control N2O emission from wastewater treatment plants.

Surface nitrous oxide (N2O) concentrations and fluxes from different rivers draining contrasting landscapes: Spatio-temporal variability, controls, and implications based on IPCC emission factor

Increasing indirect nitrous oxide (N₂O) emission from river networks as a result of enhanced human activities on landscapes has become a global issue, as N₂O has been widely recognized as an important ozone-depleting greenhouse gas. However, indirect N₂O emissions from different rivers, particularly for those that drain completely different landscapes, are poorly understood. Here, we investigated the spatial-temporal variability of N₂O emissions among the different rivers in the Chaohu Lake Basin of Eastern China. Our results showed that river reaches in urban watersheds are the hotspots of N₂O production, with a mean N₂O concentration of ∼410 nmol L^-1, which is 9-18 times greater than those mainly draining forested (23 nmol L^-1), agricultural (42 nmol L^-1) and mixed (45 nmol L^-1) landscapes. Riverine dissolved N₂O was generally supersaturated with respect to the atmosphere. Such N₂O saturation can best be explained by nitrogen availability, except for those in the forested watersheds, where dissolved oxygen is thought to be the primary predictor. The estimated N₂O fluxes in urban rivers reached ∼471 μmol m^-2 d^-1, a value of ∼22, 13, and 11 times that in forested, agricultural and mixed watersheds, respectively. Averaged riverine N₂O emission factors (EF_5r) of the forested, agricultural, urban and mixed watersheds were 0.066%, 0.12%, 0.95% and 0.16%, respectively, showing different deviations from the default EF_5r that released by IPCC in 2019. This points to a need for more field measurements with wider spatial coverage and finer frequency to further refine the EF_5r and to better reveal the mechanisms behind indirect N₂O emissions as influenced by watershed landscapes.

Are nitrous oxide emissions indirectly fueled by input of terrestrial dissolved organic nitrogen in a large eutrophic Lake Taihu, China?

Lakes actively transform nitrogen (N) and emit disproportionately large amounts of N₂O relative to their surface area. Studies have investigated the relative importance of denitrification or nitrification on N₂O emissions; however, the linkage between N₂O efflux and dissolved organic nitrogen (DON) and carbon (DOC) remains largely unknown. Long-term (2012-2017) seasonal field observations and a series of degradation experiments were used to unravel how DON composition impacts N₂O emissions from Lake Taihu, China. In the northwestern part of the lake, large riverine inflow and high N₂O emissions occur in all seasons (24.6 ± 25.2 μmol m^-2 d^-1), coincident with high levels of terrestrial DON and DOC here. The degradation of labile DON and DOC likely enhanced ammonification as supported by the correlations between NH₄⁺-N and DON, DOC, a(350), and terrestrial humic-like C3. The area with large riverine inputs in the northwestern part of the lake was characterized by low DO which may enhance incomplete aerobic nitrification and incomplete denitrification, both leading to N₂O production. Twenty days laboratory experiments indicated greater N₂O production in the northwest inflow samples (N₂O on day 20: 120.9 nmol L^-1 and 17.3 nmol L^-1 for bio- and photo-degradation samples, respectively) compared with the central lake samples (N₂O on day 20: 20.3 nmol L^-1 and 12.3 nmol L^-1 for bio- and photo-degradation samples, respectively), despite both having low Chl-a. Our DON and DOC degradation experiments confirmed the occurrence of ammonification along with consumption of NH₄⁺-N and thereafter NO₃^--N. Our results collectively suggest that terrestrial DON fueled ammonification, enhanced nitrification and incomplete denitrification, and thereby became an important contributor to the N₂O efflux from Lake Taihu.

