Nonlinear response of riverine N2O fluxes to oxygen and temperature

Seasonal variation in greenhouse gas concentrations and diffusive fluxes in three river-reservoir systems in the Seine Basin (France)

River and reservoir ecosystems have been considered as hot spots for GHG (greenhouse gas) emissions while their specific hydrological and biogeochemical processes affect GHG concentrations; however, few studies integrated river-reservoir systems to identify the dominant drivers of GHG concentrations and flux changes associated with these systems. In the present study, we examined the seasonal variations in GHG concentrations in the surface water of three river-reservoir systems in the Seine Basin. The levels and seasonal variations of GHG concentrations exhibited distinct patterns among reservoirs, upstream, and downstream rivers. The concentrations of CH4 (methane) in the reservoirs were notably higher than those observed in both upstream and downstream rivers and showed higher values in summer and autumn, which contrasted with CO2 (carbon dioxide) concentrations, while N2O (nitrous oxide) concentrations did not show an obvious seasonal pattern. A high mole ratio of CH4/CO2 was found in these reservoirs, with a value of 0.03 and was more than 30 and 10 times higher than that in the upstream and downstream rivers, respectively. The three river-reservoir systems were oversaturated with GHG during the study period, with the average diffusive fluxes (expressed as CO2eq: CO2 equivalent) of 810 ± 1098 mg CO2eq m-2 d-1, 9920 ± 2413 mg CO2eq m-2 d-1, and 7065 ± 2704 mg CO2eq m-2 d-1 in the reservoirs, upstream and downstream rivers, respectively. CO2 and CH4-CO2 were respectively the dominant contributors to GHG diffusive fluxes in river and reservoir sections, while N2O contributed negligibly to GHG diffusive fluxes in the three river-reservoir systems. Our results showed that GHG concentrations and gas transfer coefficient have varying importance in driving GHG diffusive fluxes among different sections of the river-reservoir systems. In addition, our results also show the combined effect of reservoirs and upstream rivers on the water quality variables and hydrological characteristics of downstream rivers, highlighting the future need for additional investigations of GHG processes in the river-reservoir systems.

High-Resolution Estimates of N2O Emissions from Inland Waters and Wetlands in China

Inland waters (rivers, lakes, and reservoirs) and wetlands (marshes and coastal wetlands) represent large and continuous sources of nitrous oxide (N2O) emissions, in view of adequate biomass and anaerobic conditions. Considerable uncertainties remain in quantifying spatially explicit N2O emissions from aquatic systems, attributable to the limitations of models and a lack of comprehensive data sets. Herein, we conducted a synthesis of 1659 observations of N2O emission rates to determine the major environmental drivers across five aquatic systems. A framework for spatially explicit estimates of N2O emissions in China was established, employing a data-driven approach that upscaled from site-specific N2O fluxes to robust multiple-regression models. Results revealed the effectiveness of models incorporating soil organic carbon and water content for marshes and coastal wetlands, as well as water nitrate concentration and dissolved organic carbon for lakes, rivers, and reservoirs for predicting emissions. Total national N2O emissions from inland waters and wetlands were 1.02 × 105 t N2O yr-1, with contributions from marshes (36.33%), rivers (27.77%), lakes (25.27%), reservoirs (6.47%), and coastal wetlands (4.16%). Spatially, larger emissions occurred in the Songliao River Basin and Continental River Basin, primarily due to their substantial terrestrial biomass. This study offers a vital national inventory of N2O emissions from inland waters and wetlands in China, providing paradigms for the inventorying work in other countries and insights to formulate effective mitigation strategies for climate change.

Unveiling Microbial Nitrogen Metabolism in Rivers using a Machine Learning Approach

Microbial nitrogen metabolism is a complicated and key process in mediating environmental pollution and greenhouse gas emissions in rivers. However, the interactive drivers of microbial nitrogen metabolism in rivers have not been identified. Here, we analyze the microbial nitrogen metabolism patterns in 105 rivers in China driven by 26 environmental and socioeconomic factors using an interpretable causal machine learning (ICML) framework. ICML better recognizes the complex relationships between factors and microbial nitrogen metabolism than traditional linear regression models. Furthermore, tipping points and concentration windows were proposed to precisely regulate microbial nitrogen metabolism. For example, concentrations of dissolved organic carbon (DOC) below tipping points of 6.2 and 4.2 mg/L easily reduce bacterial denitrification and nitrification, respectively. The concentration windows for NO3--N (15.9-18.0 mg/L) and DOC (9.1-10.8 mg/L) enabled the highest abundance of denitrifying bacteria on a national scale. The integration of ICML models and field data clarifies the important drivers of microbial nitrogen metabolism, supporting the precise regulation of nitrogen pollution and river ecological management.

Non-negligible N2O emission hotspots: Rivers impacted by ion-adsorption rare earth mining

Rare earth mining causes severe riverine nitrogen pollution, but its effect on nitrous oxide (N2O) emissions and the associated nitrogen transformation processes remain unclear. Here, we characterized N2O fluxes from China's largest ion-adsorption rare earth mining watershed and elucidated the mechanisms that drove N2O production and consumption using advanced isotope mapping and molecular biology techniques. Compared to the undisturbed river, the mining-affected river exhibited higher N2O fluxes (7.96 ± 10.18 mmol m-2d-1 vs. 2.88 ± 8.27 mmol m-2d-1, P = 0.002), confirming that mining-affected rivers are N2O emission hotspots. Flux variations scaled with high nitrogen supply (resulting from mining activities), and were mainly attributed to changes in water chemistry (i.e., pH, and metal concentrations), sediment property (i.e., particle size), and hydrogeomorphic factors (e.g., river order and slope). Coupled nitrification-denitrification and N2O reduction were the dominant processes controlling the N2O dynamics. Of these, the contribution of incomplete denitrification to N2O production was greater than that of nitrification, especially in the heavily mining-affected reaches. Co-occurrence network analysis identified Thiomonas and Rhodanobacter as the key genus closely associated with N2O production, suggesting their potential roles for denitrification. This is the first study to elucidate N2O emission and influential mechanisms in mining-affected rivers using combined isotopic and molecular techniques. The discovery of this study enhances our understanding of the distinctive processes driving N2O production and consumption in highly anthropogenically disturbed aquatic systems, and also provides the foundation for accurate assessment of N2O emissions from mining-affected rivers on regional and global scales.

