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Greenhouse gases (GHGs) emitted or absorbed by lakes are an important component of the global carbon cycle. However, few studies have focused on the GHG dynamics of eutrophic saline lakes, thus preventing a comprehensive understanding of the carbon cycle. Here, we conducted four sampling analyses using a floating chamber in Daihai Lake, a eutrophication saline lake in Inner Mongolia Autonomous Region, China, to explore its carbon dioxide (CO 2 ) and methane (CH 4 ) emissions. The mean CO 2 emission flux (FCO 2 ) and CH 4 emission flux (FCH 4 ) were 17.54 ± 14.54 mmol/m 2 /day and 0.50 ± 0.50 mmol/m 2 /day, respectively. The results indicated that Daihai Lake was a source of CO 2 and CH 4 , and GHG emissions exhibited temporal variability. The mean CO 2 partial pressure (pCO 2 ) and CH 4 partial pressure (pCH 4 ) were 561.35 ± 109.59 µatm and 17.02 ± 13.45 µatm, which were supersaturated relative to the atmosphere. The regression and correlation analysis showed that the main influencing factors of pCO 2 were wind speed, dissolved oxygen (DO), total nitrogen (TN) and Chlorophyll a (Chl.a), whereas the main influencing factors of pCH 4 were water temperature (WT), Chl.a, nitrate nitrogen (NO 3 - -N), TN, dissolved organic carbon (DOC) and water depth. Salinity regulated carbon mineralization and organic matter decomposition, and it was an important influencing factor of pCO 2 and pCH 4 . Additionally, the trophic level index (TLI) significantly increased pCH 4 . Our study elucidated that salinity and eutrophication play an important role in the dynamic changes of GHG emissions. However, research on eutrophic saline lakes needs to be strengthened., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2022. Published by Elsevier B.V.)

Although saline aquatic ecosystems are significant emitters of greenhouse gases (GHGs), dynamic changes in GHGs at the sediment-water interface remain unclear. The present investigation carried out a total of four sampling campaigns in Daihai Lake, which is a eutrophic saline lake situated in a semi-arid area of northern China. The aim of this study was to investigate the spatio-temporal dynamics of carbon dioxide (CO 2 ) and methane (CH 4 ) fluxes at the sediment-water interface and the influencing factors. The mean concentrations of porewater CO 2 and CH 4 were 44.98 ± 117.99 μmol L -1 and 124.36 ± 97.00 μmol L -1 , far exceeding those in water column of 11.14 ± 2.16 μmol L -1 and 0.33 ± 0.23 μmol L -1 , respectively. The CO 2 and CH 4 fluxes at the sediment-water interface (F S - W CO 2 and F S-W CH 4 ) exhibited significant spatial and temporal variations, with mean values of 9.24 ± 13.84 μmol m -2  d -1 and 3.53 ± 4.36 μmol m -2  d -1 , respectively, indicating that sediment is the source of CO 2 and CH 4 in the water column. However, CO 2 and CH 4 fluxes were much lower than those measured at the water-air interface in a companion study (17.54 ± 14.54 mmol m -2 d -1 and 0.50 ± 0.50 mmol m -2 d -1 , respectively), indicating that the diffusive flux of gases at the sediment-water interface was not the primary source of CO 2 and CH 4 emissions to the atmosphere. Regression and correlation analyses revealed that salinity (Sal) and nutrients were the most influential factors on porewater gas concentrations, and that gas fluxes increased with increasing gas concentrations and porosity. The microbial activity of sediment is greatly affected by nutrients and Sal. Additionally, Sal has the ability to regulate biogeochemical processes, thereby regulating GHG emissions. The present investigation addresses the research gap concerning GHG emissions from sediments of eutrophic saline lakes. The study suggests that controlling the eutrophication and salinization of lakes could be a viable strategy for reducing carbon emissions from lakes. However, further investigations are required to establish more conclusive results., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2023. Published by Elsevier Ltd.)

