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

Nitrous oxide emissions and soil profile responses to manure substitution in the North China Plain drylands

Substituting synthetic fertilizers with manures in agriculture enhances soil properties and crop yield. However, the impact on nitrous oxide (N2O) emissions, especially from the soil profile, remains poorly understood. This study examined emissions from 2017 to 2019 on a well-established (>10-year) maize field site in the North China Plain. Three treatments were compared: 100 % synthetic nitrogen (NPK), 50 % synthetic fertilizer N + 50 % manure N substitution (50%MNS), and 100 % manure N substitution (100%MNS). N2O emissions were monitored for three years, and in 2019, N2O concentrations at 20 cm and 40 cm soil depths were analyzed in relation to surface N2O fluxes and environmental factors. The results showed manure substitution resulted in about 13.8 %-25.2 % (50%MNS) and 40.3 %-72.2 % (100%MNS) reduction in N2O emissions over the 3-year period compared with the NPK treatment. Throughout the maize growing season, the top-dressing accompanied by rainfall was responsible for the N2O emissions. The difference in N2O concentrations between all the treatments at 20 cm depth was insignificant, but at 40 cm depth the N2O concentrations were significantly higher for the 50%MNS treatment than the other treatments. The N2O fluxes and N2O concentration were not synchronized especially in NPK. The decoupled relationship between the N2O fluxes and the N2O concentration in the soil profile depth suggested the contribution of N2O produced in the soil profile to the surface N2O fluxes is limited. This study highlights that manure substitution is an efficient measure to reduce N2O emissions.

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.

Synchronous monitoring agricultural water qualities and greenhouse gas emissions based on low-cost Internet of Things and intelligent algorithms

This study addressed the challenges of cost and portability in synchronous monitoring water quality and greenhouse gas emissions in paddy-dominated regions by developing a novel Internet of Things (IoT)-based monitoring system (WG-IoT-MS). The system, equipped with low-cost sensors and integrated intelligent algorithms, enabled real-time monitoring of dissolved N2O concentrations. Combined with an air-water gas exchange model, the system achieved efficient monitoring and simulation of CO2 and N2O emissions from agricultural water bodies while reducing monitoring costs by approximately 60 %. The proposed method was validated in paddy-dominated regions in Danyang, China. Results indicated the excellence of the dissolved N2O concentration model based on support vector regression, demonstrating accurate predictions within a concentration range of 2.003 to 13.247 μg/L. Notably, the model maintained acceptable predictive accuracy (R2 > 0.70) even when some variables were partially absent (with the number of missing variables < 2 and the missing proportion (MP) ≤ 50 %), making up for the data loss caused by sensor malfunctions. Furthermore, the model performed well (R2 > 0.80) when testing data sourced from paddy fields and lakes. Finally, CO2 and N2O emissions were successfully monitored, with the results validated using a floating chamber method (R2 > 0.70). The method provides crucial technical support for quantitative assessment of water quality and greenhouse gas emissions in paddy-dominated regions, laying a foundation for formulating effective emission reduction strategies.

Variability in N2O emission controls among different ponds within a hilly watershed

While it is well established that small water bodies like ponds play a disproportionately large role in contributing to N2O emissions, few studies have focused on lowland ponds in hilly watersheds. Here, we explored the characteristics of N2O concentrations and emissions from various typical ponds (village, tea, forested, and aquaculture ponds) in a hilly watershed and examined the specific controls influencing N2O production. Our findings revealed that tea ponds exhibited the highest N2O flux (8.42 ± 8.23 μmol m-2 d-1), which was 2.8 to 3.3 times greater than other types of ponds. Remarkable seasonal variations were observed in tea and forested ponds due to the seasonality of nutrient-enriched runoff, whereas such variations were less pronounced in village and aquaculture ponds. Key factors such as nitrogen levels, temperature, and dissolved oxygen (DO) emerged as the primary controls of N2O concentrations in ponds, heavily influenced by land use and human activities in their drainage areas. Specifically, N2O production in tea and aquaculture ponds was driven by N inputs from fertilization and feed, respectively, while DO levels governed the process in village and forested ponds, influenced by abundant algae and forest vegetation. This study emphasizes that environmental factors predominantly drive N2O production in ponds within hilly watersheds, but land use in the pond drainages acts as an indirect yet crucial influence. This highlights the need for future research to develop targeted emission reduction strategies based on land use to effectively mitigate N2O emissions, promising a path toward more sustainable and climate-friendly watershed management.

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.