Denitrification regulates spatiotemporal pattern of N2O emission in an interconnected urban river-lake network

Urban rivers are hotspots of N2O production and emission. Interconnected river-lake networks are constructed to improve the water quality and hydrodynamic conditions of urban rivers in many cities of China. However, the impact of the river-lake connectivity project on N2O production and emission remains unclear. This study investigated dissolved N2O and emission of the river-lake network in Wuhan City, China from March 2021 to December 2021. The results showed that river-lake connection greatly decreased riverine Nitrogen (N) concentration and increased dissolved oxygen (DO) concentration compare to traditional urban rivers. N2O emissions from the urban river interconnected with lakes (LUR: 67.3 ± 92.6 μmol/m2/d) were much lower than those from the traditional urban rivers (UR: 467.3 ± 1075.7 μmol/m2/d) and agricultural rivers (AR: 20.4 ± 15.3μmol/m2/d). Regression tree analysis suggested that the N2O concentrations were extremely high when hypoxia exists (DO < 1.6 mg/L), and TDN was the primary factor regulating N2O concentrations when hypoxia does not occur. Thus, we ascribe the low N2O emission in the LUR and AR to the lower N contents and higher DO concentrations. The microbial process of N2O production and consumption were quantitatively estimated by isotopic models. The mean proportion of denitrification derived N2O (fbD) was 63.5 %, 55.6 %, 42.3 % and 42.7 % in the UR, LUR, lakes and AR, suggested denitrification dominated N2O production in the urban rivers, but nitrification dominated N2O production in the lakes and AR. The positive correlation between logN2O and fbD suggested that denitrification is the key process to regulate the N2O production and emission. The abundance of denitrification genes (nirS and nirK) was much higher than that of nitrification genes (amoA and amoB), also evidenced that denitrification was the main N2O source. Therefore, river-lake interconnected projects changed the nutrients level and hypoxic condition, leading to the inhibition of denitrification and nitrification, and ultimately resulting in a decrease of N2O production and emission. These results advance the knowledge on the microbial processes that regulate N2O emissions in inland waters and illustrate the integrated management of water quality and N2O emission.

Estimating Yangtze River basin's riverine N2O emissions through hybrid modeling of land-river-atmosphere nitrogen flows

Riverine ecosystems are a significant source of nitrous oxide (N2O) worldwide, but how they respond to human and natural changes remains unknown. In this study, we developed a compound model chain that integrates mechanism-based modeling and machine learning to understand N2O transfer patterns within land, rivers, and the atmosphere. The findings reveal a decrease in N2O emissions in the Yangtze River basin from 4.7 Gg yr-1 in 2000 to 2.8 Gg yr-1 in 2019, with riverine emissions accounting for 0.28% of anthropogenic nitrogen discharges from land. This unexpected reduction is primarily attributed to improved water quality from human-driven nitrogen control, while natural factors contributed to a 0.23 Gg yr-1 increase. Notably, urban rivers exhibited a more rapid N2O efflux ( [Formula: see text] ), with upstream levels nearly 3.1 times higher than rural areas. We also observed nonlinear increases in [Formula: see text] with nitrogen discharge intensity, with urban areas showing a gradual and broader range of increase compared to rural areas, which exhibited a sharper but narrower increase. These nonlinearities imply that nitrogen control measures in urban areas lead to stable reductions in N2O emissions, while rural areas require innovative nitrogen source management solutions for greater benefits. Our assessment offers fresh insights into interpreting riverine N2O emissions and the potential for driving regionally differentiated emission reductions.

Agricultural ditches are hotspots of greenhouse gas emissions controlled by nutrient input

Agricultural ditches are pervasive in agricultural areas and are potential greenhouse gas (GHG) hotspots, since they directly receive abundant nutrients from neighboring farmlands. However, few studies measure GHG concentrations or fluxes in this particular water course, likely resulting in underestimations of GHG emissions from agricultural regions. Here we conducted a one-year field study to investigate the GHG concentrations and fluxes from typical agricultural ditch systems, which included four different types of ditches in an irrigation district located in the North China Plain. The results showed that almost all the ditches were large GHG sources. The mean fluxes were 333 μmol m-2 h-1 for CH4, 7.1 mmol m-2 h-1 for CO2, and 2.4 μmol m-2 h-1 for N2O, which were approximately 12, 5, and 2 times higher, respectively, than that in the river connecting to the ditch systems. Nutrient input was the primary driver stimulating GHG production and emissions, resulting in GHG concentrations and fluxes increasing from the river to ditches adjacent to farmlands, which potentially received more nutrients. Nevertheless, the ditches directly connected to farmlands showed lower GHG concentrations and fluxes compared to the ditches adjacent to farmlands, possibly due to seasonal dryness and occasional drainage. All the ditches covered approximately 3.3% of the 312 km2 farmland area in the study district, and the total GHG emission from the ditches in this area was estimated to be 26.6 Gg CO2-eq yr-1, with 17.5 Gg CO2, 0.27 Gg CH4, and 0.006 Gg N2O emitted annually. Overall, this study demonstrated that agricultural ditches were hotspots of GHG emissions, and future GHG estimations should incorporate this ubiquitous but underrepresented water course.