Eutrophic lakes, especially shallow eutrophic lakes, disproportionately contribute to greenhouse gas (GHG) emissions. To investigate the effects of eutrophication on GHG dynamics, we conducted field measurements every three months from January 2019 to October 2019 in Lake Ulansuhai, a shallow eutrophic lake (mean depth of 0.7 m) located in a semi-arid region in Northern China. We found that Lake Ulansuhai was a predominantly source of atmospheric carbon dioxide (CO 2 ); however, it converted to a CO 2 sink in July due to eutrophication. It was also a strong source of methane (CH 4 ) with a mean CO 2 emission of 35.7 ± 12.1 mmol m -2 d -1 and CH 4 emission of 5.9 ± 2.9 mmol m -2 d -1 . The CO 2 concentrations in most sites and CH 4 concentrations in all sites were supersaturated, with the average partial pressure of CO 2 (pCO 2 ) being 654±34 μatm and the partial pressure of CH 4 (pCH 4 ) being 157±37 μatm. The partial pressures and emissions of the greenhouse gases exhibited substantial seasonal and spatial variations. The correlation analysis between the trophic level index and the partial pressure of the greenhouse gases indicated that eutrophication could significantly decrease the CO 2 emissions but increase the CH 4 emissions from the lake, resulting in a CH 4 and CO 2 emission ratio of approximately 2 in terms of global warming potential. Eutrophication decreased the pCO 2 in the lake and subsequently increased the pCH 4 due to nutrient input, thereby enhancing primary production. The results indicated that shallow eutrophic lakes in arid regions are strong sources of CH 4 and that eutrophication could alter the greenhouse gas emission patterns., (Copyright © 2021 Elsevier Ltd. All rights reserved.)

Gradual riparian wetland drying is increasingly sensitive to global warming and contributes to climate change. Riparian wetlands play a significant role in regulating carbon and nitrogen cycles. In this study, we analyzed the emissions of carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) from riparian wetlands in the Xilin River basin to understand the role of these ecosystems in greenhouse gas (GHG) emissions. Moreover, the impact of the catchment hydrology and soil property variations on GHG emissions over time and space was evaluated. Our results demonstrate that riparian wetlands emit larger amounts of CO2 (335–2790 mgm-2h-1 in the wet season and 72–387 mgm-2h-1 in the dry season) than CH4 and N2O to the atmosphere due to high plant and soil respiration. The results also reveal clear seasonal variations and spatial patterns along the transects in the longitudinal direction. N2O emissions showed a spatiotemporal pattern similar to that of CO2 emissions. Near-stream sites were the only sources of CH4 emissions, while the other sites served as sinks for these emissions. Soil moisture content and soil temperature were the essential factors controlling GHG emissions, and abundant aboveground biomass promoted the CO2 , CH4 , and N2O emissions. Moreover, compared to different types of grasslands, riparian wetlands were the potential hotspots of GHG emissions in the Inner Mongolian region. Degradation of downstream wetlands has reduced the soil carbon pool by approximately 60 %, decreased CO2 emissions by approximately 35 %, and converted the wetland from a CH4 and N2O source to a sink. Our study showed that anthropogenic activities have extensively changed the hydrological characteristics of the riparian wetlands and might accelerate carbon loss, which could further affect GHG emissions. [ABSTRACT FROM AUTHOR]

