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

Small water body significantly contributes to nitrous oxide emissions in China's aquaculture

Aquaculture systems are expected to act as potential hotspots for nitrous oxide (N2O) emissions, largely attributed to substantial nutrient loading from aquafeed applications. However, the specific patterns and contributions of N2O fluxes from these systems to the global emissions inventory are not well characterized due to limited data. This study investigates the patterns of N2O flux across 127 freshwater systems in China to elucidate the role of aquaculture ponds and lakes/reservoirs in landscape N2O emission. Our findings show that the average N2O flux from aquaculture ponds was 3.63 times higher (28.73 μg N2O m-2 h-1) than that from non-aquaculture ponds. Additionally, the average N2O flux from aquaculture lakes/reservoirs (15.65 μg N2O m-2 h-1) increased 3.05 times compared to non-aquaculture lakes/reservoirs. The transition from non-aquaculture to aquaculture practices has resulted in a net annual increase of 7589 ± 2409 Mg N2O emissions in China's freshwater systems from 2003 to 2022, equivalent to 20% of total N2O emissions from China's inland water. Particularly, the robust negative regression relationship between N2O emission intensity and water area suggests that small ponds are hotspots of N2O emissions, a result of both elevated nutrient concentrations and more vigorous biogeochemical cycles. This indicates that N2O emissions from smaller aquaculture ponds are larger per unit area compared to equivalent larger water bodies. Our findings highlight that N2O emissions from aquaculture systems can not be proxied by those from natural water bodies, especially small water bodies exhibiting significant but largely unquantified N2O emissions. In the context of the rapid global expansion of aquaculture, this underscores the critical need to integrate aquaculture into global assessments of inland water N2O emissions to advance towards a low-carbon future.

Drainage ditches are significant sources of indirect N2O emissions regulated by available carbon to nitrogen substrates in salt-affected farmlands

Agriculture is a main source of nitrous oxide (N2O) emissions. In agricultural systems, direct N2O emissions from nitrogen (N) addition to soils have been widely investigated, whereas indirect emissions from aquatic ecosystems such as ditches are poorly known, with insufficient data available to refine the IPCC emission factor. In this contribution, in situ N2O emissions from two ditch water‒air interfaces based on a diffusion model were investigated (almost once per month) from June 2021 to December 2022 in an intensive arable catchment with high N inputs and salt-affected conditions in the Qingtongxia Irrigation District, northwestern China. Our results implied that agricultural ditches (mean 148 μg N m-2 h-1) were significant sources for N2O emissions, and were approximately 2.1 times greater than those of the Yellow River directly connected to ditches. Agronomic management strategies increased N2O fluxes in summer, while precipitation events decreased N2O fluxes. Agronomic management strategies, including fertilization (294--540 kg N hm-2) and irrigation on farmland, resulted in enhanced diffuse N loads in drain water, whereas precipitation diluted the dissolved N2O concentration in ditches and accelerated the ditch flow rate, leading to changes in the residence time of N-containing substances in water. The spatial analysis showed that N2O fluxes (202-233 μg N m-2 h-1) in the headstream and upstream regions of ditches due to livestock and aquaculture pollution sources were relatively high compared to those in the midstream and downstream regions (100-114 μg N m-2 h-1). Furthermore, high available carbon (C) relative to N reduced N2O fluxes at low DOC:DIN ratio levels by inhibiting nitrification. Spatiotemporal variations in the N2O emission factor (EF5) across ditches with higher N resulted in lower EF5 and a large coefficient of variation (CV) range. EF5 was 0.0011 for the ditches in this region, while the EF5 (0.0025) currently adopted by the IPCC is relatively high. The EF5 variation was strongly controlled by the DOC:DIN ratio, TN, and NO3--N, while salinity was also a nonnegligible factor regulating the EF5 variation. The regression model incorporating NO3--N and the DOC:DIN ratio could greatly enhance the predictions of EF5 for agricultural ditches. Our study filled a key knowledge gap regarding EF5 from agricultural ditches in salt-affected farmland and offered a field investigation for refining the EF5 currently used by the IPCC.

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

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

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.

