Riverine N2O Research Articles

Temperature has an enhanced role in sediment N2O and N2 fluxes in wider rivers.

Riverine N2O and N2 fluxes, key components of the global nitrogen budget, are known to be influenced by river size (often represented by average river width), yet the specific mechanisms behind these effects remain unclear. This study examined how environmental and microbial factors influenced sediment N2O and N2 fluxes across rivers with varying widths (2.8 to 2,000 m) in China. Sediment acted as sources of both N2O and N2 emissions, with both N2 (0.2 to 20.8 mmol m-2 d-1) and N2O fluxes (0.7-54.2 μmol m-2 d-1) decreasing significantly as river width increased. N2 fluxes were positively correlated with denitrifying bacterial abundance, whereas N2O fluxes, when normalized by the abundance of denitrifying bacteria, were negatively correlated with the abundance of N2O-reducing microbes. Water physicochemical factors, particularly temperature and nitrate, were more important drivers of these fluxes than sediment factors. Nitrate significantly increased denitrifying bacterial abundance, whereas higher temperatures enhanced cell-specific activity. Lower N2O and N2 emissions in wider rivers were attributed to decreased denitrifying microbial abundance and lower denitrification rates, in addition to the commonly assumed reduction in exogenous N2O and N2 inputs. Rolling regression analysis showed that nitrate concentration had a stronger effect on sediment N2O and N2 fluxes in narrower rivers, whereas temperature was more influential in wider rivers. This difference is attributed to more stable nitrate concentrations and decreased nitrogen removal efficiency in wider rivers, while temperature variation remained consistent across all river widths. Beyond sediments, temperature had a greater effect on excess N2O concentrations than nitrate in the overlying water of wider rivers (>165 m), highlighting its broader impact. This study provides new biogeochemical insights into how river width influences sediment N2O and N2 fluxes and highlights the importance of incorporating temperature into flux predictions, particularly for wider rivers.

Sustainable management of riverine N2O emission baselines

Abstract The riverine N2O fluxes are assumed to linearly increase with nitrate loading. However, this linear relationship with a uniform EF5r is poorly constrained, which impedes the N2O estimation and mitigation. Our meta-analysis discovered a universal N2O emission baseline (EF5r = k/[NO3−], k = 0.02) for natural rivers. Anthropogenic impacts caused an overall increase in baselines and the emergence of hotspots, which constitute two typical patterns of anthropogenic sources. The k values of agricultural and urban rivers increased to 0.09 and 0.05, respectively, with 11% and 14% of points becoming N2O hotspots. Priority control of organic and NH4+ pollution could eliminate hotspots and reduce emissions by 51.6% and 63.7%, respectively. Further restoration of baseline emissions on nitrate removal is a long-term challenge considering population growth and declining unit benefits (ΔN-N2O/N-NO3−). The discovery of EF-lines emphasized the importance of targeting hotspots and managing baseline emissions sustainably to balance social and environmental benefits.

Unveiling riverine N2O dynamics along urbanization gradients by integrating hydrological, biogeochemical and microbial processes

Human-disturbed rivers are globally significant sources of atmospheric nitrous oxide (N2O). Yet, the underlying mechanisms of urbanization impact on riverine N2O dynamics are not well understood. This study unveiled the effects of urbanization on N2O dynamics by integrating hydrological, biogeochemical and microbial processes in a river with various urbanization intensities. Riverine NO3− concentration enhanced with increasing urban land percentage, primarily because of the increased proportional contribution of sewage & manure source. The 15N site preference and relevant isotopic evidences revealed that the proportion of denitrification derived N2O increased from 60 % to 76 %, with the urban land percentage increasing from 〈 5 % to 〉 22 %, which was caused by decreases in flow velocity and dissolved oxygen saturation, increases in NO3− concentration and N2O-denitrifying genes. The non-negligible contribution of nitrification to N2O production (∼ 40 %) in lower-urbanized river stretches may be attributed to aerobic conditions and lower impermeable riparian zone facilitating the occurrence of in-river nitrification and the access of in-soil nitrification to river. Urbanization-mediated decreases in flow velocity and dissolved oxygen and increases in nitrogen availability and denitrification process resulted in an increase in N2O concentration and flux, with N2O concentration approximately four times higher in higher-urbanized river reaches (50.7 ± 26.3 nmol/L) than in lower-urbanized river reaches (14.4 ± 2.5 nmol/L). In addition, increased proportional contribution of sewage & manure source also provides the possibility for exogenous N2O inputs with urban expansion. These findings contribute to deepening our understanding of how urbanization drives N2O dynamics in river systems.

