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

Microbial mechanisms underlying the reduction of N2O emissions from submerged plant covered system

Submerged plant (SP) restoration is a crucial strategy for restoring aquatic ecosystem. However, the effect of SP on nitrous oxide (N2O) emissions remains controversial, and the impact of SP-attached biofilms on N2O emissions is often overlooked. In this study, SP and non-submerged plant (NSP) systems were set up and operated continuously for 189 days, revealing that SP reduced N2O flux by 42.4 %. By comparing the N2O net emission rates from water, sediment, and biofilms, we identified biofilms as the primary medium responsible for the reduction in N2O emissions in both SP and NSP systems. Further analysis of N2O metabolic rates from nitrification, denitrification, and abiotic processes under light and dark conditions confirmed that counter-diffusion of dissolved oxygen and nutrients in SP biofilms plays a key role in reducing denitrification-driven N2O emissions. Additionally, SP-attached biofilms increased nosZII-type denitrifiers (e.g., Bacillus) and reduced N2O production potential ((nirS+nirK)/(nosZI+nosZII)). Notably, the establishment of a SP restoration project in a typical eutrophic freshwater lake demonstrated that SP could reduce N2O fluxes by 61.5 %. This study provides significant insights for strategies aimed at mitigating N2O emissions.

Spatial distribution of potential nitrogen reduction rates and associated microbial communities revealed by metagenomic analysis in Yangtze River sediments

Understanding the intricacies of nitrogen reduction processes and the composition of associated microbial communities is crucial for illuminating the reactions of ecosystems and their functions to persistent nitrogen inputs. To enhance research on the nitrogen reduction process, we determined the potential rates, quantified the relevant genes, and analyzed the macro factors in the sediments of the Yangtze River. The results showed that dissimilatory reduction of nitrate to ammonium (DNRA) dominated the N-reduction processes in the Yangtze River sediment, with average rates of 0.89 ± 0.71 nmol N g-1 h-1. Meanwhile, denitrification and anammox rates were 0.73 ± 0.74 and 0.07 ± 0.07 nmol N g-1 h-1, respectively. The Three Gorges Dam (TGD) caused higher potential rates (nmol N g-1 h-1) of denitrification (1.38), anammox (0.12), DNRA (1.48), and N2O depletion (1.49 nmol g-1 h-1) in the Three Gorges Reservoir (TGR) compared to other river reaches. The average copy numbers (copies·g-1) of nrfA (2.96 × 106), narG (8.17 × 105), nirS (6.10 × 106), nosZ (2.77 × 106), and hzsB (3.68 × 105) in TGR sediments were higher than those in the other reaches. The TGD's interception of fine sediments and nutrients enhanced microbial gene abundance, thereby favoring N-reduction processes and resulting in N2O depletion in reservoir sediments. Moreover, the TGD caused a decreased contribution gap between DNRA and denitrification in the TGR (2%) compared with the upper (35%) and lower (18%) reaches, while causing predominant anammox (50%) in the middle reach. Metagenomic results suggested that sediment particle size, along with organic carbon and inorganic nitrogen concentrations, influenced N reduction rates by affecting narG, norB and C, nrfA and H, and hzsB and C. This study reveals the spatial pattern of the N-reduction rate in the Yangtze River sediments and quantitatively defines the intensity of dam effects on sediment N-reduction rate.

Freshwater Salinization Mitigated N2O Emissions in Submerged Plant-Covered Systems: Insights from Attached Biofilms

Submerged plants (SMPs) play a critical role in improving water quality and reducing N2O greenhouse gas emissions. However, freshwater salinization represents a major environmental challenge in aquatic systems. To investigate the impact of salinization on N2O emissions, this study conducted indoor mesocosm experiments simulating SMP and nonsubmerged plant (Non_SMP) areas in freshwater lakes. The objective was to explore the effects and microbial mechanisms of the attached biofilm on N2O emission in freshwater salinization. Salinization systems (700-1500 μS cm-1) reduced N2O flux by 37.0 and 40.5% compared to freshwater systems (<700 μS cm-1) of SMPs and Non_SMPs, respectively. Kinetic experiments showed that the reduction in N2O emissions was mainly attributed to the attached biofilm rather than the sediment or water. The N2O net emission rates of the attached biofilm decreased by 47.1 and 71.8% in salinization systems of SMPs and Non_SMPs, respectively, compared with freshwater systems. Additionally, biofilms in salinization systems exhibited lower denitrification rates. Furthermore, salinization reduced the N2O production potential ((nirS + nirK)/(nosZI + nosZII)), thereby further decreasing N2O emissions. This study provides valuable insights into the role and mechanisms of biofilms in mitigating N2O emissions in salinized freshwater lakes.