Despite growing research on greenhouse gas (GHG) emissions from inland waters, few systematic efforts have been made to assess the regional‐scale GHG emissions from Asian rivers under increasing anthropogenic stress. We examined factors controlling longitudinal and seasonal variations in the partial pressure of CO2 (pCO2), and CH4 and N2O concentrations in the Ganges, Mekong, and Yellow River by simultaneously measuring gas concentrations and stable C isotopes, and optical properties of dissolved organic matter (DOM) from 2016 to 2019. The levels of pCO2 and CH4 were distinctively higher in polluted tributaries and affected reaches of the Ganges and Mekong than in the Yellow River. The highest levels of N2O were found in the Ganges, followed by the Yellow River and Mekong. Across these basins, dry‐season mean concentrations of CO2, CH4, and N2O were 1.6, 2, and 7 times higher than those measured in the monsoon season, respectively. This seasonality was consistent with that of δ13C‐CO2, while δ13C‐CH4 showed an opposite pattern. GHG concentrations exhibited significant positive relationships with DOM concentrations and optical properties including fluorescence index and protein‐like fluorescence, implying the contribution of anthropogenic, labile DOM to production of GHGs in the polluted reaches. Graphical mixing models of δ13C‐CO2 and δ13C‐CH4 support the stronger impact of wastewater on the Ganges and Mekong than on the Yellow River. The overall results suggest that neglecting localized pollution impacts on GHG emissions from increasingly urbanized river basins can result in a substantial underestimation of global riverine GHG emissions. Plain Language Summary: Rapid urbanization and poor wastewater infrastructure are turning many large rivers in Asia into significant sources of greenhouse gases (GHGs). We examined controlling factors for the concentrations of CO2, CH4, and N2O in the Ganges, Mekong, and Yellow River by simultaneously measuring gas concentrations and stable carbon isotopes, and optical properties of dissolved organic matter (DOM). The concentrations of CO2 and CH4 were distinctively higher in polluted tributaries and affected sections of the Ganges and Mekong than in the Yellow River. N2O concentration was highest in the Ganges, followed by the Yellow River and Mekong. Across these basins, the concentrations of the three gases were generally higher during dry periods than in the monsoon season. GHG concentrations were positively related to DOM optical properties indicating the contribution of easily degradable DOM from pollution sources. The stable carbon isotope ratios of CO2 and CH4 support the stronger impact of wastewater on the Ganges and Mekong than on the Yellow River. The overall results suggest that neglecting local pollution impacts on GHG emissions from increasingly urbanized river basins can result in a substantial underestimation of global riverine GHG emissions. Key Points: High CO2, CH4, and N2O concentrations in urbanized river reaches reflect influences from local tributaries carrying wastewaterSeasonality in stable C isotopes and dissolved organic matter properties corresponds to higher dry‐season greenhouse gas concentrations than in the monsoonStable C isotopes and their relationships with CO2 and CH4 concentrations support localized wastewater effects on urbanized rivers [ABSTRACT FROM AUTHOR]

• Reservoir drawdown zone was a hotspot of GHG emissions. • CH 4 fluxes went to zero due to dam decommissioning. • Soil volumetric moisture content and soil bulk density were the major factors contributing to GHG emissions. • The desiccation of reservoir sediments can result in loss of C from reservoir. Reservoirs are a category of artificial ecosystem that play a key role in the global carbon cycle. However, there has been limited research on the impacts of dam decommissioning (DD) on greenhouse gas (GHG) emissions over a reservoir life cycle. This study analyzed the effects of DD on CO 2 and CH 4 fluxes emitted from impounded water zone (area of inundated sediments), the drawdown zone (area of sediment exposure during DD), and the margin zone (area of long-term sediment exposure) before, during, and after the drawdown of a retired reservoir in Inner Mongolia. The margin zone encompassed ± 90 % of the total reservoir area over the study period, whereas the impounded water and drawdown zones accounted for the remainder. CO 2 emissions decreased during DD process in the margin zone (325.24 ± 313.80–128.81 ± 128.78 mg m−2h−1) and impounded water zones (286.00 ± 242.00–0.00 ± 0.00 mg m−2h−1), whereas they increased in the drawdown zone (0.00 ± 0.00–239.52 ± 104.35 mg m−2h−1). CH 4 emissions gradually declined over time after DD in the impounded water zone and this zone was the main contributor of CH 4 fluxes before DD (57.36 ± 12.77 mg m−2h−1), and finally emissions went to zero. Exposure of sediments gradually resulted in a significant increase in CO 2 emissions. The major factors affecting GHG emissions were soil volumetric moisture content (SMC) and soil bulk density (ρb). The CO 2 -eq of the reservoir ecosystem decreased after DD. This happened despite CO 2 emissions increasing because CH 4 , which has a higher global warming potential, decreased following DD. The consideration of DD in the carbon footprint of reservoirs for understanding of reservoir carbon dynamics and the global carbon balance requires further study of the long-term effects of DD on carbon fluxes. [ABSTRACT FROM AUTHOR]