Diffusive flux of CH4 and N2O from agricultural river networks: Regression tree and importance analysis

River networks in subtropical agricultural hilly region become an inconvenient greenhouse gas (GHG, methane and nitrous oxide) source because of the influence of human activities, which has caused large uncertainties for refinement of national GHG inventories and their global budget. Based on field monitoring experiments at high temporal resolution, we employed regression tree and importance analysis to identify quantitatively factors that influence the diffusive flux of GHGs to provide a scientific basis for reducing GHG emissions and controlling regional carbon and nitrogen losses. The results indicate that significant spatiotemporal variation of methane (CH₄) nitrous oxide (N₂O) diffusion occurs in all the four reaches (W1, W2, W3 and W4) of Tuojia river networks. Among them, W1 contributed lowest CH₄ (22.55 μg C m^-2 h^-1) and N₂O (5.00 μg N m^-2 h^-1) diffusive flux than the other three (P < 0.05), while W4 offered highest CH₄ (166.15 μg C m^-2 h^-1) and N₂O (30.47 μg N m^-2 h^-1) diffusive flux but with no statistically significant difference between W2 and W3 due to homogeneous extraneous nutrition loading into the two reaches. W4 also contributed largest cumulative flux of CH₄ (14.55 kg C ha^-1 yr^-1) and N₂O (2.69 kg N ha^-1 yr^-1) in Tuojia River networks (P < 0.05). Furthermore, the regression tree and importance analysis indicate that, in the anaerobic environment, dissolved oxygen saturation controlled the production and diffusion for both CH₄ and N₂O. The findings of this investigation highlighted that decision support tools provide an effective pathway to enhance the GHG mitigation technology research in agroecosystems and simultaneously shed light on the global campaign on refinement of national GHG inventories as well as regional nutrient management.

Methane and nitrous oxide porewater concentrations and surface fluxes of a regulated river

Greenhouse gas (GHG) emissions from rivers are a critical missing component of current global GHG models. Their exclusion is mainly due to a lack of in-situ measurements and a poor understanding of the spatiotemporal dynamics of GHG production and emissions, which prevents optimal model parametrization. We combined simultaneous observations of porewater concentrations along different beach positions and depths, and surface fluxes of methane and nitrous oxide at a plot scale in a large regulated river during three water stages: rising, falling, and low. Our goal was to gain insights into the interactions between hydrological exchanges and GHG emissions and elucidate possible hypotheses that could guide future research on the mechanisms of GHG production, consumption, and transport in the hyporheic zone (HZ). Results indicate that the site functioned as a net source of methane. Surface fluxes of methane during river water stages at three beach positions (shallow, intermediate and deep) correlated with porewater concentrations of methane. However, fluxes were significantly higher in the intermediate position during the low water stage, suggesting that low residence time increased methane emissions. Vertical profiles of methane peaked at different depths, indicating an influence of the magnitude and direction of the hyporheic mixing during the different river water stages on methane production and consumption. The site acted as either a sink or a source of nitrous oxide depending on the elevation of the water column. Nitrous oxide porewater concentrations peaked at the upper layers of the sediment throughout the different water stages. River hydrological stages significantly influenced porewater concentrations and fluxes of GHG, probably by influencing heterotrophic respiration (production and consumption processes) and transport to and from the HZ. Our results highlight the importance of including dynamic hydrological exchanges when studying and modeling GHG production and consumption in the HZ of large rivers.

Role of sludge retention time in mitigation of nitrous oxide emission from a pilot-scale oxidation ditch

Nitrous oxide (N₂O) emission from wastewater treatment plants (WWTPs) has become a focus of attention due to its significant greenhouse effect. In this study, the role of sludge retention time (SRT) in mitigation of N₂O emission from a pilot-scale oxidation ditch was systematically investigated. The activated sludge system that operated at SRT of 25 days demonstrated significantly lower N₂O emission factor, higher resistance to ammonia overload and aeration failure shock than those obtained at SRT of 15 days no matter which hydraulic retention time (HRT) was adopted. Batch experiments revealed that nitrifier denitrification (ND) was the primary mechanism of N₂O generation. However, more microbes affiliated with Nitrospira genera were harbored in the system at SRT 25 d, which could effectively avoid nitrite accumulation, a key factor promoting N₂O generation by ND. PICRUSt results further suggested the system at SRT 25 d possessed higher genetic potential for N₂O reduction reflected by the more abundant nitrous-oxide reductase.