IPCC Emission Factor Overestimates N2O Emissions from Agricultural Ditches

Agricultural ditches emit disproportionate amounts of nitrous oxide (N2O), but their contributions to regional or global N2O emissions remain unclear due to limited data. The Intergovernmental Panel on Climate Change (IPCC) recommends using emission factors (EFs) to estimate indirect N2O emission, but the EF for ditches (EF5g) is categorized as groundwater, which potentially introduces a significant bias. This study conducted a regional-scale campaign in the North China Plain, one of the world's most intensive agricultural regions, and calculated the EF5g values from agricultural ditches by the concentration method (N2O-N/NO3--N). The results found that the regional-scale mean EF5g value (0.0028) was less than half of the IPCC default value (0.006), illustrating that the current IPCC methodology significantly overestimates N2O emissions from agricultural ditches. Despite the relatively small EF5g values, agricultural ditches exhibited a high mean N2O concentration (3.36 μg L-1) and a large regional emission (1.14 ± 0.86 Gg N2O-N yr-1), which is equal to 3.8 ± 2.9% of direct N2O emission from the croplands in the North China Plain. Since ditches are ubiquitous in agricultural regions and are likely to expand under climate change, refining EF5g is crucial to accurately assess their contribution to global N2O budgets.

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.

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.

Boosting N2O Catalytic Decomposition by the Synergistic Effect of Multiple Elements in Cobalt-Based High-Entropy Oxides

Nitrous oxide (N2O) has a detrimental impact on the greenhouse effect, and its efficient catalytic decomposition at low temperatures remains challenging. Herein, the cobalt-based high-entropy oxide with a spinel-type structure (Co-HEO) is successfully fabricated via a facile coprecipitation method for N2O catalytic decomposition. The obtained Co-HEO catalyst displays more remarkable catalytic performance and higher thermal stability compared with single and binary Co-based oxides, as the temperature of 90% N2O decomposition (T90) is 356 °C. A series of characterization results reveal that the synergistic effect of multiple elements enhances the reducibility and augments oxygen vacancy in the high-entropy system, thus boosting the activity of the Co-HEO catalyst. Moreover, density functional theory (DFT) calculations and the temperature-programmed surface reaction (TPSR) with isotope labeling demonstrate that N2O decomposition on the Co-HEO catalyst follows the Langmuir-Hinshelwood (L-H) mechanism with the promotion of abundant oxygen vacancies. This work provides a fundamental understanding of the synergistic catalytic effect in N2O decomposition and paves the way for the novel environmental catalytic applications of HEO.

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.

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.

Nitrous oxide emission mitigation from biological wastewater treatment - A review

Nitrous oxide (N₂O) emitted from wastewater treatment processes has emerged as a focal point for academic and practical research amidst pressing environmental issues. This review presents an updated view on the biological pathways for N₂O production and consumption in addition to the critical process factors affecting N₂O emission. The current research trends including the strain and reactor aspects were then outlined with discussions. Last but not least, the research needs were proposed. The holistic life cycle assessment needs to be performed to evaluate the technical and economic feasibility of the proposed mitigation strategies or recovery options. This review also provides the background information for the proposed future research prospects on N₂O mitigation and recovery technologies. As pointed out, dilution effects of the produced N₂O gas product would hinder its use as renewable energy; instead, its use as an effective oxidizing agent is proposed as a promising recovery option.

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.

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 (EF_5r, 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 EF_5r 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 EF_5r and better reveal the mechanisms behind indirect N₂O emissions as influenced by suburban agriculture.

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.