Unravelling spatiotemporal N2O dynamics in an urbanized estuary system using natural abundance isotopes

Estuaries are strong sources of N2O to the atmosphere; yet we still lack insights into the impact of their biogeochemical dynamics on the emissions of this powerful greenhouse gas. Here, we investigated the spatiotemporal dynamics of the N cycle in an estuary with a focus on the emission mechanisms and pathways of N2O. By coupling N2O isotopocule analysis and substrate NO3- isotope analysis, we found that nutrient availability, oxygen level, salinity gradient and temperature variation were major drivers of the N2O emissions from the Scheldt Estuary. In winter, lower temperature and higher O2 concentration diminished denitrification rates and reduction of N2O to N2, while both were enhanced in warmer summer, causing higher fraction of reduced N2O. As a result, we found comparable N2O fluxes and dissolved concentrations between the two seasons. Decrease in salinity level and increase in NO3- concentration accelerated N2O production when moving upstream of the estuary where more urbanization and higher NO3- from wastewater discharges were found. However, these drivers had no significant effect on the fraction of N2O derived by either denitrification or nitrification and/or fungal denitrification since the fractional proportion of these pathways showed no spatiotemporal variations, remaining around 89 % and 11 %, respectively. These findings challenge the conventional notion that N2O fluxes are generally higher in summer because of higher denitrification rates while confirming that denitrification is the most important pathway of N2O production in the estuaries. Furthermore, our study highlight the importance of combining various isotope analyses to gain in-depth understanding about N2O emission pathways and N cycling in dynamic systems like estuaries.

Spatiotemporal variations and dominated environmental parameters of nitrous oxide (N2O) concentrations from cascade reservoirs in southwest China

Anthropogenic activity has caused rivers and reservoirs to become sources of nitrous oxide (N2O), which is thought to play an important role in global climate change. There are thermal and DO stratification in deep-water reservoirs with long hydraulic retention time, which change N2O production mechanism compared with shallow-water reservoirs. To promote our understanding of the relationship of N2O production in reservoirs at different depths, spatiotemporal variations in water environmental factors and N2O from cascade reservoirs of Chaishitan (CST), Longtan (LT), Yantan (YT) and Dahua (DH) reservoirs in the Zhujiang River were detected, and the LT and YT reservoirs were compared as representatives of deep-water and shallow-water reservoirs in April and July 2019. The average N2O concentrations in the LT and YT reservoirs were 22.82 ± 2.21 and 21.55 ± 1.65 nmol L-1, respectively. During spring and summer, the WT (water temperature) and DO (dissolved oxygen) concentrations in the YT reservoir were well mixed. In contrast, the LT reservoir, as a deep-water reservoir, had thermal and DO stratifications in both the shallow and middle water, especially in the summer when the solar radiation intensity was high. During summer stratification, the DO concentration in the LT reservoir showed obvious spatial variation, ranging from 1.23 to 9.84 mg L-1, while the DO concentration in the YT reservoir showed very little variation, ranging from 6.45 to 7.09 mg L-1. Structural equation modeling results showed that NH4+ was the main determinant of the N2O concentration in the YT reservoir, and DO was the most influential factor of the N2O concentration in the LT reservoir. These results suggest significant variations in the factors influencing N2O concentration among reservoirs. Additionally, the mechanisms of N2O production differ between deep-water and shallow-water reservoirs. This study highlights the spatio-temporal variations and influential factors contributing to N2O concentration. Furthermore, it discusses the production mechanisms of N2O in different types of reservoirs. These findings contribute to our understanding of N2O distribution in hydropower systems and provide valuable data for the management of hydropower facilities and research on greenhouse gas emissions.

Projected Increases in Precipitation Are Expected To Reduce Nitrogen Use Efficiency and Alter Optimal Fertilization Timings in Agriculture in the South East of England

Nitrogen fertilization is vital for productive agriculture and efficient land use. However, globally, approximately 50% of the nitrogen applied is lost to the environment, causing inefficiencies, pollution, and greenhouse gas emissions. Rainfall and its effect on soil moisture are the major components controlling nitrogen losses in agriculture. Thus, changing rainfall patterns could accelerate nitrogen inefficiencies. We used a mechanistic modeling platform to determine how precipitation-optimal nitrogen fertilization timings and resulting crop nitrogen uptake have changed historically (1950-2020) and how they are predicted to change under the RCP8.5 climate scenario (2021-2069) in the South East of England. We found that historically, neither precipitation-optimal fertilization timings nor resulting plant uptake changed significantly. However, there were large year-to-year variations in both. In the 2030s, where it is projected to get wetter, precipitation-optimal fertilization timings are predicted to be later in the season and the resulting plant uptake noticeably lower. After 2040, the precipitation-optimal uptakes are projected to increase with earlier precipitation-optimal timings closer to historical values, corresponding to the projected mean daily rainfall rates decreasing to the historical values in these growing seasons. It seems that the interannual variation in precipitation-optimal uptake is projected to increase. Ultimately, projected changes in precipitation patterns will affect nitrogen uptake and precipitation-optimal fertilization timings. We argue that the use of bespoke fertilization timings in each year can help recuperate the reduced N uptake due to changing precipitation.