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.

River metabolic fingerprints and regimes reveal ecosystem responses to enhanced wastewater treatment

Although many studies have examined how improvements in wastewater treatment impact river nutrient concentrations and loads, there has been much less focus on measuring river metabolism to evaluate the wider aquatic ecosystem benefits of reducing nutrient inputs to rivers. The objectives of this study were to evaluate the effects of enhanced wastewater treatment (nitrification) on river metabolism in the Grand River, Canada's largest river draining into Lake Erie. Metabolic fingerprints and regimes (calculated from high-frequency dissolved oxygen [DO] measurements) were used to visualize whole-river ecosystem functional responses to these wastewater treatment upgrades. There was a 60% reduction in ecosystem respiration during summer, in response to reductions in effluent total ammonia inputs, causing a shift from net heterotrophy to net autotrophy, and contraction of river metabolic fingerprints. This resulted in major improvements in summer DO concentrations, with reductions in the percentage of days during summer that DO minima fell below water-quality guidelines for protection of aquatic early life stages, from 88% to ≤16%. The results also point to potential cascading impacts on coupled phosphorus and nitrogen cycles, which may generate further improvements in river water quality. During the summer, high rates of river metabolism and nutrient retention may result in measured water-column nutrient concentrations potentially underestimating nutrient pressures. This study also demonstrates the value of combining river metabolism with nutrient monitoring for a more holistic understanding of the role of nutrients in river ecosystem health and function.

Dissolved oxygen isotope modelling refines metabolic state estimates of stream ecosystems with different land use background

Dissolved oxygen (DO) is crucial for aerobic life in streams and rivers and mostly depends on photosynthesis (P), ecosystem respiration (R) and atmospheric gas exchange (G). However, climate and land use changes progressively disrupt metabolic balances in natural streams as sensitive reflectors of their catchments. Comprehensive methods for mapping fundamental ecosystem services become increasingly important in a rapidly changing environment. In this work we tested DO and its stable isotope (¹⁸O/¹⁶O) ratios as novel tools for the status of stream ecosystems. For this purpose, six diel sampling campaigns were performed at three low-order and mid-latitude European streams with different land use patterns. Modelling of diel DO and its stable isotopes combined with land use analyses showed lowest P rates at forested sites, with a minimum of 17.9 mg m⁻² h⁻¹. Due to high R rates between 230 and 341 mg m⁻² h⁻¹ five out of six study sites showed a general heterotrophic state with P:R:G ratios between 0.1:1.1:1 and 1:1.9:1. Only one site with agricultural and urban influences showed a high P rate of 417 mg m⁻² h⁻¹ with a P:R:G ratio of 1.9:1.5:1. Between all sites gross G rates varied between 148 and 298 mg m⁻² h⁻¹. In general, metabolic rates depend on the distance of sampling locations to river sources, light availability, nutrient concentrations and possible exchanges with groundwater. The presented modelling approach introduces a new and powerful tool to study effects of land use on stream health. Such approaches should be integrated into future ecological monitoring.

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

Urban rivers receive used water derived from anthropogenic activities and are a crucial source of the potent greenhouse gas nitrous oxide (N₂O). However, considerable uncertainties still exist regarding the variation and mechanisms of N₂O production in response to the discharge of treated sewage from municipal wastewater treatment plants (WWTPs). This study investigated N₂O concentrations and microbial processes responsible for nitrogen conversion upstream and downstream of WWTPs along the Tama River flowing through Tokyo, Japan. We evaluated the effect of treated sewage on dissolved N₂O concentrations and inherent N₂O consumption activities in the river sediments. In summer and winter, the mean dissolved N₂O concentrations were 0.67 µg-N L^-1 and 0.82 µg-N L^-1, respectively. Although the dissolved N₂O was supersaturated (mean 288.7% in summer, mean 240.7% in winter) in the river, the N₂O emission factors (EF_5r, 0.013%-0.025%) were significantly lower than those in other urban rivers and the Intergovernmental Panel on Climate Change default value (0.25%). The nitrate (NO₃^-) concentration in the Tama River increased downstream of the WWTPs discharge sites, and it was the main nitrogen constituent. An increasing trend of NO₃^- concentration was observed from upstream to downstream, along with an increase in the N₂O consumption potential of the river sediment. A multiple regression model showed that NO₃^- is the crucial factor influencing N₂O saturation. The diversity in the upstream microbial communities was greater than that in the downstream ones, indicating the involvement of treated sewage discharge in shaping the microbial communities. Functional gene quantification for N₂O production and consumption suggested that nirK-type denitrifiers likely contributed to N₂O production. Structural equation models (SEMs) revealed that treated sewage discharged from WWTPs increased the NO₃^- loading from upstream to downstream in the river, inducing changes in the microbial communities and enhancing the N₂O consumption activities. Collectively, aerobic conditions limited denitrification and in turn facilitated nitrification, leading to low N₂O emissions even despite high NO₃^- loadings in the Tama River. Our findings unravel an overestimation of the N₂O emission potential in an urban oxygen-rich river affected by treated sewage discharge.