Assessment of Indirect N2O Emission Factors from Agricultural River Networks Based on Long-Term Study at High Temporal Resolution

Assessment of indirect emission factors (EF_5r) of nitrous oxide (N₂O) from agricultural river networks remains challenging, and results are uncertain due to limited data availability. This study compared two methods of assessing EF_5r using data from long-term observations at high temporal resolution in a typical agricultural catchment in subtropical central China. The concentration method (method 1) and the Intergovernmental Panel on Climate Change (IPCC) 2006 method (method 2) were employed to evaluate the emission factor. EF_5r estimated using method 1 (i.e., EF_5r1) was 0.00077 ± 0.00025 (0.00038-0.00097). EF_5r calculated using method 2 (i.e., EF_5r2) was lower than EF_5r1, with a mean value of 0.00004 (0.000015-0.00012). Both EF_5r1 and EF_5r2 were significantly lower than the IPCC 2006 default value of 0.0025, suggesting that N₂O emissions from China and world river networks may be grossly overestimated. A complex N₂O production pathway and diffusion mechanism were responsible for the transfer of N₂O from the sediment to river water and then to the atmosphere. These findings provide essential data for refining national greenhouse gas inventories and contribute evidence for downward revision of indirect emission factors adopted by the IPCC.

Surface nitrous oxide concentrations and fluxes from water bodies of the agricultural watershed in Eastern China

Agriculture is one of major emission sources of nitrous oxide (N₂O), an important greenhouse gas dominating stratospheric ozone destruction. However, indirect N₂O emissions from agriculture watershed water surfaces are poorly understood. Here, surface-dissolved N₂O concentration in water bodies of the agricultural watershed in Eastern China, one of the most intensive agricultural regions, was measured over a two-year period. Results showed that the dissolved N₂O concentrations varied in samples taken from different water types, and the annual mean N₂O concentrations for rivers, ponds, reservoir, and ditches were 30 ± 18, 19 ± 7, 16 ± 5 and 58 ± 69 nmol L^-1, respectively. The N₂O concentrations can be best predicted by the NO₃^--N concentrations in rivers and by the NH₄⁺-N concentrations in ponds. Heavy precipitation induced hot moments of riverine N₂O emissions were observed during farming season. Upstream waters are hot spots, in which the N₂O production rates were two times greater than in non-hotspot locations. The modeled watershed indirect N₂O emission rates were comparable to direct emission from fertilized soil. A rough estimate suggests that indirect N₂O emissions yield approximately 4% of the total N₂O emissions yield from N-fertilizer at the watershed scale. Separate emission factors (EF) established for rivers, ponds, and reservoir were 0.0013, 0.0020, and 0.0012, respectively, indicating that the IPCC (Inter-governmental Panel on Climate Change) default value of 0.0025 may overestimate the indirect N₂O emission from surface water in eastern China. EF was inversely correlated with N loading, highlighting the potential constraints in the IPCC methodology for water with a high anthropogenic N input.

Methane and nitrous oxide emission from different treatment units of municipal wastewater treatment plants in Southwest Germany

We measured the atmospheric emission rates of methane (CH₄) and nitrous oxide (N₂O) in two wastewater treatment plants in Southwest Germany, which apply different treatment technologies. Dissolved gas concentrations and fluxes were measured during all processing steps as well as in the discharge receiving streams. N₂O isotopocule analysis revealed that NH₂OH oxidation during nitrification contributed 86–96% of the N₂O production in the nitrification tank, whereas microbial denitrification was the main production pathway in the denitrification tank in a conventional activated sludge (CAS) system. During wastewater treatment using a modified Ludzack-Ettinger system (MLE) with energy recovery, N₂O was predominantly produced by the NO₂^- reduction by nitrifier-denitrification process. For both systems, N₂O emissions were low, with emission factors of 0.008% and 0.001% for the MLE and the CAS system, respectively. In the effluent-receiving streams, bacterial denitrification and nitrification contributed nearly equally to N₂O production. The CH₄ emission from the MLE system was estimated as 118.1 g-C d^-1, which corresponds to an emission factor of 0.004%, and was three times lower than the emission from the CAS system with 0.01%.