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.

Greenhouse Gases Trade-Off from Ponds: An Overview of Emission Process and Their Driving Factors

Inland water bodies (particularly ponds) emit a significant amount of greenhouse gases (GHGs), particularly methane (CH4), carbon dioxide (CO2), and a comparatively low amount of nitrous oxide (N2O) to the atmosphere. In recent decades, ponds (<10,000 m2) probably account for about 1/3rd of the global lake perimeter and are considered a hotspot of GHG emissions. High nutrients and waterlogged conditions provide an ideal environment for CH4 production and emission. The rate of emissions differs according to climatic regions and is influenced by several biotic and abiotic factors, such as temperature, nutrients (C, N, & P), pH, dissolved oxygen, sediments, water depth, etc. Moreover, micro and macro planktons play a significant role in CO2 and CH4 emissions from ponds systems. Generally, in freshwater bodies, the produced N2O diffuses in the water and is converted into N2 gas through different biological processes. There are several other factors and mechanisms which significantly affect the CH4 and CO2 emission rate from ponds and need a comprehensive evaluation. This study aims to develop a decisive understanding of GHG emissions mechanisms, processes, and methods of measurement from ponds. Key factors affecting the emissions rate will also be discussed. This review will be highly useful for the environmentalists, policymakers, and water resources planners and managers to take suitable mitigation measures in advance so that the climatic impact could be reduced in the future.

Effects of water level on nitrous oxide emissions from vegetated ditches

Nitrous oxide (N₂O) is considered a powerful greenhouse gas. Vegetated ditches are an important source of N₂O emissions in the agricultural systems. However, few studies have examined on the relationship between N₂O emissions and the water level in vegetated ditches. To investigate the effect of water level on the N₂O emissions, three pilot-scale ditches vegetated with Myriophyllum aquaticum were constructed with low (LW), medium (MW), and high (HW) water levels. The examined results indicated that the M. aquaticum ditches decreased N₂O emissions by 38.4% and 67.9% in MW and HW, respectively, as compared to the LW ditch. In addition, the N₂O emission factor decreased with increasing water level in the order of: LW (0.18%) > MW (0.11%) > HW (0.06%). The MW and HW ditches reduced the N₂O emissions by controlling the sediment nitrogen contents, in which the ammonia nitrogen increased with increasing the level of water, while nitrate nitrogen decreased with increasing the level of water. The increase in the level of water significantly reduced the gene abundance of ammonia-oxidizing archaea (AOA) (p < 0.05), thereby reducing the N₂O emissions in the MW and HW conditions due to the positive correlation between N₂O emissions and AOA gene abundances. The unclassified_k_norank_d_Bacteria was the dominant denitrifying bacterial genus observed in the M. aquaticum ditches, and its highly relative abundance yielded low N₂O emissions in the HW ditch. These findings indicate that reducing N₂O emissions may be achieved by controlling the water level in vegetated ditches.

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

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

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.

Characteristics of annual N2O and NO fluxes from Chinese urban turfgrasses

Urban turfgrass ecosystems are expected to increase at unprecedented rates in upcoming decades, due to the increasing population density and urban sprawl worldwide. However, so far urban turfgrasses are among the least understood of all terrestrial ecosystems concerning their impact on biogeochemical N cycling and associated nitrous oxide (N₂O) and nitric oxide (NO) fluxes. In this study, we aimed to characterize and quantify annual N₂O and NO fluxes from urban turfgrasses dominated by either C4, warm-season species or C3, cool-season and shade-enduring species, based on year-round field measurements in Beijing, China. Our results showed that soil N₂O and NO fluxes varied substantially within the studied year, characterizing by higher emissions during the growing season and lower fluxes during the non-growing season. The regression model fitted by soil temperature and soil water content explained approximately 50%-70% and 31%-38% of the variance in N₂O and NO fluxes, respectively. Annual cumulative emissions for all urban turfgrasses ranged from 0.75 to 1.27 kg N ha^-1 yr^-1 for N₂O and from 0.30 to 0.46 kg N ha^-1 yr^-1 for NO, both are generally higher than those of Chinese natural grasslands. Non-growing season fluxes contributed 17%-37% and 23%-30% to the annual budgets of N₂O and NO, respectively. Our results also showed that compared to the cool-season turfgrass, annual N₂O and NO emissions were greatly reduced by the warm-season turfgrass, with the high root system limiting the availability of inorganic N substrates to soil microbial processes of nitrification and denitrification. This study indicates the importance of enhanced N retention of urban turfgrasses through the management of effective species for alleviating the potential environmental impacts of these rapidly expanding ecosystems.