Dissolved N2O concentrations in oil palm plantation drainage in a peat swamp of Malaysia

Oil palm plantations in Southeast Asia are the largest supplier of palm oil products and have been rapidly expanding in the last three decades even in peat-swamp areas. Oil palm plantations on peat ecosystems have a unique water management system that lowers the water table and, thus, may yield indirect N₂O emissions from the peat drainage system. We conducted two seasons of spatial monitoring for the dissolved N₂O concentrations in the drainage and adjacent rivers of palm oil plantations on peat swamps in Sarawak, Malaysia, to evaluate the magnitude of indirect N₂O emissions from this ecosystem. In both the dry and wet seasons, the mean and median dissolved N₂O concentrations exhibited over-saturation in the drainage water, i.e., the oil palm plantation drainage may be a source of N₂O to the atmosphere. In the wet season, the spatial distribution of dissolved N₂O showed bimodal peaks in both the unsaturated and over-saturated concentrations. The bulk δ¹⁵N of dissolved N₂O was higher than the source of inorganic N in the oil palm plantation (i.e., N fertilizer and soil organic nitrogen) during both seasons. An isotopocule analysis of the dissolved N₂O suggested that denitrification was a major source of N₂O, followed by N₂O reduction processes that occurred in the drainage water. The δ¹⁵N and site preference mapping analysis in dissolved N₂O revealed that a significant proportion of the N₂O produced in peat and drainage is reduced to N₂ before being released into the atmosphere.

Characteristics of dissolved N2O and indirect N2O emission factor in the groundwater of high nitrate leaching areas in northwest China

Numerous studies have demonstrated high concentrations of dissolved N₂O and indirect N₂O emission factors in groundwater affected by agriculture. However, the characteristics of seasonal and vertical dimensional difference in groundwater in high nitrate leaching areas are relatively lacking. We monitored the concentrations of dissolved and wellhead N₂O of 23 groundwater wells over a one year period to understand the seasonal characteristics of dissolved and wellhead N₂O concentrations and indirect N₂O emission factors (EF₅) of the shallow and deep groundwater in a high nitrogen leaching area and analyze the reasons for their differences. The mean dissolved N₂O concentration in groundwater was 9.71 (9.03) μg/L, which was 1.5-fold higher during the wet season relative to the dry season. Furthermore, the leaching of soil N₂O caused by rainfall and irrigation could be a pivotal factor affecting seasonal variation in the dissolved N₂O. Shallow wells were found to have higher dissolved and wellhead N₂O concentrations compared with deep wells in all seasons. The low wellhead N₂O concentrations during the dry season were attributed to the seasonal decrease of the groundwater table and dissolved N₂O concentrations. We concluded that indirect N₂O emission factors did not vary in the vertical dimension but were higher during the wet season than that during the dry season. In addition, the mean indirect N₂O emission factor in the groundwater was 0.025 %, which was one order of magnitude below the current IPCC value (0.25 %). Thus, we proposed that such a low indirect N₂O emissions factor could imply a low indirect N₂O emission potential in groundwater with high dissolved oxygen and nitrogen loads. Our study further indicated that seasonal differences in dissolved N₂O concentrations and indirect N₂O emission factors should be considered when estimating the potential emissions of dissolved N₂O in groundwater.

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 (EF_5r ) model, one of the bottom-up approaches, adopts a fixed EF_5r value to estimate riverine N₂ O emissions based on IPCC methodology. However, the estimates have considerable uncertainty due to the large spatiotemporal variations in EF_5r values. Factors regulating EF_5r 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 EF_5r values. Our results show that the EF_5r values in China and other regions with high N loads are lower than those for regions with lower N loads. Although the global mean EF_5r value is comparable to the IPCC default value, the global EF_5r values are highly skewed with large variations, indicating that adopting region-specific EF_5r 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 EF_5r 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 EF_5r 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 EF_5r values at the global scale and provides a pathway to estimate global riverine N₂ O emissions more accurately based on IPCC methodology.

Conversion of coastal wetland to aquaculture ponds decreased N2O emission: Evidence from a multi-year field study