Wastewater-effluent discharge and incomplete denitrification drive riverine CO2, CH4 and N2O emissions

Rivers are well-known sources of the greenhouse gasses (GHG) carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O). These emissions from rivers can increase because of anthropogenic activities, such as agricultural fertilizer input or the discharge of treated wastewater, as these often contain elevated nutrient concentrations. Yet, the specific effects of wastewater effluent discharge on river GHG emissions remain poorly understood. Here, we studied two lowland rivers which both receive municipal wastewater effluent: river Linge and river Kromme Rijn. Dissolved concentrations and fluxes of CH4, N2O and CO2 were measured upstream, downstream and at discharge locations, alongside water column properties and sediment composition. Microbial communities in the sediment and water column were analysed using 16S rRNA gene sequencing. In general, observed GHG emissions from Linge and Kromme Rijn were comparable to eutrophic rivers in urban and agricultural environments. CO2 emissions peaked at most discharge locations, likely resulting from dissolved CO2 present in the effluent. CH4 emission was highest 2 km downstream, suggesting biological production by methanogenic activity stimulated by the effluents' carbon and nutrient supply. Dissolved N2O concentrations were strongly related to NO3− content of the water column which points towards incomplete riverine denitrification. Notably, methanogenic archaea were more abundant downstream of effluent discharge locations. However, overall microbial community composition remained relatively unaffected in both rivers. In conclusion, we demonstrate a clear link between wastewater effluent discharge and enhanced downstream GHG emission of two rivers. Mitigating the impact of wastewater effluent on receiving rivers will be crucial to reduce riverine GHG contributions.

Hydrologic Connectivity Regulates Riverine N2O Sources and Dynamics.

Indirect nitrous oxide (N2O) emissions from streams and rivers are a poorly constrained term in the global N2O budget. Current models of riverine N2O emissions place a strong focus on denitrification in groundwater and riverine environments as a dominant source of riverine N2O, but do not explicitly consider direct N2O input from terrestrial ecosystems. Here, we combine N2O isotope measurements and spatial stream network modeling to show that terrestrial-aquatic interactions, driven by changing hydrologic connectivity, control the sources and dynamics of riverine N2O in a mesoscale river network within the U.S. Corn Belt. We find that N2O produced from nitrification constituted a substantial fraction (i.e., >30%) of riverine N2O across the entire river network. The delivery of soil-produced N2O to streams was identified as a key mechanism for the high nitrification contribution and potentially accounted for more than 40% of the total riverine emission. This revealed large terrestrial N2O input implies an important climate-N2O feedback mechanism that may enhance riverine N2O emissions under a wetter and warmer climate. Inadequate representation of hydrologic connectivity in observations and modeling of riverine N2O emissions may result in significant underestimations.

Ammonium-derived nitrous oxide is a global source in streams

Global riverine nitrous oxide (N2O) emissions have increased more than 4-fold in the last century. It has been estimated that the hyporheic zones in small streams alone may contribute approximately 85% of these N2O emissions. However, the mechanisms and pathways controlling hyporheic N2O production in stream ecosystems remain unknown. Here, we report that ammonia-derived pathways, rather than the nitrate-derived pathways, are the dominant hyporheic N2O sources (69.6 ± 2.1%) in agricultural streams around the world. The N2O fluxes are mainly in positive correlation with ammonia. The potential N2O metabolic pathways of metagenome-assembled genomes (MAGs) provides evidence that nitrifying bacteria contain greater abundances of N2O production-related genes than denitrifying bacteria. Taken together, this study highlights the importance of mitigating agriculturally derived ammonium in low-order agricultural streams in controlling N2O emissions. Global models of riverine ecosystems need to better represent ammonia-derived pathways for accurately estimating and predicting riverine N2O emissions.