Coastal reservoirs as a source of nitrous oxide: Spatio-temporal patterns and assessment strategy

Coastal reservoirs are widely regarded as a viable solution to the water scarcity problem faced by coastal cities with growing populations. As a result of the accumulation of anthropogenic wastes and the alteration of hydroecological processes, these reservoirs may also become the emission hotspots of nitrous oxide (N₂O). Hitherto, accurate global assessment of N₂O emission suffers from the scarcity and low spatio-temporal resolution of field data, especially from small coastal reservoirs with high spatial heterogeneity and multiple water sources. In this study, we measured the surface water N₂O concentrations and emissions at a high spatial resolution across three seasons in a subtropical coastal reservoir in southeastern China, which was hydrochemically highly heterogeneous because of the combined influence of river runoff, aquacultural discharge, industrial discharge and municipal sewage. Both N₂O concentration and emission exhibited strong spatio-temporal variations, which were correlated with nitrogen loading from the river and wastewater discharge. The mean N₂O concentration and emission were found to be significantly higher in the summer than in spring and autumn. The results of redundancy analysis showed that NH₄⁺-N explained the greatest variance in N₂O emission, which implied that nitrification was the main microbial pathway for N₂O production in spite of the potentially increasing importance of denitrification of NO₃^--N in the summer. The mean N₂O emission across the whole reservoir was 107 μg m^-2 h^-1, which was more than an order of magnitude higher than that from global lakes and reservoirs. Based on our results of Monte Carlo simulations, a minimum of 15 sampling points per km² would be needed to produce representative and reliable N₂O estimates in such a spatially heterogeneous aquatic system. Overall, coastal reservoirs could play an increasingly important role in future climate change via their N₂O emission to the atmosphere as water demand and anthropogenic pressure continue to rise.

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

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

Is rice field a nitrogen source or sink for the environment?

Rice field has been traditionally considered as a nonpoint source of reactive nitrogen (N) for the environment, while it, with surrounding ditches and ponds, also contributes to receiving N inputs from atmosphere and waterbodies and intercepting N outputs from rice field. However, a comprehensive assessment of the N source or sink of rice field for the environment is lacking. Here, we conducted the 3-year systematic observations and process-based simulations of N budget at the Jingzhou site in Central China. We identified the roles of rice field and evaluated the opportunities for shifting its role from N source (i.e., outputs > inputs) to sink (i.e., outputs ≤ inputs). Rice field was found to be a N source of 24.2-28.7 kg N ha^-1 for waterbodies (including surface and ground waters), but a N sink (2.2-8.8 kg N ha^-1) for the atmosphere for the wet and normal year climatic scenarios. The "4R-nutrient stewardship" (i.e., using the right type of N fertilizers, at right rate, right time, and in right place) applied in rice field was sufficient for the source-to-sink shift for the atmosphere for dry year climatic scenario, but needed to implement together with improvements of irrigation and drainage for waterbodies. Furthermore, with the combination of these improved management technologies, rice field played a role as a N sink of up to 22.8 kg N ha^-1 for the atmosphere and up to 2.0 kg N ha^-1 for waterbodies, along with 24% decrease in irrigation water use and 21% decrease in N application rate without affecting rice yield and soil fertility. Together these findings highlight a possibility to achieve an environmental-friendly rice field by improving agricultural management technologies.

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

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

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.