Land reclamation is a major threat to the world's coastal wetlands, and it may influence the biogeochemical cycling of nitrogen in coastal regions. Conversion of coastal marshes into aquaculture ponds is common in the Asian Pacific region, but its impacts on the production and emission of nitrogen greenhouse gases remain poorly understood. In this study, we compared N₂O emission from a brackish marsh and converted shrimp aquaculture ponds in the Shanyutan wetland, the Min River Estuary in Southeast China over a three-year period. We also measured sediment and porewater properties, relevant functional gene abundance, sediment N₂O production potential and denitrification potential in the two habitats. Results indicated that the pond sediment had lower N-substrate availability, lower ammonia oxidation (AOA and comammox Nitrospira amoA), nitrite reduction (nirK and nirS) and nitrous oxide reduction (nosZ Ⅰ and nosZ Ⅱ) gene abundance and lower N₂O production and denitrification potentials than in marsh sediments. Consequently, N₂O emission fluxes from the aquaculture ponds (range 5.4-251.8 μg m^-2 h^-1) were significantly lower than those from the marsh (12.6-570.7 μg m^-2 h^-1). Overall, our results show that conversion from marsh to shrimp aquaculture ponds in the Shanyutan wetland may have diminished nutrient input from the catchment, impacted the N-cycling microbial community and lowered N₂O production capacity of the sediment, leading to lower N₂O emissions. Better post-harvesting management of pond water and sediment may further mitigate N₂O emissions caused by the aquaculture operation.

Global methane and nitrous oxide emissions from inland waters and estuaries

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

Environmental drivers of nitrous oxide emission factor for a coastal reservoir and its catchment areas in southeastern China

While Asia is projected to be one of the major nitrous oxide (N₂O) sources in the coming decades, a more accurate assessment of N₂O budget has been hampered by low data resolution and poorly constrained emission factor (EF). Since urbanized coastal reservoirs receive high nitrogen loads from diverse sources across a heterogeneous landscape, the use of a single fixed EF may lead to large errors in N₂O assessment. In this study, we conducted high spatial resolution sampling of dissolved N₂O, nitrate-nitrogen (NO₃^--N) and other physico-chemical properties of surface water in Wenwusha Reservoir and other types of water bodies (river, drainage channels, and aquaculture ponds) in its catchment areas in southeastern China between November 2018 and June 2019. The empirically derived EF (calculated as N₂O-N:NO₃^--N) for the reservoir showed considerable spatial variations, with a 10-fold difference ranging from 0.8 × 10^-3 to 8.8 × 10^-3. The average EF varied significantly among the four types of water bodies in the following descending order: aquaculture ponds > river > drainage channels > reservoir. Across all the water bodies, the mean EF in summer was 1.8-3.5 and 1.7-2.8 fold higher than that in autumn and spring, respectively, owing to the elevated water temperature. Overall, our derived EF deviated considerably from the IPCC default value, which implied that the use of default EF could result in over- or under-estimation of N₂O emissions by up to 42%. We developed a multiple regression model that could explain 82% of the variance in EF based on water temperature and the ratio between dissolved organic carbon and nitrate-nitrogen (p < 0.001), which could be used to improve the estimate of EF for assessing N₂O emission from coastal reservoirs and other similar environments.

Large variations in indirect N2O emission factors (EF5) from coastal aquaculture systems in China from plot to regional scales

Aquaculture ponds are important anthropogenic sources of nitrous oxide (N₂O). Direct N₂O emissions arising from feed application to ponds have been widely investigated, but indirect emissions from N₂O production from residual feeds in pond water are much less understood and characterized to refine the IPCC emission factor. In this study, we determined the concentrations and spatiotemporal variations of dissolved N₂O and NO₃^--N in situ in three aquaculture ponds at the Min River Estuary in southeastern China during the culture period over two years, and calculated the indirect N₂O emission factor (EF₅) for aquaculture ponds using the N₂O-N/NO₃^--N mass ratio methodology. Our results indicated that the EF₅ values in the ponds over the culture period ranged between 0.0007 and 0.0543, with a clear seasonal pattern which closely followed that of the DOC:NO₃^-N ratio. We also observed significant spatial variations in EF₅ among the three ponds, which could be attributed to the difference in feed conversion rate. In addition, we assessed the EF₅ values from aquaculture ponds in five regions of the Chinese coastline across the latitudinal gradient from the tropical to the temperate zones. The average EF₅ value from aquaculture ponds across the five coastal regions was 0.0093±0.0024, which was approximately 3.7 times of the IPCC default value for rivers and estuaries (0.0025). Moreover, the EF₅ values demonstrated considerable spatial variations across these coastal regions with a coefficient of variation of 59%, which were largely related to the difference in water salinity. Our findings filled a key knowledge gap about the indirect N₂O emission factor from aquaculture ponds, and provided field evidence for the refinement of EF₅ value currently adopted by IPCC in the national greenhouse gas inventory.