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 (FN2O), with upstream levels nearly 3.1 times higher than rural areas. We also observed nonlinear increases in FN2O 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.

Electrical conductivity as a reliable indicator for assessing land use effects on stream N2O concentration

While it is widely acknowledged that different categories of terrestrial land use significantly affect N2O emissions from streams and rivers, a suitable indicator to comprehensively reflect such influences is still lacking. This study examined the effectiveness of stream environmental and microbial factors in reflecting the influence of land use types on stream N2O concentrations, taking into account both endogenous N2O production and exogenous N2O inputs. Our results showed that human-disturbed (urban and agricultural) streams had nearly four times higher excess N2O concentrations (ΔN2O) than forest streams (5.01 versus 1.32 nmol/L N2O). By combining our observations with previous studies, we identify electrical conductivity as a significant and even the strongest predictor of (excess) N2O concentration and flux in streams across different land uses, surpassing the predictive power of nitrogen content. Elevated ΔN2O was observed at higher conductivities (r = 0.69, p < 0.01), which coincided with higher carbon and nutrient levels across different types of land use. Also, conductivity was positively correlated with the abundance of N2O-producing microbes (p < 0.05), including both nitrifying and denitrifying microbes. Denitrifiers adapted to human disturbance and the genetic potential for net N2O production, as evidenced by the positive correlation between conductivity and nir:nosZ ratio, were enhanced under high-conductivity conditions (p < 0.05). Furthermore, electrical conductivity had great potential to qualitatively indicate the magnitude of exogenous N2O input across different land uses. Overall, our results highlight the value of electrical conductivity as a reliable indicator for assessing the effect of land use types on stream N2O concentrations. This opens opportunities for the development of simple field-based assessments of riverine N2O emissions across regional landscapes.

Tracing Microbial Production and Consumption Sources of N2O in Rivers on the Qinghai-Tibet Plateau via Isotopocule and Functional Microbe Analyses

Nitrous oxide (N2O), a potent greenhouse gas, is produced in rivers through a series of microbial metabolic pathways. However, the microbial source of N2O production and the degree of N2O reduction in river systems are not well understood and quantified. This work investigated isotopic compositions (δ15N-N2O and δ18O-N2O) and N2O site preference as well as N2O-related microbial features, thereby differentiating the importance of nitrification, denitrification, and N2O reduction in controlling N2O emissions from five rivers on the eastern Qinghai-Tibet Plateau (EQTP). The average N2O concentration in overlying water (15.2 nmol L-1) was close to that in porewater (17.5 nmol L-1), suggesting that both overlying water and sediment are potentially important sources of N2O. Canonical and nitrifier denitrification dominated riverine N2O production, with contribution being approximately 90%. Nitrification is a non-negligible source of N2O production, and N2O concentration was positively correlated with nitrification genetic potential. The degree of N2O reduction ranged from 78.1 to 94.1% (averaging 90%), significantly exceeding the reported values (averaging 70%) in other freshwaters, which was attributed to the higher ratios of organic carbon to nitrogen and lower ratio of (nirS + nirK)/nosZ in EQTP rivers. This study indicates that a combination of isotopic and isotopocule values with functional microbe analysis is useful for quantifying the microbial sources of N2O in rivers, and the intense microbial reduction of N2O significantly accounts for the low N2O emissions observed in EQTP rivers, suggesting that both the production and consumption of N2O in rivers should be considered in the future.