Cryptic Sulfur and Oxygen Cycling Potentially Reduces N2O-Driven Greenhouse Warming: Underlying Revision Need of the Nitrogen Cycle

Increasing global deoxygenation has widely formed oxygen-limited biotopes, altering the metabolic pathways of numerous microbes and causing a large greenhouse effect of nitrous oxide (N₂O). Although there are many sources of N₂O, denitrification is the sole sink that removes N₂O from the biosphere, and the low-level oxygen in waters has been classically thought to be the key factor regulating N₂O emissions from incomplete denitrification. However, through microcosm incubations with sandy sediment, we demonstrate here for the first time that the stress from oxygenated environments does not suppress, but rather boosts the complete denitrification process when the sulfur cycle is actively ongoing. This study highlights the potential of reducing N₂O-driven greenhouse warming and fills a gap in pre-cognitions on the nitrogen cycle, which may impact our current understanding of greenhouse gas sinks. Combining molecular techniques and kinetic verification, we reveal that dominant inhibitions in oxygen-limited environments can interestingly undergo triple detoxification by cryptic sulfur and oxygen cycling, which may extensively occur in nature but have been long neglected by researchers. Furthermore, reviewing the present data and observations from natural and artificial ecosystems leads to the necessary revision needs of the global nitrogen cycle.

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.

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.

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

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

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.

Soil Redox Controls CO2, CH4 and N2O Efflux from White-Rot Fungi in Temperate Forest Ecosystems

Microaerophilic white-rot fungi (WRF) are impacted by oxygen depletion because of fluctuating redox occurrence in southern temperate forest soils of Chile (1500-5000 mm year^-1). How these conditions influence WRF survival has been scarcely examined. We explored the contributions of WRF to greenhouse gas (GHG) emissions of N₂O and CH₄ and soil organic C oxidation (CO₂) in five sterilized and inoculated forest soils derived from various parent materials and climates. The soil was incubated for 20 days following (i) oxic, (ii) anoxic, and (iii) fluctuating redox conditions. Fungi contributed to 45% of the total GHG under redox fluctuating conditions, including the contribution of bacteria, while the opposite (26%) was valid for oxic treatment. On average, the highest gas emission (62%) was N₂O for WRF under redox treatment, followed by anoxic (22%) and oxic (16%) treatments, while CO₂ and CH₄ emissions followed oxic > redox > anoxic. These data suggest that indigenous microbial WRF communities are well adapted to fluctuating redox milieu with a significant release of GHG emissions in humid temperate forests of the southern cone.

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.

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.

Functional structures of soil microbial community relate to contrasting N2O emission patterns from a highly acidified forest

Nitrous oxide (N₂O) is an important greenhouse gas contributing to global climate change. Emissions of N₂O from acidic forests are increasing rapidly; however, little is known about the mechanisms driving these emissions. We analyzed soil samples from a high N₂O emission area (HEA, 224-601 μg N m^-2 h^-1) and an adjacent low emission area (LEA, 20-30 μg N m^-2 h^-1) of a highly acidified forest. HEA showed similar carbon and nitrogen (N) pools and microbial biomass to LEA, but significantly higher moisture and extractable nutrients than LEA did. GeoChip 4 detected 298 gene families (unadjusted P < 0.05; 94, adjusted P < 0.05) showing significantly different structures between HEA and LEA. Both areas had highly diverse N cycling functional genes. However, HEA had higher relative abundances of nor, P450nor, and archaeal nitrifier nirK, which provided evidence for the importance of denitrifiers in N₂O emission. HEA also showed significantly higher relative abundances of lignin- and cellulose-degrading genes, oxygen-limitation-response genes and denitrifier ppk, but lower abundances of N- and phosphorus (P) -limitation-response genes especially denitrifier pstS, corresponding to the higher moisture and extractable nutrients conducive to denitrification. The moisture, extractable nutrients and pH explained over 50% variation in microbial communities, and extractable P appeared as the key factor driving community variation and consequently regulated N₂O production. CAPSULE ABSTRACT: N₂O emission in highly acidified forest soils was related to the diverse N functional genes, especially denitrification genes, and was affected by soil properties.

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.

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.