Indirect emissions of nitrous oxide in a cropland watershed with contrasting hydrology in central France

Nitrous oxide (N₂O) is an important greenhouse gas. Its atmospheric concentration have increased with the industrialisation and the use of N fertilizer. The contribution of freshwater systems to N₂O emissions is still very uncertain, while regional transfer of nitrogen depends on soil and hydrology. Riverine and spring N₂O dissolved in water was therefore measured over two years in the 3453 km² Haut-Loir watershed (France). This temperate cropland watershed is characterized by two different hydrological systems east and west of the Loir River. The eastern rivers, fed by the emergence of the deep Beauce aquifer, exhibited significantly higher dissolved N₂O concentrations (Beauce region, mean: 2.93 μg-N L^-1) than the western rivers (Perche region, mean: 0.87 μg-N L^-1), which were largely influenced by runoff during winter flooding. The eastern rivers had large nitrate concentrations all over the year; in the Perche, nitrate underwent a seasonal cycle with large loads during winter floods, but there were no consistent seasonal patterns in N₂O. The ratios of N₂O in excess of equilibrium on nitrate, often used as a proxy of emission factor (EF), were much smaller than the default IPCC values, both for rivers (0.014% versus 0.25% for IPCC EF_5r) and the Loir spring (0.085% versus 0.6% for the IPCC EF_5g for groundwater and springs). EF_5r were significantly different between the two parts of the watershed only in winter, because of the seasonal variability of NO₃^-. Moreover dissolved N₂O is controlled not only by NO₃^-, as it is considered in the calculation of the EF₅, but also by water pH and dissolved organic carbon. A good prediction of dissolved N₂O was obtained using these physicochemical variables and hydrological regions. Thus, these results suggest that the spatial variability of riverine N₂O depends on local hydrology, while further research is needed to understand the seasonal variability.

Marginal Trade-Offs for Improved Agro-Ecological Efficiency Using Data Envelopment Analysis

Today’s agricultural management decisions impact food security and sustainable ecosystems, even when operating with back-to-basic operations. In such endeavors, policymakers usually need a quantitative tool, such as trade-offs margins, to effectively adjust resource consumption or production. This paper applies the weighted slack-based measurement (SBM-DEA) program to 136 developing countries’ agricultural performance. First, it finds the current agricultural efficiency and then makes marginal trade-offs on desirable-output variables (such as crop yield and forest area) to see the effective changes in undesirable-output (such as methane and nitrous oxide emissions). The results show that choosing effective marginal trade-offs does not deteriorate the relative efficiency of the decision-making units (DMUs) below the efficient frontier line. Thus, such a method enables the decision-makers to determine the best marginal trade-off points to reach the optimal efficiencies and decide which output factor needs special brainstorming to design effective policy.

Methane and nitrous oxide have separated production zones and distinct emission pathways in freshwater aquaculture ponds

Aquaculture systems receive intensive carbon (C) and nitrogen (N) loadings, and are therefore recognized as major anthropogenic sources of methane (CH₄) and nitrous oxide (N₂O) emissions. However, the extensively managed aquaculture ponds were identified as a hotspot of CH₄ emission but just a weak N₂O source. Here, we investigate annual CH₄ and N₂O fluxes from three earthen ponds used for crab culture, of different sizes, in southeast China. Our purposes are to identify the spatiotemporal variations of CH₄ and N₂O emissions and their components among ponds and to evaluate the zone for CH₄ and N₂O production. Static chamber-measured CH₄ flux ranged from 0.03 to 64.7 mg CH₄ m^‒2 h^‒1 (average: 9.02‒14.3 mg CH₄ m^‒2 h^‒1), and temperature, followed by dissolved organic C (DOC) concentration, and redox potential, were the primary drivers of seasonal CH₄ flux patterns. Annual mean diffusive CH₄ flux was 1.80‒2.34 mg CH₄ m^‒2 h^‒1, and that by ebullition was up to 7.20‒12.0 mg CH₄ m^‒2 h^‒1 (79.1‒83.5% of the total CH₄ flux). Annual CH₄ emission was positively correlated with sediment DOC concentration but negatively (P < 0.05) correlated with water depth across ponds, with the highest CH₄ emission occurred in a pond with low water depth and high DOC concentration. The calculated diffusive N₂O flux by the gas transfer velocity was 0.32‒0.60 times greater than the measured N₂O emission, suggesting that N₂O in water column can not only evade as water-air fluxes but diffuse downwards and to be consumed in anaerobic sediments. This also indicates that N₂O was primarily produced in water column. The highly reduced condition and depletion of NO₃^‒-N in sediments, can limit N₂O production from both nitrification and denitrification but favor N₂O consumption, leading the ponds to become a weak source of N₂O annually and even a sink of N₂O in summer. Our results highlight that the current global CH₄ budget for inland waters is probably underestimated due to a lack of data and underestimation of the contribution of ebullitive CH₄ flux in small lentic waters. The downwards N₂O diffusion from the water column into sediment also indicates that the extensively-used model approach based on gas transfer velocity potentially overestimates N₂O fluxes, especially in small eutrophic aquatic ecosystems.