Watershed land use change indirectly dominated the spatial variations of CH4 and N2O emissions from two small suburban rivers

Urban rivers have been proved to be the hot spots of atmospheric methane (CH4) and nitrous oxide (N2O) emissions. However, the rivers across rural-urban interface, which are undergoing significant environmental changes in the process of urbanization, received little attention. In this study, we conducted seasonal investigations in two small suburban rivers in mountainous southwest China, to clarify the spatial-temporal characteristics of CH4 and N2O concentrations and fluxes in relation to watershed land use change and their potential controls. The results showed that, the small suburban rivers acted as strong CH4 and N2O emitters to atmosphere with averaged fluxes of 960.2±966.6 and 205.1±240.8 µmol·m−2·d−1, respectively. The concentrations and fluxes of CH4 and N2O presented a sharp increase and spatial variability when the rivers cross the rural-urban interface. Such discontinuous spatial pattern of CH4 and N2O emission in river continuum were closely related to watershed land use, and could be modeled as functions of the urban land use proportion. The urban land proportion in the basins could explain 70% and 59% of the total spatial variations of CH4 and N2O emissions, respectively. Stepwise multiple regression models (SMRM) based on water environment parameters were established in our study and indicate that DTC, NH4+, NO3−, TP and conductivity play key controls on CH4 and N2O emissions in the two studied mountainous suburban rivers. Moreover, SMRM and urban land use based functions were tested to have good predictions based on constrained mountainous samples, could provide a possible path to extrapolate CH4 and N2O emissions in mountainous urbanized-rivers at the whole regional network scale. We also found evidences for the sewage-dominated tributary with abnormally high CH4 and N2O concentrations, which was partially responsible for the sharp increase of CH4 and N2O fluxes in urban reaches. The results of structural equation model provided strong evidence for direct and indirect controls of urban expansion on longitudinal differentiation of riverine CH4 and N2O emissions crossing rural-urban interface. We argued that CH4 and N2O emissions from rivers in the mountainous rural-urban area are sensitive to the watershed urbanization and water pollution, and suggested that land use management, nutrient and organic matter control can be promising avenues to mitigate riverine CH4 and N2O emissions.

Spatiotemporal variability and controlling factors of indirect N2O emission in a typical complex watershed

The mechanisms of N2O emissions from inland rivers remain poorly understood because of the high variability of dissolved N2O concentration and the complexity of influencing factors. Thus, it is necessary to research the spatiotemporal patterns and influencing factors to understand the driving mechanisms of riverine N2O emissions. We combine the Soil and Water Assessment Tool (SWAT) outputs with established empirical equations to identify the spatiotemporal fluctuations of riverine N2O emissions and evaluate model performance through field measurements in a typical watershed. The spatiotemporal hotspots of N2O emissions are then determined, and the relative importance of environmental variables is further determined by correlation and attribution analysis. The results indicate that the riverine N2O emissions are relatively high from August to October, which accounted for 35.38% of the annual emissions. Temporal changes are attributed to agricultural activities and meteorological factors. Agricultural activities such as planting and fertilization lead to increased diffuse nitrogen loads on the land surface. Meantime, heavy precipitation events enhance the transport of nutrients, resulting in changes in nitrogen levels in the river. Spatial analysis shows that the urban watersheds (191.22 ± 156.19 μmol m−2 d−1) are the hotspots of riverine N2O emission, which are 1.55–3.03 times that of non-urban rivers. Spatial variations are mainly affected by riverine physicochemical indicators for different watersheds. Sewage from various sources received by urban rivers provides appropriate environmental conditions for N2O production, and transports large exogenous dissolved N2O. Furthermore, salinity (r = 0.80; p < 0.001) and nitrogen nutrients in riverine physicochemical indicators show a significant correlation with N2O fluxes. It emphasizes that N-related (TN, NH4+, NO3−) indicators are important reactants for N2O generation, which can promote nitrification and denitrification. Meanwhile, the results of structural equation modeling (SEM) also demonstrate that N2O emissions follow a similar pattern to riverine dissolved N2O concentration (r = 0.841, p < 0.001), and non-point source (r = 0.678, p < 0.001) play an important role in the changes of dissolved N2O concentrations. Our results highlight that certain hot moments and hot spots of rivers play a disproportionate role in year-round and basin-wide N2O emissions, respectively. It is necessary to implement more effective management measures by controlling key environmental factors to reduce N2O emissions.