Distinctive Microbial Processes and Controlling Factors Related to Indirect N2O Emission from Agricultural and Urban Rivers in Taihu Watershed

Inland rivers are hotspots of anthropogenic indirect nitrous oxide (N₂O) emissions, but the underlying microbial processes remain poorly understood. This study measured N₂O fluxes from agricultural and urban rivers in Taihu watershed and investigated the microbial processes driving N₂O production and consumption. The N₂O fluxes were significantly higher in agricultural rivers (140.1 ± 89.1 μmol m^-2 d^-1) than in urban rivers (25.1 ± 27.0 μmol m^-2 d^-1) (p < 0.001). All wind-based models significantly underestimated N₂O flux in urban rivers (p < 0.05) when using the Intergovernmental Panel on Climate Change method because they underestimated the N₂O emission factor (EF_5r). Wind speed and nitrate were the key factors affecting N₂O fluxes in agricultural and urban rivers, respectively. NirK-type denitrifiers produced N₂O in urban river water, while nirS-type denitrifiers consumed N₂O in the sediments of all rivers. Co-occurrence network analysis indicated organics from Microcystis served as electron donors for denitrifiers (dominated by Flavobacterium) in water, while direct interspecies electron transfer between Thiobacillus and methanogens and between Dechloromonas and sulfate-reducing bacteria enhanced N₂O reduction in sediments. This study advances our knowledge on the distinctive microbial processes that determine N₂O emissions in inland rivers and illustrates the need to revise EF_5r for N₂O estimation in urban rivers.

Submerged macrophytes regulate diurnal nitrous oxide emissions from a shallow eutrophic lake: A case study of Lake Wuliangsuhai in the temperate arid region of China

Submerged macrophytes can increase oxygen concentrations of water and promote diel oxygen fluctuations, and this phenomenon is hypothesized to play a vital role in regulating nitrous oxide (N₂O) emissions from eutrophic lakes. However, the effects of submerged macrophytes on N₂O emissions in shallow eutrophic lakes remain poorly investigated. In this study, Lake Wuliangsuhai, a typical shallow eutrophic lake, was investigated to study the role of submerged macrophytes in regulating N₂O emissions. We measured the N₂O fluxes and related parameters through continual 72-h in situ diel monitoring in two sampling sections that covered dense submerged macrophyte areas and open water. In this study, the dissolved oxygen (DO) concentration of the water in the submerged macrophyte area exhibited significant diurnal variations, with significantly higher water oxygen concentrations than the open water area during the daytime. The N₂O fluxes of Lake Wuliangsuhai ranged from 0.01 to 0.24 μmol m^-2 h^-1, with an average value of 0.11 μmol m^-2 h^-1. Moreover, significant diel variations in the N₂O flux and net N₂O production were observed in the submerged macrophyte areas, where the maximum N₂O flux occurred at midday. The molar ratios of NH₄⁺-N to oxygen (N/O ratio) of the water were responsible for the diel variations in the N₂O production in the lake. However, the high oxygen concentration of the water was the major regulator of the N₂O flux of Lake Wuliangsuhai. Therefore, submerged macrophyte restoration is significant not only for water quality improvement in shallow eutrophic lakes but also for N₂O emission mitigation by increasing the DO concentration of the water.

Nitrous oxide flux observed with tall-tower eddy covariance over a heterogeneous rice cultivation landscape

Although croplands are known to be strong sources of anthropogenic N₂O, large uncertainties still exist regarding their emission factors, that is, the proportion of N in fertilizer application that escapes to the atmosphere as N₂O. In this study, we report the results of an experiment on the N₂O flux in a landscape dominated by rice cultivation in the Yangtze River Delta, China. The observation was made with a closed-path eddy covariance system on a 70-m tall tower from October 2018 to December 2020 (27 months). Temperature and precipitation explained 78% of the seasonal and interannual variability in the observed N₂O flux. The growing season (May to October) mean flux (1.14 nmol m^-2 s^-1) was much higher than the median flux found in the literature for rice paddies. The mean N₂O flux during the observational period was 0.90 ± 0.71 nmol m^-2 s^-1, and the annual cumulative N₂O emission was 7.6 and 9.1 kg N₂O-N ha^-1 during 2019 and 2020, respectively. The corresponding landscape emission factor was 3.8% and 4.6%, respectively, which were much higher than the IPCC default direct (0.3%) and indirect emission factors (0.75%) for rice paddies.

Unexpectedly minor nitrous oxide emissions from fluvial networks draining permafrost catchments of the East Qinghai-Tibet Plateau

Streams and rivers emit substantial amounts of nitrous oxide (N₂O) and are therefore an essential component of global nitrogen (N) cycle. Permafrost soils store a large reservoir of dormant N that, upon thawing, can enter fluvial networks and partly degrade to N₂O, yet the role of waterborne release of N₂O in permafrost regions is unclear. Here we report N₂O concentrations and fluxes during different seasons between 2016 and 2018 in four watersheds on the East Qinghai-Tibet Plateau. Thawing permafrost soils are known to emit N₂O at a high rate, but permafrost rivers draining the East Qinghai-Tibet Plateau behave as unexpectedly minor sources of atmospheric N₂O. Such low N₂O fluxes are associated with low riverine dissolved inorganic N (DIN) after terrestrial plant uptake, unfavorable conditions for N₂O generation via denitrification, and low N₂O yield due to a small ratio of nitrite reductase: nitrous oxide reductase in these rivers. We estimate fluvial N₂O emissions of 0.432 - 0.463 Gg N₂O-N yr^-1 from permafrost landscapes on the entire Qinghai-Tibet Plateau, which is marginal (~0.15%) given their areal contribution to global streams and rivers (0.7%). However, we suggest that these permafrost-affected rivers can shift from minor sources to strong emitters in the warmer future, likely giving rise to the permafrost non-carbon feedback that intensifies warming.