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.

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.

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.

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.

Emission of nitrous oxide from plain multi-ditch system and its impact factors

Multi-level ditch area is a major component of the hydrographic net of plain area, China. Given the high concentration of nitrogen (N) in the surface water and vigorous biogeochemical interactions, ditch is likely to be the hot spots of N₂O emission. However, N₂O emission flux and emission factor (EF_5r) of multi-level ditches have not been determined. To address this knowledge gap, a 1-year field work in three ditches with different levels in Chengdu Plain was conducted. It is found that the annual flux of N₂O emission and EF_5r was higher in the lateral (0.0020 and 83.94 μg m^-2 h^-1) and field ditches (0.0019 and 110.75 μg m^-2 h^-1) than in the branch ditch (0.0016 and 46.38 μg m^-2 h^-1, P < 0.05). It is found that parameters of groundwater level, discharge, precipitation, and NH₄⁺ were the primary factors, and these parameters can model the N₂O flux well. Furthermore, the content of NH₄⁺ in the surface water of ditches presented better correlation with the emission of N₂O than the content of NO₃^-. Therefore, controlling NH₄⁺ emission and lessening fertilizer usage in summer may be key solutions for indirect reduction of N₂O in Chengdu Plain.

Widespread nitrous oxide undersaturation in farm waterbodies creates an unexpected greenhouse gas sink

Nitrogen pollution and global eutrophication are predicted to increase nitrous oxide (N2O) emissions from freshwater ecosystems. Surface waters within agricultural landscapes experience the full impact of these pressures and can contribute substantially to total landscape N2O emissions. However, N2O measurements to date have focused on flowing waters. Small artificial waterbodies remain greatly understudied in the context of agricultural N2O emissions. This study provides a regional analysis of N2O measurements in small (<0.01 km2) artificial reservoirs, of which an estimated 16 million exist globally. We show that 67% of reservoirs were N2O sinks (-12 to -2 μmol N2O⋅m-2⋅d-1) in Canada's largest agricultural area, despite their highly eutrophic status [99 ± 289 µg⋅L-1 chlorophyll-a (Chl-a)]. Generalized additive models indicated that in situ N2O concentrations were strongly and nonlinearly related to stratification strength and dissolved inorganic nitrogen content, with the lowest N2O levels under conditions of strong water column stability and high algal biomass. Predicted fluxes from previously published models based on lakes, reservoirs, and agricultural waters overestimated measured fluxes on average by 7- to 33-fold, challenging the widely held view that eutrophic N-enriched waters are sources of N2O.

A review of indirect N2O emission factors from agricultural nitrogen leaching and runoff to update of the default IPCC values

Indirect N₂O emissions from agricultural nitrogen (N) leaching and runoff in water bodies contribute significantly to the global atmospheric N₂O budget. However, considerable uncertainty regarding this source remains in the bottom-up N₂O inventory. Indirect N₂O emission factor associated with N leaching and runoff (EF₅; kg N₂ON per kg of NO₃^--N) incorporate three components for groundwater and surface drainage (EF_5g), rivers (EF_5r), and estuaries (EF_5e). The 2006 IPCC default EF₅ value was based on a small number of studies available at the time. Here we present the synthesis of 254 measurements of EF₅, dissolved N₂O, and nitrate from 106 studies. Our results do not support the further downward revision of EF_5g by the IPCC and suggest an upward revision of EF_5g of 0.0060. The emission factors for groundwater and springs (0.0079) was higher than that for surface drainage (0.0040). The emission factor for lakes, ponds, and reservoirs was 0.0012, whereas that for rivers was 0.0030, and a combined EF_5r was 0.0026. Estimated EF_5r and EF_5e (0.0026) values from the study were close to the current IPCC default values (0.0025 each). We estimated an updated default EF₅ value of 0.01 for the refinement of IPCC guidelines.