Nitrogen removal rates in a frigid high-altitude river estimated by measuring dissolved N2 and N2O

Rivers are important sites of both nitrogen removal and emission of nitrous oxide (N₂O), a powerful greenhouse gas. Previous measurements have focused on nitrogen-rich temperate rivers, with cold, low-nitrogen river systems at high-altitude receiving less attention. Here, nitrogen removal rates were estimated by directly measuring dissolved N₂ and N₂O of the Yellow River in its source region of the Tibetan Plateau, a frigid high-altitude environment. We measured the dissolved N₂ and N₂O using N₂:Ar ratio method and headspace equilibrium technique, respectively. Dissolved N₂ in the river water ranged from 337 to 513 μmol N₂ L^-1, and dissolved N₂O ranged from 10.4 to 15.4 nmol N₂O L^-1. Excess dissolved N₂ (△N₂) ranged from -8.6 to 10.5 μmol N₂ L^-1, while excess dissolved N₂O (△N₂O) ranged from 2.1 to 8.3 nmol N₂O L^-1; they were relatively low compared with most other rivers in the world. However, N₂ removal fraction (△N₂/DIN, average 21.6%) and EF_5r values (N₂O - N/NO₃ - N, range 1.6 × 10^-4-5.0 × 10^-2) were comparable with many other rivers despite the high altitude for the Yellow River source region. Furthermore, the EF_5r values increased with altitude. Estimated fluxes of N₂ and N₂O to the atmosphere from the river surface ranged from -67.5 to 93.5 mmol N m^-2 d^-1 and from 4.8 to 93.8 μmol N m^-2 d^-1, respectively, and the nitrogen removal from rivers was estimated to be 1.87 × 10⁷ kg N yr^-1 for the Yellow River source region. This is the first report of nitrogen removal for a frigid high-altitude river; the results suggest that N removal and N₂O emission from cold high-altitude rivers should be considered in the global nitrogen budget.

Contribution of dissolved N2O in total N2O emission from sewage treatment plant

The characteristics of nitrous oxide, N₂O, from a sewage treatment plant, which conducts nitrogen removal, and the river that receives its effluent water, were investigated by intensive daily surveys in summer and winter. N₂O production in the sewage treatment plant was promoted in winter when nitrite accumulated in the reaction tank. The dissolved N₂O concentration in the effluent water was also high in winter, which caused the dissolved N₂O concentration to increase in the river downstream. In contrast, the N₂O production inside the plant and the dissolved N₂O emission through the effluent water, the dissolved N₂O discharge, was controlled in summer when the nitrogen removal was more complete and there was no-nitrite accumulation. The dissolved N₂O in the effluent water was rapidly lost after leaving the plant by as much as 26% in summer and 59% in winter. Additionally, the amount of the dissolved N₂O discharge in winter was almost equal to that of the indirect N₂O emission. When the nitrogen removal proceeded successfully, the amount of dissolved N₂O discharge was small. In contrast, when the nitrogen removal was insufficient, the dissolved N₂O discharge became an important N₂O source.

Street-level emissions of methane and nitrous oxide from the wastewater collection system in Cincinnati, Ohio

Recent studies have indicated that urban streets can be hotspots for emissions of methane (CH₄) from leaky natural gas lines, particularly in cities with older natural gas distribution systems. The objective of the current study was to determine whether leaking sewer pipes could also be a source of street-level CH₄ as well as nitrous oxide (N₂O) in Cincinnati, Ohio, a city with a relatively new gas pipeline network. To do this, we measured the carbon (δ¹³C) and hydrogen (δ²H) stable isotopic composition of CH₄ to distinguish between biogenic CH₄ from sewer gas and thermogenic CH₄ from leaking natural gas pipelines and measured CH₄ and N₂O flux rates and concentrations at sites from a previous study of street-level CH₄ enhancements (77 out of 104 sites) as well as additional sites found through surveying sewer grates and utility manholes (27 out of 104 sites). The average isotopic signatures for δ¹³C-CH₄ and δ²H-CH₄ were -48.5‰ ± 6.0‰ and -302‰ ± 142‰. The measured flux rates ranged from 0.0 to 282.5 mg CH₄ day^-1 and 0.0-14.1 mg N₂O day^-1 (n = 43). The average CH₄ and N₂O concentrations measured in our study were 4.0 ± 7.6 ppm and 392 ± 158 ppb, respectively (n = 104). 72% of sites where fluxes were measured were a source of biogenic CH₄. Overall, 47% of the sampled sites had biogenic CH₄, while only 13% of our sites had solely thermogenic CH₄. The other sites were either a source of both biogenic and thermogenic CH₄ (13%), and a relatively large portion of sites had an unresolved source (29%). Overall, this survey of emissions across a large urban area indicates that production and emission of biogenic CH₄ and N₂O is considerable, although CH₄ fluxes are lower than those reported for cities with leaky natural gas distribution systems.