Varying water column stability controls the denitrification process in a subtropical reservoir, Southwest China

Reservoirs are regarded as hotspots of nitrogen transformation and potential sources of nitrous oxide (N₂O). However, it remains unclear how the hydrological conditions due to dam construction control the processes of nitrogen transformation in reservoir waters. To address this issue, we examined the spatial-temporal characteristics of nitrate concentrations, δ¹⁵N-NO₃^-, δ¹⁸O-NO₃^-, δ¹⁸O-H₂O, relative water column stability (RWCS), and related environmental factors in a subtropical eutrophic reservoir (Hongfeng Reservoir, HFR), Southwest China. We found that denitrification was the most important nitrogen transformation process in the HFR and that higher denitrification intensity was associated with increased RWCS in summer, which suggested hydrological control of the denitrification process. In contrast, low RWCS conditions favored the nitrification process in the HFR in winter. Additionally, dissolved oxygen (DO; p < 0.05) and nitrate concentrations (p < 0.01) had significant impacts on the denitrification rate. We also found that the spatiotemporal RWCS variations were a prerequisite for regulating DO/nitrate stratification and the coupling/decoupling of nitrification-denitrification at the local and global scales. This study would advances our knowledge of the impacts of RWCS and thermal stratification on nitrogen transformation processes in reservoirs.

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.

Restored riverine wetlands in a headwater stream can simultaneously behave as sinks of N2O and hotspots of CH4 production

Wetlands can improve water quality, but they are also recognized as important sources of greenhouse gases (GHG) such as nitrous oxide (N₂O) and methane (CH₄). Emissions of these gases from wetland ecosystems, especially those in headwaters, are poorly understood. Here, we determined monthly concentrations of dissolved N₂O and CH₄ in a headwater stream of the Taihu Lake basin of China that contains both wetland and non-wetland reaches. Daily GHG dynamics in the wetland reach were also investigated. Riverine N₂O and CH₄ concentrations generally varied within 10-30 nmol L^-1 and 0.1-1.5 μmol L^-1, respectively. CH₄ saturation levels in the wetland reach were about seven times higher than those in the non-wetland reach, but there was no difference in N₂O saturation. In the wetland reach, saturation levels of CH₄ peaked in July, coincident with a dip in N₂O saturation to levels below its saturated solubility. This underscores that hotspots of CH₄ production and sinks for N₂O can occur occasionally in wetlands in mid-summer, when vegetative growth and microbial activities are high. Diurnal measurements indicated that CH₄ saturation in water flows passing through the wetlands from midnight through the early morning can surge to levels 10 times higher than those detected at other times of the day. Simultaneously, saturation levels of N₂O decreased by 75%, indicating a net consumption of N₂O. Changes in nutrient supply determined by upstream inflows, as well as dissolved oxygen, pH, and other environmental factors mediated by the wetlands, correlate with the differentiated behavior of N₂O and CH₄ production in wetlands. Additional work will be necessary to confirm the roles of these factors in regulating GHG emissions in riverine wetlands.

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.

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.

Power law scaling model predicts N2O emissions along the Upper Mississippi River basin

Nitrous oxide (N₂O) is widely recognized as one of the most important greenhouse gases, and responsible for stratospheric ozone destruction. A significant fraction of N₂O emissions to the atmosphere is from rivers. Reliable catchment-scale estimates of these emissions require both high-resolution field data and suitable models able to capture the main processes controlling nitrogen transformation within surface and subsurface riverine environments. Thus, this investigation tests and validates a recently proposed parsimonious and effective model to predict riverine N₂O fluxes with measurements taken along the main stem of the Upper Mississippi River (UMR). The model parameterizes N₂O emissions by means of two denitrification Damköhler numbers; one accounting for processes occurring within the hyporheic and benthic zones, and the other one within the water column, as a function of river size. Its performance was assessed with several statistical quantitative indexes such as: Absolute Error (AE), Nash-Sutcliffe efficiency (NSE), percent bias (PBIAS), and ratio of the root mean square error to the standard deviation of measured data (RSR). Comparison of predicted N₂O gradients between water and air (ΔN₂O) with those quantified from field measurements validates the predictive performance of the model and allow extending previous findings to large river networks including highly regulated rivers with cascade reservoirs and locks. Results show the major role played by the water column processes in contributing to N₂O emissions in large rivers. Consequently, N₂O productions along the UMR, characterized by regulated flows and large channel size, occur chiefly within this surficial riverine compartment, where the suspended particles may create anoxic microsites, which favor denitrification.

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.

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.

Considering atmospheric N2O dynamic in SWAT model avoids the overestimation of N2O emissions in river networks

Modeling studies have focused on N₂O emissions in temperate rivers under static atmospheric N₂O (N₂O_airc), with cold temperate river networks under dynamic N₂O_airc receiving less attention. To address this knowledge and methodological gap, the dissolved N₂O concentration (N₂O_disc) and N₂O_airc algorithms were integrated with an air-water gas exchange model (F_N2O) into the SWAT (Soil and Water Assessment Tool). This new model (SWAT-F_N2O) allows users to simulate daily riverine N₂O emissions under dynamic atmospheric N₂O. The spatiotemporal fluctuations in the riverine N₂O emissions was simulated and its response to the static and dynamic atmospheric N₂O were analyzed in a middle-high latitude agricultural watershed in northeastern China. The results show that the SWAT-F_N2O model is a useful method for capturing the hotspots in riverine N₂O emissions. The model showed strong riverine N₂O absorption and weak N₂O emissions from September to February, which acted as a sink for atmospheric N₂O in this cold temperate area. High N₂O emissions occurred from April to July, which accounted for 83.34% of the yearly emissions. Spatial analysis indicated that the main stream and its tributary could contribute 302.3-1043.7 and 41.5-163.4 μg N₂O/(m²·d) to the total riverine N₂O emissions (15.02 t/a), respectively. The riverine N₂O emissions rates in the subbasins dominated by forests and paddy fields were lower than those in the subbasins dominated by arable and residential land. Riverine N₂O emissions can be overestimated under the static atmospheric N₂O rather than under the increasing atmospheric N₂O. This overestimation has increased from 1.52% to 23.97% from 1990 to 2016 under the static atmospheric N₂O. The results of this study are valuable for water quality and future climate change assessments that aim to protect aquatic and atmospheric environments.