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

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

Indirect N2O emissions with seasonal variations from an agricultural drainage ditch mainly receiving interflow water

Nitrogen (N)-enriched leaching water may act as a source of indirect N₂O emission when it is discharged to agricultural drainage ditches. In this study, indirect N₂O emissions from an agricultural drainage ditch mainly receiving interflow water were measured using the static chamber-gas chromatography technique during 2012-2015 in the central Sichuan Basin in southwestern China. We found the drainage ditch was a source of indirect N₂O emissions contributing an inter-annual mean flux of 6.56 ± 1.12 μg N m^-2 h^-1 and a mean indirect N₂O emission factor (EF_5g) value of 0.03 ± 0.003%. The mean EF_5g value from literature review was 0.51%, which was higher than the default EF_5g value (0.25%) proposed by the Intergovernmental Panel on Climate Change (IPCC) in 2006. Our study demonstrated that, more in situ observations of N₂O emissions as regards N leaching are required, to account for the large variation in EF_5g values and to improve the accuracy and confidence of the default EF_5g value. Indirect N₂O emissions varied with season, higher emissions occurred in summer and autumn. These seasonal variations were related to drainage water NO₃^--N concentration, temperature, and precipitation. Our results showed that intensive precipitation increased NO₃^--N concentrations and N₂O emissions, and when combined with warmer water temperatures, these may have increased the denitrification rate that led to the higher summer and autumn N₂O emissions in the studied agricultural drainage ditch.

Diel and seasonal nitrous oxide fluxes determined by floating chamber and gas transfer equation methods in agricultural irrigation watersheds in southeast China

Agricultural nitrate leaching and runoff incurs high nitrogen loads in agricultural irrigation watersheds, constituting one of important sources of atmospheric nitrous oxide (N₂O). Two independent sampling campaigns of N₂O flux measurement over diel cycles and N₂O flux measurements once a week over annual cycles were carried out in an agricultural irrigation watershed in southeast China using floating chamber (chamber-based) and gas transfer equation (model-based) methods. The diel and seasonal patterns of N₂O fluxes did not differ between the two measurement methods. The diel variation in N₂O fluxes was characterized by the pattern that N₂O fluxes were greater during nighttime than daytime periods with a single flux peak at midnight. The diel variation in N₂O fluxes was closely associated with water environment and chemistry. The time interval of 9:00-11:00 a.m. was identified to be the sampling time best representing daily N₂O flux measurements in agricultural irrigation watersheds. Seasonal N₂O fluxes showed large variation, with some flux peaks corresponding to agricultural irrigation and drainage episodes and heavy rainfall during the crop-growing period of May to November. On average, N₂O fluxes calculated by model-based methods were 27% lower than those determined by the chamber-based techniques over diel or annual cycles. Overall, more measurement campaigns are highly needed to assess regional agricultural N₂O budget with low uncertainties.

Hydrogeological Controls on Regional-Scale Indirect Nitrous Oxide Emission Factors for Rivers

Indirect nitrous oxide (N₂O) emissions from rivers are currently derived using poorly constrained default IPCC emission factors (EF_5r) which yield unreliable flux estimates. Here, we demonstrate how hydrogeological conditions can be used to develop more refined regional-scale EF_5r estimates required for compiling accurate national greenhouse gas inventories. Focusing on three UK river catchments with contrasting bedrock and superficial geologies, N₂O and nitrate (NO₃^-) concentrations were analyzed in 651 river water samples collected from 2011 to 2013. Unconfined Cretaceous Chalk bedrock regions yielded the highest median N₂O-N concentration (3.0 μg L^-1), EF_5r (0.00036), and N₂O-N flux (10.8 kg ha^-1 a^-1). Conversely, regions of bedrock confined by glacial deposits yielded significantly lower median N₂O-N concentration (0.8 μg L^-1), EF_5r (0.00016), and N₂O-N flux (2.6 kg ha^-1 a^-1), regardless of bedrock type. Bedrock permeability is an important control in regions where groundwater is unconfined, with a high N₂O yield from high permeability chalk contrasting with significantly lower median N₂O-N concentration (0.7 μg L^-1), EF_5r (0.00020), and N₂O-N flux (2.0 kg ha^-1 a^-1) on lower permeability unconfined Jurassic mudstone. The evidence presented here demonstrates EF_5r can be differentiated by hydrogeological conditions and thus provide a valuable proxy for generating improved regional-scale N₂O emission estimates.