• The dynamics of CO 2 and CH 4 partial pressures are co-regulated by various nutrients. • CO 2 and CH 4 emission from lakes decreased and increased with longitude, respectively. • Freshwater lakes emit mainly CH 4 , while salt lakes are main contributor of CO 2. • NPP, NDVI and human activities enhance CH 4 emissions but reduce CO 2 emissions. Lakes are important component of the regional carbon cycle and a significant sources of greenhouse gases (GHGs) to the atmosphere. However, the regional variations in carbon dioxide (CO 2) and methane (CH 4) emissions, as well as the factors regulating these emissions are poorly constrained. To investigate the regional GHG emission from lakes and their potential influencing factors, we conducted field measurements in 13 lakes in Inner Mongolia from July to August 2020. We found that the studied lakes were predominantly sources of atmosphere CO 2 and CH 4. The CO 2 partial pressure (p CO 2) exhibited supersaturation in 80 % of samplings, but all samplings of CH 4 partial pressure (p CH 4) showed supersaturation. Moreover, the average p CH 4 is 40 times greater than that in the atmosphere. The lakes emitted substantial amounts CO 2 and CH 4 with average values of 25.8 ± 4.9 mmol m−2 d−1 and 2.1 ± 0.3 mmol m−2 d−1, respectively. At the regional scale, the dynamics of p CO 2 and p CH 4 are regulated by combination of various environmental factors. CO 2 emissions from studied lakes decreased with the catchment net primary productivity (NPP), normalized difference vegetation index (NDVI), and nighttime lights which is indicative of urbanization. Conversely, CH 4 fluxes increased with NPP, NDVI, and nighttime lights. We conclude that terrestrial carbon-cycling related variables and human activities significantly affect CO 2 and CH 4 emissions from lakes by transporting different nutrients, but they show opposite trends. Our results also exhibited that the freshwater lakes are the main contributors of CH 4 , while saline lakes are the main contributors of CO 2. We suggest that increased human activity leads to changes in terrestrial carbon-cycle related processes which affects lake carbon emissions by altering the amount of nutrients input to the lakes. [ABSTRACT FROM AUTHOR]

Shallow eutrophic lakes with submerged aquatic vegetation (SAV) are a large source of atmospheric methane (CH 4) emissions. However, current estimates of CH 4 emissions from lakes are uncertain owing to the lack of data on the diel and monthly variability of CH 4 emissions. In this study, we conducted monthly diel measurements in Ulansuhai Lake, China during the ice-free period (April to October, 2019) to determine the diel and monthly variability of CH 4 emissions. The diffusive CH 4 emission flux (FCH 4d) from June to September was significantly higher (∼5- to 10-fold) than that from April, May, and October, accounting for > 92% of the total emissions. Daytime measurements would overestimate emissions if extrapolated to the whole day because FCH 4d during daytime was higher than that during nighttime. Mean daily FCH 4d emission estimates are overestimated by 25%, ranging from 11% in summer to 46% in spring based solely on daytime measurement. The sampling sites were net sources of CH 4 emissions, with total FCH 4 of 9.74 ± 9.06 mmol m−2 d−1. FCH 4d increased sharply when the water temperature was above 20 °C. FCH 4 (CO 2 eq) accounted for over 90% of the total greenhouse gas emissions (CH 4 plus CO 2), with the majority occurring from June to September. Our findings indicate that diel and monthly variations should be considered for more accurate estimation of CH 4 emissions from the lakes with SAV. [ABSTRACT FROM AUTHOR]