Indirect nitrous oxide emission factors of fluvial networks can be predicted by dissolved organic carbon and nitrate from local to global scales.

Streams and rivers are important sources of nitrous oxide (N2 O), a powerful greenhouse gas. Estimating global riverine N2 O emissions is critical for the assessment of anthropogenic N2 O emission inventories. The indirect N2 O emission factor (EF5r ) model, one of the bottom-up approaches, adopts a fixed EF5r value to estimate riverine N2 O emissions based on IPCC methodology. However, the estimates have considerable uncertainty due to the large spatiotemporal variations in EF5r values. Factors regulating EF5r are poorly understood at the global scale. Here, we combine 4-year in situ observations across rivers of different land use types in China, with a global meta-analysis over six continents, to explore the spatiotemporal variations and controls on EF5r values. Our results show that the EF5r values in China and other regions with high N loads are lower than those for regions with lower N loads. Although the global mean EF5r value is comparable to the IPCC default value, the global EF5r values are highly skewed with large variations, indicating that adopting region-specific EF5r values rather than revising the fixed default value is more appropriate for the estimation of regional and global riverine N2 O emissions. The ratio of dissolved organic carbon to nitrate (DOC/NO3 - ) and NO3 - concentration are identified as the dominant predictors of region-specific EF5r values at both regional and global scales because stoichiometry and nutrients strictly regulate denitrification and N2 O production efficiency in rivers. A multiple linear regression model using DOC/NO3 - and NO3 - is proposed to predict region-specific EF5r values. The good fit of the model associated with easily obtained water quality variables allows its widespread application. This study fills a key knowledge gap in predicting region-specific EF5r values at the global scale and provides a pathway to estimate global riverine N2 O emissions more accurately based on IPCC methodology.

Nitrous oxide concentration and flux in Min River Basin of southeast China: Effects of land use, stream order and water variables

Estimates of riverine nitrous oxide (N2O) emissions have great uncertainty partly due to the fact that the data is still sparse for stream sizes and land uses. Here we determined water dissolved N2O concentration and water–air interface N2O flux across the land uses and stream orders in Min River Basin of southeast China. N2O concentrations ranged from 11.1 to 27.4 nmol/L, increasing from first- to eighth-order rivers. The first- and second-order streams showed negative N2O flux (−0.33 and −0.05 µg m−2h−1, respectively) compared to third- to eighth-order rivers (from 0.29 to 2.85 µg m−2h−1). Average N2O flux was significantly higher in urban (1.60 µg m−2h−1) and cropland rivers (0.89 µg m−2h−1) than in forest rivers (−0.47 µg m−2h−1), which could be due to the main effect of land use was on the nitrogen load. Indirect N2O emission factor varied between 0.035 and 0.14 % across the rivers, which decreased from the urban to forest rivers and were lower than the IPCC default value (0.26 %). N2O concentration and flux were positively correlated with NO3−, NH4+, depth, velocity and temperature, suggesting the importance of stream surface hydromoprhology to N2O emissions. Land use indirectly affected N2O flux by affecting water NO3−, NH4+, DO, DOC, pH and temperature, which could further explain the land use dependence of N2O emissions on water variables. High DOC:NO3− ratio (>6) was the main factor of N2O sink in the small streams, which may be due to the enhanced N2O consumption and low N2O production. Therefore, riverine N2O emissions are highly variable across stream sizes and land uses, which are not only increased by inorganic nitrogen load and are also dependent on stream hydraulics and morphology.