The comparison of greenhouse gas emissions in sewage treatment plants with different treatment processes

Greenhouse gas emissions from different sewage treatment plants: oxidation ditch process, double-circulated anoxic-oxic process and anoxic-oxic process were evaluated based on the survey. The methane and nitrous oxide characteristics were discussed based on the gaseous and dissolved gas profiles. As a result, it was found that methane was produced in the sewer pipes and the primary sedimentation tank. Additionally, a ventilation system would promote the gasification of dissolved methane in the first treatment units. Nitrous oxide was produced and emitted in oxic tanks with nitrite accumulation inside the sewage treatment plant. A certain amount of nitrous oxide was also discharged as dissolved gas through the effluent water. If the amount of dissolved nitrous oxide discharge is not included, 7-14% of total nitrous oxide emission would be overlooked. Based on the greenhouse gas calculation, electrical consumption and the N₂O emission from incineration process were major sources in all the plants. For greenhouse gas reduction, oxidation ditch process has an advantage over the other advanced systems due to lower energy consumption, sludge production, and nitrogen removal without gas stripping.

Greenhouse gases emission from the sewage draining rivers

Carbon dioxide (CO₂), methane (CH₄) and nitrous oxide (N₂O) concentration, saturation and fluxes in rivers (Beitang drainage river, Dagu drainage rive, Duliujianhe river, Yongdingxinhe river and Nanyunhe river) of Tianjin city (Haihe watershed) were investigated during July and October in 2014, and January and April in 2015 by static headspace gas chromatography method and the two-layer model of diffusive gas exchange. The influence of environmental variables on greenhouse gases (GHGs) concentration under the disturbance of anthropogenic activities was discussed by Spearman correlative analysis and multiple stepwise regression analysis. The results showed that the concentration and fluxes of CO₂, CH₄ and N₂O were seasonally variable with >winter>fall>summer, spring>summer>winter>fall and summer>spring>winter>fall for concentrations and spring>summer>fall>winter, spring>summer>winter>fall and summer>spring>fall>winter for fluxes respectively. The GHGs concentration and saturation were higher in comprehensively polluted river sites and lower in lightly polluted river sites. The three GHGs emission fluxes in two sewage draining rivers of Tianjin were clearly higher than those of other rivers (natural rivers) and the spatial variation of CH₄ was more obvious than the others. CO₂ and N₂O air-water interface emission fluxes of the sewage draining rivers in four seasons were about 1.20-2.41 times and 1.13-3.12 times of those in the natural rivers. The CH₄ emission fluxes of the sewage draining rivers were 3.09 times in fall to 10.87 times in spring of those in the natural rivers in different season. The wind speed, water temperature and air temperature were related to GHGs concentrations. Nitrate and nitrite (NO₃^-+NO₂^--N) and ammonia (NH₄⁺-N) were positively correlated with CO₂ concentration and CH₄ concentration; and dissolved oxygen (DO) concentration was negatively correlated with CH₄ concentration and N₂O concentration. The effect of human activities on carbon and nitrogen cycling in river is great.

Methane and nitrous oxide annual emissions from an old eutrophic temperate reservoir

Two greenhouse gases -methane (CH₄) and nitrous oxide (N₂O) - were monitored monthly during one year (2011) at the Eguzon Reservoir in France. The objective of the study was to quantify for the first time in a temperate area the total emissions of these gases through the main emission pathways (diffusion and bubbling from the reservoir, degassing and downstream diffusion). The reservoir was impounded in 1926 and had, in 2011, a eutrophic status promoting high organic matter degradation and nitrification-denitrification, all favouring CH₄ and N₂O production. CH₄ and N₂O emissions were dominated by diffusion from the reservoir surface (respectively 78.0% and 92.3%). Ebullition was only observed for CH₄ and accounted for 14.0% of total CH₄ emissions. Downstream degassing and diffusion represented 8.1% of the total CH₄ emissions and 7.7% of the total N₂O emissions.