Human Activities Inducing High CH4 Diffusive Fluxes in an Agricultural River Catchment in Subtropical China

Methane (CH4) is one of the key greenhouse gases (GHGs) in the atmosphere with current concentration of 1859 ppb in 2017 due to climate change and anthropogenic activities. Rivers are of increasing concern due to sources of atmospheric CH4. However, knowledge and data limitations exist for field studies of subtropical agricultural river catchments, particularly in southern China. The headspace balance method and the diffusion model method were employed to assess spatiotemporal variations of CH4 diffusive fluxes from April 2015 to January 2016 in four order reaches (S1, S2, S3, and S4) of the Tuojia River, Hunan, China. Results indicated that both the dissolved concentrations and diffusive fluxes of CH4 showed obvious spatiotemporal variations. The observed mean concentration and diffusive flux of CH4 were 0.40 ± 0.02 μmol L−1 and 41.19 ± 2.50 µg m−2 h−1, respectively, showing the river to be a strong source of atmospheric CH4. The CH4 diffusive fluxes during the rice-growing seasons were significantly greater than the winter fallow season (an increase of 80.26%). The spatial distribution of CH4 diffusive fluxes increased gradually from (17.58 ± 1.42) to (55.56 ± 4.32) µg m−2 h−1 due to the organic and nutrient loading into the river waterbodies, with the maximum value at location S2 and the minimum value at location S1. Correlation analysis showed that the CH4 diffusive fluxes exhibited a positive relationship with the dissolved organic carbon (DOC), salinity, and water temperature (WT), while a negative correlation occurred between CH4 diffusive fluxes and the dissolved oxygen (DO) concentration, as well as the pH value. Our findings highlighted that a good understanding of exogenous nutrient loading in agricultural catchments will clarify the influence of human activities on river water quality and then constrain the global CH4 budget.

Seasonal variability of sediment controls of carbon cycling in an agricultural stream

Streams and rivers are 'active pipelines' where high rates of carbon (C) turnover can lead to globally important emissions of carbon dioxide (CO₂) and methane (CH₄) from surface waters to the atmosphere. Streambed sediments are particularly important in affecting stream chemistry, with rates of biogeochemical activity, and CO₂ and CH₄ concentrations far exceeding those in surface waters. Despite an increase in research on CO₂ and CH₄ in streambed sediments there is a lack of knowledge and insight on seasonal dynamics. In this study the seasonally variable effect of sediment type (sand-dominated versus gravel-dominated) on porewater C cycling, including CO₂ and CH₄ concentrations, was investigated. We found high concentrations of CO₂ and CH₄ in the streambed of a small agricultural stream. Sand-dominated sediments were characterised by higher microbial activity and CO₂ and CH₄ concentrations than gravel-dominated sediments, with CH₄:CO₂ ratios higher in sand-dominated sediments but rates of recalcitrant C uptake highest in gravel-dominated sediments. CO₂ and CH₄ concentrations were unexpectedly high year-round, with little variation in concentrations among seasons. Our results indicate that small, agricultural streams, which generally receive large amounts of fine sediment and organic matter (OM), may contribute greatly to annual C cycling in freshwater systems. These results should be considered in future stream management plans where the removal of sandy sediments may perform valuable ecosystem services, reducing C turnover, CO₂ and CH₄ concentrations, and mitigating greenhouse gas (GHG) production.

Methane and nitrous oxide temporal and spatial variability in two midwestern USA streams containing high nitrate concentrations

Concentrations and emissions of greenhouse gases CO₂, CH₄, and N₂O commonly are examined individually in aquatic environments in which each is expected to be relatively important; however, their co-occurrence and dynamic interactions in fluvial settings could provide important information about their controlling biogeochemical processes and potential contributions to global climate change. Spatial and temporal variability of CH₄, N₂O, and CO₂ concentrations were measured from June 1999 to September 2003 in two nitrate-rich (40-1200 μM) streams draining agricultural land in the midwestern USA that differed ~13-fold in flow. Seasonal (biweekly), diel (hourly), and transport-oriented (reach-scale) sampling approaches were compared. Dissolved gas concentrations exceeded atmospheric equilibrium values up to 700- and 16-fold, for CH₄ and N₂O, respectively. Mean concentrations were higher in the larger stream than in the smaller stream. In both streams, CH₄ emissions were generally higher in summer-fall and negatively correlated with flow and NO₃^- concentration while N₂O emissions were generally higher in winter/spring and positively correlated with flow and NO₃^-. In the small stream, diel variations in the concentrations, emissions, and isotopic compositions of CH₄, N₂O, and NO₂^- resulted from diel variations in sources, sinks, and air-water gas exchange velocities. Seasonal mean total (CH₄ + N₂O) area-normalized emission rates, expressed as CO₂ warming potential equivalents, were similar for the two streams, but the total reach-scale emission rate for the larger stream, including CO₂, was about 2.9 times that of the smaller stream (131.6 vs 46.0 kg CO₂ equivalents km^-1 day^-1, respectively). The CH₄ contribution to this flux was 9-28%, despite the relatively high NO₃^- and O₂ concentrations in the streams, indicating contributions from upwelling groundwater or reactions in streambed sediment.

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.