A highly agricultural river network in Jurong Reservoir watershed as significant CO2 and CH4 sources

Freshwaters are receiving growing concerns on atmospheric carbon dioxide (CO₂) and methane (CH₄) budget; however, little is known about the anthropogenic sources of CO₂ and CH₄ from river network in agricultural-dominated watersheds. Here, we chose such a typical watershed and measured surface dissolved CO₂ and CH₄ concentrations over 2 years (2015-2017) in Jurong Reservoir watershed for different freshwater types (river network, ponds, reservoir, and ditches), which located in Eastern China and were impacted by agriculture with high fertilizer N application. Results showed that significantly higher gas concentrations occurred in river network (CO₂: 112 ± 36 μmol L^-1; CH₄: 509 ± 341 nmol L^-1) with high nutrient concentrations. Dissolved CO₂ and CH₄ concentrations were supersaturated in all of the freshwater types with peak saturation ratios generally occurring in river network. Temporal variations in the gas saturations were positively correlated with water temperature. The saturations of CO₂ and CH₄ were positively correlated with each other in river network, and both of these saturations were also positively correlated with nutrient loadings, and negatively correlated with dissolved oxygen concentration. The highly agricultural river network acted as significant CO₂ and CH₄ sources with estimated emission fluxes of 409 ± 369 mmol m^-2 d^-1 for CO₂ and 1.6 ± 1.2 mmol m^-2 d^-1 for CH₄, and made a disproportionately large, relative to the area, contribution to the total aquatic carbon emission of the watershed. Our results suggested the aquatic carbon emissions accounted for 6% of the watershed carbon budget, and fertilizer N and watersheds land use played a large role in the aquatic carbon emission.

Large Spatial Variations in Diffusive CH4 Fluxes from a Subtropical Coastal Reservoir Affected by Sewage Discharge in Southeast China

Coastal reservoirs are potentially CH₄ emission hotspots owing to their biogeochemical role as the sinks of anthropogenic carbon and nutrients. Yet, the fine-scale spatial variations in CH₄ concentrations and fluxes in coastal reservoirs remain poorly understood, hampering an accurate determination of reservoir CH₄ budgets. In this study, we examined the spatial variability of diffusive CH₄ fluxes and their drivers at a subtropical coastal reservoir in southeast China using high spatial resolution measurements of dissolved CH₄ concentrations and physicochemical properties of the surface water. Overall, this reservoir acted as a consistent source of atmospheric CH₄, with a mean diffusive flux of 16.1 μmol m^-2 h^-1. The diffusive CH₄ flux at the reservoir demonstrated considerable spatial variations, with the coefficients of variation ranging between 199 and 426% over the three seasons. The shallow water zone (comprising 23% of the reservoir area) had a disproportionately high contribution (56%) to the whole-reservoir diffusive CH₄ emissions. Moreover, the mean CH₄ flux in the sewage-affected sectors was significantly higher than that in the nonsewage-affected sectors. The results of bootstrap analysis further showed that increasing the sample size from 10 to 100 significantly reduced the relative standard deviation of mean diffusive CH₄ flux from 73.7 to 3.4%. Our findings highlighted the role of sewage in governing the spatial variations in reservoir CH₄ emissions and the importance of high spatial resolution data to improve the reliability of flux estimates for assessing the contribution of reservoirs to the regional and global CH₄ budgets.

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.

Environmental investments decreased partial pressure of CO2 in a small eutrophic urban lake: Evidence from long-term measurements

Inland waters emit large amounts of carbon dioxide (CO₂) to the atmosphere, but emissions from urban lakes are poorly understood. This study investigated seasonal and interannual variations in the partial pressure of CO₂ (pCO₂) and CO₂ flux from Lake Wuli, a small eutrophic urban lake in the heart of the Yangtze River Delta, China, based on a long-term (2000-2015) dataset. The results showed that the annual mean pCO₂ was 1030 ± 281 μatm (mean ± standard deviation) with a mean CO₂ flux of 1.1 ± 0.6 g m^-2 d^-1 during 2000-2015, suggesting that compared with other lakes globally, Lake Wuli was a significant source of atmospheric CO₂. Substantial interannual variability was observed, and the annual pCO₂ exhibited a decreasing trend due to improvements in water quality driven by environmental investment. Changes in ammonia nitrogen and total phosphorus concentrations together explained 90% of the observed interannual variability in pCO₂ (R² = 0.90, p < 0.01). The lake was dominated by cyanobacterial blooms and showed nonseasonal variation in pCO₂. This finding was different from those of other eutrophic lakes with seasonal variation in pCO₂, mostly because the uptake of CO₂ by algal-derived primary production was counterbalanced by the production of CO₂ by algal-derived organic carbon decomposition. Our results suggested that anthropogenic activities strongly affect lake CO₂ dynamics and that environmental investments, such as ecological restoration and reducing nutrient discharge, can significantly reduce CO₂ emissions from inland lakes. This study provides valuable information on the reduction in carbon emissions from artificially controlled eutrophic lakes and an assessment of the impact of inland water on the global carbon cycle.