Influence of infrastructure on water quality and greenhouse gas dynamics in urban streams

Streams and rivers are significant sources of nitrous oxide (N₂O), carbon dioxide (CO₂), and methane (CH₄) globally, and watershed management can alter greenhouse gas (GHG) emissions from streams. We hypothesized that urban infrastructure significantly alters downstream water quality and contributes to variability in GHG saturation and emissions. We measured gas saturation and estimated emission rates in headwaters of two urban stream networks (Red Run and Dead Run) of the Baltimore Ecosystem Study Long-Term Ecological Research project. We identified four combinations of stormwater and sanitary infrastructure present in these watersheds, including: (1) stream burial, (2) inline stormwater wetlands, (3) riparian/floodplain preservation, and (4) septic systems. We selected two first order catchments in each of these categories and measured GHG concentrations, emissions, and dissolved inorganic and organic carbon (DIC and DOC) and nutrient concentrations biweekly for 1 year. From a water quality perspective, the DOC : NO₃ ^- ratio of streamwater was significantly different across infrastructure categories. Multiple linear regressions including DOC : NO₃ ^- and other variables (dissolved oxygen, DO; total dissolved nitrogen, TDN; and temperature) explained much of the statistical variation in nitrous oxide (N₂O, r² = 0.78), carbon dioxide (CO₂, r² = 0.78) and methane (CH4, r 2 = 0.50) saturation in stream water. We measured N₂O saturation ratios, which were among the highest reported in the literature for streams, ranging from 1.1 to 47 across all sites and dates. N₂O saturation ratios were highest in streams draining watersheds with septic systems and strongly correlated with TDN. The CO₂ saturation ratio was highly correlated with the N₂O saturation ratio across all sites and dates, and the CO₂ saturation ratio ranged from 1.1 to 73. CH₄ was always supersaturated, with saturation ratios ranging from 3.0 to 2157. Longitudinal surveys extending form headwaters to third-order outlets of Red Run and Dead Run took place in spring and fall. Linear regressions of these data yielded significant negative relationships between each gas with increasing watershed size as well as consistent relationships between solutes (TDN or DOC, and DOC : TDN ratio) and gas saturation. Despite a decline in gas saturation between the headwaters and stream outlet, streams remained saturated with GHGs throughout the drainage network, suggesting that urban streams are continuous sources of CO₂, CH₄, and N₂O. Our results suggest that infrastructure decisions can have significant effects on downstream water quality and greenhouse gases, and watershed management strategies may need to consider coupled impacts on urban water and air quality.

Dissolved nitrous oxide and emission relating to denitrification across the Poyang Lake aquatic continuum

Most aquatic ecosystems contribute elevated N₂O to atmosphere due to increasing anthropogenic nitrogen loading. To further understand the spatial heterogeneity along an aquatic continuum from the upriver to wetland to lake to downriver, the study was conducted on spatial variations in N₂O emission along Poyang Lake aquatic continuum during the flood season from 15 July 2013 to 10 August 2013. The results showed the N₂O concentrations, the ratio of N₂O/dinitrogen (N₂) gases production, N₂O emission and denitrification rates ranged from 0.10 to 1.11μgN/L, -0.007% to 0.051%, -9.73 to 127μgN/m²/hr and 1.33×10⁴ to 31.9×10⁴μgN₂/m²/hr, respectively, across the continuum. The average N₂O concentrations, the ratio of N₂O/N₂ and N₂O emission was significantly lower in wetlands as compared to the rivers and lake (p<0.01). The significantly high denitrification rate and low N₂O emission together highlighted that most N₂O can be converted into N₂ via near complete denitrification in the Poyang Lake wetlands. Our study suggests that the wetlands might impact N₂O budget in an integrated aquatic ecosystems. Moreover, N₂O emission from different aquatic ecosystem should be considered separately when quantifying the regional budget in aquatic ecosystem.