Basin-scale control on N2O loss rate and emission in the Changjiang River network, China

Global riverine N2O emissions have been made by several studies with great uncertainty. However, the regional N2O budgets and patterns in large river networks is still unclear, due to the lacking understanding of in-river N2O emission rate and well-classified river network water areas. Furthermore, the mass ratio of N2O emission against nitrogen(N) load in river networks remains controversial. Here we report N2O emissions from the largest river of China, the Changjiang River network, emphasizing the basin-scale control on riverine N2O loss rate in response to increasing N loads and river size. We find the N2O emission rate is negatively related to Strahler river orders, and positively related to N loading. The velocity (Vf) of N conversion into N2O was 0.131-0.436 m yr-1, and N2O loss rate (ζ) was 0.27-37.64 ×10-4 d-1 and declined exponentially with water discharge. Both the loss rate and the mass ratio of N conversion into N2O varied significantly at basin scale, indicating the diminishing capacity of river ecosystems to convert excess DIN into N2O when N load increased as a direct result of human activities. Our study shows N2O emission was 0.66 Gg N2O-N (1Gg=109g) in 1986 and increased to 10.3 Gg N2O-N in 2014 for the whole Changjiang River network. We identified the headwater streams are hotspots of N2O emission across the headwater stream to the estuary aquatic continuum. N2O emission was about 0.82% - 5.31% of global riverine N2O budget during 2010-2014. Our study suggested that an integrated approach in view of the riverine N loads and river hydrology is needed to improve estimates of riverine N2O emissions.

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.

Suburban agriculture increased N levels but decreased indirect N2O emissions in an agricultural-urban gradient river

The effects of land use on riverine N2O emissions are not well understood, especially in suburban zones between urban and rural with distinct anthropogenic perturbations. Here, we investigated in situ riverine N2O emissions among suburban, urban, and rural sections of a typical agricultural-urban gradient river, the Qinhuai River of Southeastern China from June 2010 to September 2012. Our results showed that suburban agriculture greatly increased riverine N concentration compared to traditional agricultural rivers (TAR). The mean total dissolved nitrogen (TDN) concentration was 8.18 mg N L−1 in the suburban agricultural rivers (SUAR), which was almost the same as that in the urban rivers (UR, of 8.50 mg N L−1), compared to that in TAR (0.92 mg N L−1). However, the annual average indirect N2O flux from the SUAR was only 27.15 μg N2O-N m−2 h−1, which was slightly higher than that from the TAR (13.14 μg N2O-N m−2 h−1) but much lower than that from the UR (131.10 μg N2O-N m−2 h−1). Moreover, the average N2O emission factor (EF5r, N2O-N/DIN-N) in the SUAR (0.0002) was significantly lower than those in the TAR (0.0028) and UR (0.0004). The limited indirect N2O fluxes from the SUAR are best explained by the high riverine dissolved organic carbon (DOC) and low dissolved oxygen, which probably reduced the denitrification source N2O by favoring complete denitrification to produce N2 and inhibited the nitrification source N2O, respectively. An exponential decrease model incorporating dissolved inorganic nitrogen and DOC could greatly improve our EF5r predictions in the agricultural-urban gradient river. Given the unprecedented suburban agriculture in the world, more studies in suburban agricultural rivers are needed to further refine the EF5r and better reveal the mechanisms behind indirect N2O emissions as influenced by suburban agriculture.

Estimation of Nitrous Oxide Emission from River System Based on Water Discharge and Dissolved Nitrous Oxide Concentration