Suspended particles potentially enhance nitrous oxide (N2O) emissions in the oxic estuarine waters of eutrophic lakes: Field and experimental evidence

Estuaries are considered hot spots for the production and emissions of nitrous oxide (N₂O) and easily occur suspended particles (SPS), however, current understanding about the role of SPS in the N₂O emissions from the oxic estuarine waters of lacustrine ecosystems is still limited. In this study, field investigations were performed in the estuaries of hypereutrophic Taihu Lake, and laboratory simulations were simultaneously conducted to ascertain the characteristics of N₂O emissions with different SPS concentrations. The results showed that the N₂O emission fluxes ranged from 9.75 to 118.38 μg m^-2 h^-1, indicating a high spatial heterogeneity for the N₂O emissions from the estuaries of Taihu Lake. Although the dissolved oxygen (DO) concentrations were up to 7.85 mg L^-1 in the estuarine waters, from where the N₂O emissions fluxes were approximately three times that of the lake regions. Multiple regression model selected the total nitrogen (TN), SPS, and DO concentrations as the crucial factors influencing the N₂O emission fluxes. Particularly for SPS, the simulation results showed that the N₂O concentrations increased gradually with the increase in the SPS concentrations of an oxic water column containing 4 mg L^-1 of NO₃^--N, indicating that a high SPS concentration can accelerate the N₂O emissions. It was related to the change of denitrifying bacteria population in the SPS, as evidenced by its significantly positive correlation with N₂O emissions (p < 0.01). Our findings will draw attentions to the role of SPS playing in the N₂O productions and emissions in eutrophic lakes, and its effect on nitrogen cycle should be considered in the future study.

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.

Quantifying the fate of wastewater nitrogen discharged to a Canadian river

Addition of nutrients, such as nitrogen, can degrade water quality in lakes, rivers, and estuaries. To predict the fate of nutrient inputs, an understanding of the biogeochemical cycling of nutrients is needed. We develop and employ a novel, parsimonious, process-based model of nitrogen concentrations and stable isotopes that quantifies the competing processes of volatilization, biological assimilation, nitrification, and denitrification in nutrient-impacted rivers. Calibration of the model to nitrogen discharges from two wastewater treatment plants in the Grand River, Ontario, Canada, show that ammonia volatilization was negligible relative to biological assimilation, nitrification, and denitrification within 5 km of the discharge points.

Distribution and emission of N2O in the largest river-reservoir system along the Yellow River

Rivers and reservoirs are affected by human activities and are sources of the greenhouse gas nitrous oxide (N₂O). Concentrations of N₂O in the middle and lower reaches of the Yellow River and Xiaolangdi Reservoir, China, were measured in June and December 2017. Fluxes were estimated by boundary layer method to explore their controlling factors, especially the impact of damming and reservoir operation. In the middle and lower reaches of the Yellow River, N₂O concentrations in surface waters were 26.65 ± 14.67 nmol L^-1 in summer and 21.16 ± 5.35 nmol L^-1 in winter. In comparison, the concentrations of N₂O in the reservoir were 32.94 ± 17.32 nmol L^-1 in summer and 23.73 ± 5.60 nmol L^-1 in winter. The longitudinal distribution of N₂O along the river exhibited different patterns with surface N₂O decreasing downstream towards the dam in summer but increasing in winter. Vertical profiles of N₂O concentrations in the reservoir showed an increase with depth in summer but were almost vertically uniform in winter. In winter, N₂O that had accumulated in the bottom water in summer was transported to the surface by vertical mixing and released into the atmosphere. Dissolved oxygen (DO), water temperature, and in situ biological production were the main factors affecting the distribution of N₂O. The mean emissions rates of N₂O from the surface waters were 13.7 ± 8.8 μmol m^-2 d^-1 in summer and 13.2 ± 7.6 μmol m^-2 d^-1 in winter. Approximately 1.31 × 10⁶ mol N₂O was released from the reservoir surface in 2017, which represents 0.12% of the annual N₂O emissions from global reservoirs. The construction of dams increased N₂O emission from the lower reaches of the river by 4.53 × 10⁵ mol and 1.22 × 10⁵ mol due to the discharge of the bottom water and the water and sediment regulation, respectively. This study demonstrates that the construction of dams and reservoir operation practices have made the Xiaolangdi Reservoir a key area for N₂O emissions.

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%.

Effects of seasonality, transport pathway, and spatial structure on greenhouse gas fluxes in a restored wetland

Wetlands can influence global climate via greenhouse gas (GHG) exchange of carbon dioxide (CO₂ ), methane (CH₄ ), and nitrous oxide (N₂ O). Few studies have quantified the full GHG budget of wetlands due to the high spatial and temporal variability of fluxes. We report annual open-water diffusion and ebullition fluxes of CO₂ , CH₄ , and N₂ O from a restored emergent marsh ecosystem. We combined these data with concurrent eddy-covariance measurements of whole-ecosystem CO₂ and CH₄ exchange to estimate GHG fluxes and associated radiative forcing effects for the whole wetland, and separately for open-water and vegetated cover types. Annual open-water CO₂ , CH₄ , and N₂ O emissions were 915 ± 95 g C-CO₂  m^-2  yr^-1 , 2.9 ± 0.5 g C-CH₄  m^-2  yr^-1 , and 62 ± 17 mg N-N₂ O m^-2  yr^-1 , respectively. Diffusion dominated open-water GHG transport, accounting for >99% of CO₂ and N₂ O emissions, and ~71% of CH₄ emissions. Seasonality was minor for CO₂ emissions, whereas CH₄ and N₂ O fluxes displayed strong and asynchronous seasonal dynamics. Notably, the overall radiative forcing of open-water fluxes (3.5 ± 0.3 kg CO₂ -eq m^-2  yr^-1 ) exceeded that of vegetated zones (1.4 ± 0.4 kg CO₂ -eq m^-2  yr^-1 ) due to high ecosystem respiration. After scaling results to the entire wetland using object-based cover classification of remote sensing imagery, net uptake of CO₂ (-1.4 ± 0.6 kt CO₂ -eq yr^-1 ) did not offset CH₄ emission (3.7 ± 0.03 kt CO₂ -eq yr^-1 ), producing an overall positive radiative forcing effect of 2.4 ± 0.3 kt CO₂ -eq yr^-1 . These results demonstrate clear effects of seasonality, spatial structure, and transport pathway on the magnitude and composition of wetland GHG emissions, and the efficacy of multiscale flux measurement to overcome challenges of wetland heterogeneity.