Modeling nitrous oxide emission from rivers: a global assessment

Estimates of global riverine nitrous oxide (N₂ O) emissions contain great uncertainty. We conducted a meta-analysis incorporating 169 observations from published literature to estimate global riverine N₂ O emission rates and emission factors. Riverine N₂ O flux was significantly correlated with NH₄ , NO₃ and DIN (NH₄  + NO₃ ) concentrations, loads and yields. The emission factors EF(a) (i.e., the ratio of N₂ O emission rate and DIN load) and EF(b) (i.e., the ratio of N₂ O and DIN concentrations) values were comparable and showed negative correlations with nitrogen concentration, load and yield and water discharge, but positive correlations with the dissolved organic carbon : DIN ratio. After individually evaluating 82 potential regression models based on EF(a) or EF(b) for global, temperate zone and subtropical zone datasets, a power function of DIN yield multiplied by watershed area was determined to provide the best fit between modeled and observed riverine N₂ O emission rates (EF(a): R²  = 0.92 for both global and climatic zone models, n = 70; EF(b): R²  = 0.91 for global model and R²  = 0.90 for climatic zone models, n = 70). Using recent estimates of DIN loads for 6400 rivers, models estimated global riverine N₂ O emission rates of 29.6-35.3 (mean = 32.2) Gg N₂ O-N yr^-1 and emission factors of 0.16-0.19% (mean = 0.17%). Global riverine N₂ O emission rates are forecasted to increase by 35%, 25%, 18% and 3% in 2050 compared to the 2000s under the Millennium Ecosystem Assessment's Global Orchestration, Order from Strength, Technogarden, and Adapting Mosaic scenarios, respectively. Previous studies may overestimate global riverine N₂ O emission rates (300-2100 Gg N₂ O-N yr^-1 ) because they ignore declining emission factor values with increasing nitrogen levels and channel size, as well as neglect differences in emission factors corresponding to different nitrogen forms. Riverine N₂ O emission estimates will be further enhanced through refining emission factor estimates, extending measurements longitudinally along entire river networks and improving estimates of global riverine nitrogen loads.

Export of Dissolved Methane and Carbon Dioxide with Effluents from Municipal Wastewater Treatment Plants

Inland waters play an important role for regional and global scale carbon cycling and are significant sources of the atmospheric greenhouse gases methane (CH4) and carbon dioxide (CO2). Although most studies considered the input of terrestrially derived organic and inorganic carbon as the main sources for these emissions, anthropogenic sources have rarely been investigated. Municipal wastewater treatment plants (WWTPs) could be additional sources of carbon by discharging the treated wastewater into the surrounding aquatic ecosystems. Here we analyze seasonally resolved measurements of dissolved CH4 and CO2 concentrations in effluents and receiving streams at nine WWTPs in Germany. We found that effluent addition significantly altered the physicochemical properties of the streamwater. Downstream of the WWTPs, the concentrations of dissolved CH4 and CO2 were enhanced and the atmospheric fluxes of both gases increased by a factor of 1.2 and 8.6, respectively. The CH4 exported with discharged effluent, however, accounted for only a negligible fraction (0.02%) of the estimated total CH4 emissions during the treatment process. The CH4 concentration in the effluent water was linearly related to the organic load of the wastewater, which can provide an empirical basis for future attempts to add WWTPs inputs to regional-scale models for inland water-carbon fluxes.

Modeling Source Water TOC Using Hydroclimate Variables and Local Polynomial Regression

To control disinfection byproduct (DBP) formation in drinking water, an understanding of the source water total organic carbon (TOC) concentration variability can be critical. Previously, TOC concentrations in water treatment plant source waters have been modeled using streamflow data. However, the lack of streamflow data or unimpaired flow scenarios makes it difficult to model TOC. In addition, TOC variability under climate change further exacerbates the problem. Here we proposed a modeling approach based on local polynomial regression that uses climate, e.g. temperature, and land surface, e.g., soil moisture, variables as predictors of TOC concentration, obviating the need for streamflow. The local polynomial approach has the ability to capture non-Gaussian and nonlinear features that might be present in the relationships. The utility of the methodology is demonstrated using source water quality and climate data in three case study locations with surface source waters including river and reservoir sources. The models show good predictive skill in general at these locations, with lower skills at locations with the most anthropogenic influences in their streams. Source water TOC predictive models can provide water treatment utilities important information for making treatment decisions for DBP regulation compliance under future climate scenarios.