• CO 2 and CH 4 concentrations in sediment enhanced by nutrients and temperature. • Sediment – water fluxes increased with nutrient availability from inflow to outflow. • CO 2 and CH 4 fluxes from the sediment share similar drivers. • Sediment – water fluxes were not the major pathway for atmospheric emissions. • Eutrophication and warming increase greenhouse gas fluxes from the sediment. Shallow eutrophic lakes contribute disproportional to the emissions of CO 2 and CH 4 from inland waters. The processes that contribute to these fluxes, their environmental controls, and anthropogenic influences, however, are poorly constrained. Here, we studied the spatial variability and seasonal dynamics of CO 2 and CH 4 fluxes across the sediment-water interface, and their relationships to porewater nutrient concentrations in Lake Ulansuhai, a shallow eutrophic lake located in a semi-arid region in Northern China. The mean concentrations of CO 2 and CH 4 in porewater were 877.8 ± 31.0 µmol L−1 and 689.2 ± 45.0 µmol L−1, which were more than 50 and 20 times higher than those in the water column, respectively. The sediment was always a source of both gases for the water column. Porewater CO 2 and CH 4 concentrations and diffusive fluxes across the sediment-water interface showed significant temporal and spatial variations with mean diffusive fluxes of 887.3 ±124.7 µmol m−2 d−1 and 607.1 ± 68.0 µmol m−2 d−1 for CO 2 and CH 4 , respectively. The temporal and spatial variations of CO 2 and CH 4 concentrations in porewater were associated with corresponding variations in dissolved organic carbon and dissolved nitrogen species. Temperature and dissolved organic carbon in surface porewater were the most important drivers of temporal variations in diffusive fluxes, whereas dissolved organic carbon and nitrogen were the main drivers of their spatial variations. Diffusive fluxes generally increased with increasing dissolved organic carbon and nitrogen in the porewater from the inflow to the outflow region of the lake. The estimated fluxes of both gases at the sediment-water interface were one order of magnitude lower than the emissions at the water surface, which were measured in a companion study. This indicates that diffusive fluxes across the sediment-water interface were not the main pathway for CO 2 and CH 4 emissions to the atmosphere. To improve the mechanistic understanding and predictability of greenhouse gas emissions from shallow lakes, future studies should aim to close the apparent gap in the CO 2 and CH 4 budget by combining improved flux measurement techniques with process-based modeling. [Display omitted] [ABSTRACT FROM AUTHOR]

Groundwater discharge to river networks makes up a major source of riverine CO 2 emission, available evidence however comes mainly from headwater streams which are directly connected to terrestrial ecosystems and spatially limited in terms of system size. Here relying on coupled water and CO 2 mass balances, we quantified the groundwater-mediated CO 2 input to the Yangtze River mainstem on an annual basis, where the mass balance of water provided physical constraints on CO 2 exchange between the river and groundwater. A landscape topographic control of the groundwater-river interaction was proposed where mountain reaches preferentially receive water and CO 2 discharge from the groundwater while plain alluvial reaches predominantly lose water to the aquifers. Groundwater CO 2 inputs were however small in magnitude on all reaches (0.3–14% of the total CO 2 emission and transport by the river) and unable to account for the discrepancy between surface evasion and internal metabolism in the river. Minor direct groundwater discharge to the reaches in comparison to smaller streams (negative to < 3.5% of the surface water flows) was concluded to be the main reason for low groundwater-sourced CO 2 in the large river reaches. Unlabelled Image • Coupled water and CO 2 mass balances were conducted on a large river on an annual basis. • Groundwater CO 2 inputs are small in magnitude on all reaches and not able to explain all discrepancies. • Groundwater CO 2 inputs depends largely on landscape terrains the river reaches flow through. • River damming reverts groundwater and CO 2 exchanges in years of dam construction. [ABSTRACT FROM AUTHOR]