Due to increasing active nitrogen pollution loads, river systems have become an important source of nitrous oxide (N2O) in many areas. Due to the lack of monitoring data in many studies as well as the difficulty in estimating intermediate parameters and expressing temporal-spatial variability in current methods, a high level of uncertainty remains in the estimates of riverine N2O emission quantity. Based on the monthly monitoring efforts conducted for 10 sampling sites across the Yonganxi River system in Zhejiang Province from June 2016 to July 2019, the temporal and spatial dynamics of riverine N2O dissolved concentrations ρ(N2O), N2O fluxes, and their influencing factors were addressed. A multiple regression model was then developed for predicating riverine N2O emission flux to estimate annual N2O emission quantity for the entire river system. The results indicated that observed riverine ρ(N2O) (0.03-2.14 μg·L-1) and the N2O fluxes[1.32-82.79 μg·(m2·h)-1] varied by 1-2 orders of magnitude of temporal-spatial variability. The temporal and spatial variability of ρ(N2O) were mainly influenced by the concentrations of nitrate, ammonia, and dissolved organic carbon, whereas the N2O emission fluxes were mainly affected by river water discharges and ρ(N2O). A multiple regression model that incorporates variables of river water discharge and ρ(N2O) could explain 90% of the variability in riverine N2O emission fluxes and has high accuracy. The model estimated N2O emission quantity from the entire Yonganxi River system of 3.67 t·a-1, with 29% from the main stream and 71% from the tributaries. The IPCC default emission factor method might greatly overestimate and underestimate N2O emission quantities for rivers impacted by low and high pressures of human activities, respectively. This study advances our quantitative understanding of N2O emission for the entire river system and provides a reference method for estimating riverine N2O emission with more accuracy.

Continuous Damming Increases Riverine N2o Emissions: A Case Study of Cascade Reservoirs in the Yunnan-Guizhou Plateau

Continuous Damming Increases Riverine N2o Emissions: A Case Study of Cascade Reservoirs in the Yunnan-Guizhou Plateau

Nitrous Oxide Emissions From Drying Streams and Rivers

AbstractStreams and rivers are suffering more extreme and prolonged low flows than those naturally occurring without human intervention with potential relevant worldwide effects on the production of nitrous oxide (N2O), a greenhouse gas 300 times more potent than carbon dioxide. Here, we address the question of whether droughts and management‐induced low flows increase riverine N2O emissions causing positive feedback in response to climate change. Supported by field data, we model riverine N2O emissions for decreasing mean summer flows in a forested and agricultural watershed under chemostatic and chemodynamic dissolved inorganic reactive nitrogen (DIN) concentrations. Our results demonstrate that total N2O emissions decrease with increasing low flow severity in both watersheds regardless of DIN scenario; which imparts a form of climate resilience.

Spatiotemporal Dynamics of Nitrous Oxide Emission Hotspots in Heterogeneous Riparian Sediments

AbstractNitrous oxide (N2O) is a potent ozone‐depleting greenhouse gas produced by incomplete denitrification. Recent works on riverine N2O emissions focus mainly on contributions from in‐channel, benthic, and fluvial hyporheic environments under assumptions of steady‐state conditions and homogeneous sediment hydraulic conductivity (K). However, riparian floodplains are also a potentially important N2O source characterized by complex sediment heterogeneity and dynamic surface and groundwater interactions. We use numerical flow and reactive transport models to investigate the influence of complex sedimentary architecture and high‐flow events (e.g., storms) on N2O production. We interpret the correlation between flow and solute fields with the flow topological Okubo‐Weiss metric (OW) and the scalar dissipation rate weighted by soil organic matter (OM) fraction and soil saturation. We model a heterogeneous riparian floodplain based on field observations from the Theis Environmental Monitoring and Modeling Site, Ohio, USA. N2O production is greatest within intermediate‐K sediments (e.g., sands) where denitrification rates are highest, and emissions increase by more than an order of magnitude during storms. Sensitivity analysis reveals that the denitrification rate is most influential for N2O flux, accounting for nearly 46% of the variance in production rates. Denitrification rates adapt to spatial changes in the flow topology (measured by OW) related to sediment heterogeneity and are strongly influenced by subsurface mixing dynamics. Mixing is greatest in shear strain‐dominated regions, while vorticity promotes OM dissolution and prolongs residence times. Accurate lithologic representation is imperative for characterizing subsurface N2O production dynamics, especially given growing concern regarding climate change driven hydrologic changes within watersheds worldwide.