Fighting global warming by greenhouse gas removal: destroying atmospheric nitrous oxide thanks to synergies between two breakthrough technologies

Even if humans stop discharging CO2 into the atmosphere, the average global temperature will still increase during this century. A lot of research has been devoted to prevent and reduce the amount of carbon dioxide (CO2) emissions in the atmosphere, in order to mitigate the effects of climate change. Carbon capture and sequestration (CCS) is one of the technologies that might help to limit emissions. In complement, direct CO2 removal from the atmosphere has been proposed after the emissions have occurred. But, the removal of all the excess anthropogenic atmospheric CO2 will not be enough, due to the fact that CO2 outgases from the ocean as its solubility is dependent of its atmospheric partial pressure. Bringing back the Earth average surface temperature to pre-industrial levels would require the removal of all previously emitted CO2. Thus, the atmospheric removal of other greenhouse gases is necessary. This article proposes a combination of disrupting techniques to transform nitrous oxide (N2O), the third most important greenhouse gas (GHG) in terms of current radiative forcing, which is harmful for the ozone layer and possesses quite high global warming potential. Although several scientific publications cite "greenhouse gas removal," to our knowledge, it is the first time innovative solutions are proposed to effectively remove N2O or other GHGs from the atmosphere other than CO2.

Spatial and temporal variability in nitrous oxide and methane emissions in urban riparian zones of the Pearl River Delta

Spatial and temporal variability in nitrous oxide and methane emissions were quantified in three seasons using closed chambers in three riparian zone locations of three branches of the Pearl River, Guangzhou, China. The sampling sites were selected in a rapidly developing urban area of Guangzhou and represented a pollution gradient. The results show that urban riparian landscapes can be large source areas for CH4 and N2O, with fluxes of -0.035∼32.30 mg m(-2) h(-1) and -5.49∼37.31 μg m(-2) h(-1), respectively. River water quality, sediment texture, and NH4-N and NO3-N concentrations correlated with N2O and CH4 emission rates. The riparian zones of the more seriously polluted tributaries showed higher greenhouse gas fluxes than that of the less polluted main stem of the Pearl River. Rain events increased emissions of CH4 by 6.5∼21.3 times and N2O by 2.2∼5.7 times. The lower concentrations of heavy metals increased the activity of denitrifying enzymes while inhibited the methane producing pathways. This work demonstrates that rapidly developing urban areas are an important source of greenhouse gas emissions, which is conditioned by various environmental factors.

Indirect nitrous oxide emissions from streams within the US Corn Belt scale with stream order

N2O is an important greenhouse gas and the primary stratospheric ozone depleting substance. Its deleterious effects on the environment have prompted appeals to regulate emissions from agriculture, which represents the primary anthropogenic source in the global N2O budget. Successful implementation of mitigation strategies requires robust bottom-up inventories that are based on emission factors (EFs), simulation models, or a combination of the two. Top-down emission estimates, based on tall-tower and aircraft observations, indicate that bottom-up inventories severely underestimate regional and continental scale N2O emissions, implying that EFs may be biased low. Here, we measured N2O emissions from streams within the US Corn Belt using a chamber-based approach and analyzed the data as a function of Strahler stream order (S). N2O fluxes from headwater streams often exceeded 29 nmol N2O-N m(-2) ⋅ s(-1) and decreased exponentially as a function of S. This relation was used to scale up riverine emissions and to assess the differences between bottom-up and top-down emission inventories at the local to regional scale. We found that the Intergovernmental Panel on Climate Change (IPCC) indirect EF for rivers (EF5r) is underestimated up to ninefold in southern Minnesota, which translates to a total tier 1 agricultural underestimation of N2O emissions by 40%. We show that accounting for zero-order streams as potential N2O hotspots can more than double the agricultural budget. Applying the same analysis to the US Corn Belt demonstrates that the IPCC EF5r underestimation explains the large differences observed between top-down and bottom-up emission estimates.

Nitrogen loading and nitrous oxide emissions from a river with multiple hydroelectric reservoirs

River networks receive a large fraction of the anthropogenic nitrogen applied to river catchments. The different impacts of the stream nitrogen (N) loading on nitrous oxide (N2O) emissions from various of aquatic ecosystems are still unknown. In this study, direct measurements of water-air interface N2O exchange in different water bodies were conducted. Results showed that the water-air interface N2O exchange from tributaries, hydropower station reservoirs, a main stream, and its estuary were 10.14 ± 13.51, 15.64 ± 10.72, 27.59 ± 20.99, and 15.98 ± 12.26 µg N2O-N m(-2) h(-1), respectively, indicating the strong impacts of human activities on N2O emission rates. The water NO2 (-)-N values predicted the dissolved N2O concentrations better than did the NO3 (-)-N and NH4 (+)-N values, indicating strong denitrification and nitrification processes. The dissolved inorganic N explained 36 % of the variations in the N2O emissions for the whole river network.