Role of surface and subsurface processes in scaling N2O emissions along riverine networks

Riverine environments, such as streams and rivers, have been reported as sources of the potent greenhouse gas nitrous oxide ([Formula: see text]) to the atmosphere mainly via microbially mediated denitrification. Our limited understanding of the relative roles of the near-surface streambed sediment (hyporheic zone), benthic, and water column zones in controlling [Formula: see text] production precludes predictions of [Formula: see text] emissions along riverine networks. Here, we analyze [Formula: see text] emissions from streams and rivers worldwide of different sizes, morphology, land cover, biomes, and climatic conditions. We show that the primary source of [Formula: see text] emissions varies with stream and river size and shifts from the hyporheic-benthic zone in headwater streams to the benthic-water column zone in rivers. This analysis reveals that [Formula: see text] production is bounded between two [Formula: see text] emission potentials: the upper [Formula: see text] emission potential results from production within the benthic-hyporheic zone, and the lower [Formula: see text] emission potential reflects the production within the benthic-water column zone. By understanding the scaling nature of [Formula: see text] production along riverine networks, our framework facilitates predictions of riverine [Formula: see text] emissions globally using widely accessible chemical and hydromorphological datasets and thus, quantifies the effect of human activity and natural processes on [Formula: see text] production.

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.

Indirect Nitrous Oxide Emission Factors for Agricultural Field Drains and Headwater Streams

Agriculture is a major source of nitrous oxide (N₂O) emissions, a potent greenhouse gas. While direct N₂O emissions from soils have been widely investigated, indirect N₂O emissions from nitrogen (N) enriched surface water and groundwater bodies are poorly understood. In this contribution, indirect N₂O emissions from subsurface agricultural field drains and headwater streams were monitored over a two-year period (2013-2015) in an intensive arable catchment in eastern England. Indirect N₂O emission factors for groundwater (EF_5g) and surface runoff (EF_5r) were calculated for both field drain and streamwater samples, respectively, using two approaches: the N₂O-N/NO₃-N ratio and the IPCC (2006) methodology. Mean EF_5g values derived from the N₂O-N/NO₃-N ratio were 0.0012 for field drains and 0.0003 for streamwater. Using the IPCC (2006) methodology, the mean EF_5g values were 0.0011 for field drains and 0.0001 for streamwater. Thus, EF values derived from both methods were below the current IPCC (2006) default value of 0.0025 and a downward revision to 0.0012 for EF_5g and 0.0002 for EF_5r is recommended. Such revision would halve current estimates of N₂O emissions associated with nitrogen leaching and runoff from agriculture for both the UK and globally.

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.

Genetic associations as indices of nitrogen cycling rates in an aerobic denitrification biofilter used for groundwater remediation

An aerobic denitrification biofilter (ADB) for groundwater remediation was developed with high removal efficiencies (total nitrogen (TN): 82.3-95.8%; NO3(-)-N: 93.2-98.2%). Nitrate (NO3(-)-N) transformation rates stabilized between 21.0 and 23.4 g/(m(3) h), whereas nitrite (NO2(-)-N) and ammonium (NH4(+)-N) transformation rates remained less than 6.0 g/(m(3) h) as the dissolved oxygen (DO) level increased from 1.0 mg/L to 6.0 mg/L. Nitric oxide (NO) and nitrous oxide (N2O) accumulated with great fluctuations (NO: 0-1.6×10(-3) g/(m(3) h); N2O: 0.1-1.1g/(m(3)h)) throughout the experiment. This study suggested that gene associations reflect quantitative relationships with aerobic denitrification rates and can provide useful information regarding aerobic denitrification processes in groundwater. Especially, the qnorB/nosZ ratio acts as the main driver for NO3(-)-N and NH4(+)-N transformation, while the qnorB/nosZ ratio followed by the (nirS+nirK)/nosZ ratio serve a dominant role in the accumulation of N2O and NO.

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.

Proper interpretation of dissolved nitrous oxide isotopes, production pathways, and emissions requires a modelling approach

Stable isotopes ([Formula: see text]15N and [Formula: see text]18O) of the greenhouse gas N2O provide information about the sources and processes leading to N2O production and emission from aquatic ecosystems to the atmosphere. In turn, this describes the fate of nitrogen in the aquatic environment since N2O is an obligate intermediate of denitrification and can be a by-product of nitrification. However, due to exchange with the atmosphere, the [Formula: see text] values at typical concentrations in aquatic ecosystems differ significantly from both the source of N2O and the N2O emitted to the atmosphere. A dynamic model, SIDNO, was developed to explore the relationship between the isotopic ratios of N2O, N2O source, and the emitted N2O. If the N2O production rate or isotopic ratios vary, then the N2O concentration and isotopic ratios may vary or be constant, not necessarily concomitantly, depending on the synchronicity of production rate and source isotopic ratios. Thus prima facie interpretation of patterns in dissolved N2O concentrations and isotopic ratios is difficult. The dynamic model may be used to correctly interpret diel field data and allows for the estimation of the gas exchange coefficient, N2O production rate, and the production-weighted [Formula: see text] values of the N2O source in aquatic ecosystems. Combining field data with these modelling efforts allows this critical piece of nitrogen cycling and N2O flux to the atmosphere to be assessed.