Floodplain methane (CH4) emissions represent a significant component of the global CH4 budget. However, their response to escalating extreme drought events remains poorly understood, mainly due to high temporal variability under alternating wet-dry conditions. To address this gap, we conducted two years of in-situ CH4 flux measurements using the chamber technique across alternating hydrological cycles (2022-2023) in the Poyang Lake floodplain, during which the region experienced a prolonged drought. Our results showed that CH4 emissions during non-flooding periods (1.82 f 1.36 mg CH4 m-2 h-1) (mean f standard deviation) were significantly higher than those during flooding periods (1.26 f 0.96 mg CH4 m-2 h-1). Notably, CH4 fluxes in the autumn growing period (2.04 f 1.43 mg CH4 m-2 h-1) were 35 % higher than in the spring (1.51 f 1.21 mg CH4 m-2 h-1) under drought conditions. Further analysis revealed that, apart from air temperature, CH4 fluxes were primarily regulated by vegetation during non-flooding periods and by fluctuating water levels and flooding duration that influence biogeochemical processes during flooding periods. The enhanced temperature sensitivity of CH4 emissions emerged as a key factor for the higher autumn emissions compared to spring, which is directly linked to the shortened flooding period in the Poyang Lake floodplain. These findings underscore the critical role of extreme drought in reshaping hydrological conditions and CH4 emissions in floodplain wetlands, with important implications for predicting wetland responses under future climate change scenarios.

Abstract The carbon budget of floodplain lakes is regulated by natural hydrological processes, yet the role of human interventions, including cascade river‐lake hydraulic projects, remains unclear. We evaluated the combined effects of the Three Gorges Dam (TGD) located on the Yangtze River and the proposed lake‐specific hydraulic projects (LHPs) on the carbon budgets of the respective Dongting and Poyang Lakes downstream of the TGD. As a result of TGD's operation, Dongting Lake saw a weakened carbon sink, with carbon dioxide (CO2) uptake reduced by 6% and methane (CH4) emissions increasing by 4%. The opposite is true for the downstream Poyang Lake, with CO2 uptake increasing by 8% and CH4 emissions decreasing by 1%. This divergence stems from the lake‐specific vegetation composition and TGD's regulation strategy. The Phragmites in Dongting Lake is sensitive to hydrological regulation during the pre‐flooding seasons, while the Carex in Poyang Lake is sensitive to hydrological regulation during the post‐flooding seasons. In contrast, the operation of LHPs will uniformly reduce the carbon sink due to elevated lake water levels, leading to a sharp decline in CO2 uptake in Poyang Lake (36.98 gC m−2 yr−1) and Dongting Lake (11.87 gC m−2 yr−1). A cascade operation of these projects will increase the global warming potential (GWP) by 0.88 Tg CO2eq yr−1. Our findings underscore the necessity of integrating ecological consequences into water resources management to reconcile water security with climate stability goals.

Floodplain methane (CH₄) emissions constitute a substantial component of the global CH₄ budget. Nevertheless, their response to the increasing frequency of extreme drought events remains insufficiently understood, largely owing to the pronounced variability associated with alternating wet–dry hydrological regimes. To address this research gap, we conducted two years of in-situ CH₄ flux measurements across alternating hydrological cycles (2022–2023) in the Poyang Lake floodplain wetland, a period during which the region experienced an extended drought. Our results reveal that CH₄ emissions during non-flooding periods (1.82 ± 1.36 mg CH₄ m⁻² h⁻¹, mean ± standard deviation) were significantly higher than those during flooding periods (1.26 ± 0.96 mg CH₄ m⁻² h⁻¹). Notably, under drought conditions, CH₄ fluxes during the autumn growing season (2.04 ± 1.43 mg CH₄ m⁻² h⁻¹) were 35% greater than those observed in the spring growing season (1.51 ± 1.21 mg CH₄ m⁻² h⁻¹). Further analysis indicates that, apart from air temperature, CH₄ fluxes were primarily regulated by vegetation during non-flooding periods, whereas during flooding periods they were predominantly governed by water level fluctuations and inundation duration, factors that modulate key biogeochemical processes. The enhanced temperature sensitivity of CH₄ emissions emerged as a critical mechanism underlying the elevated autumn emissions relative to spring, a pattern directly attributable to the shortened flooding duration in the Poyang Lake floodplain. These findings highlight the pivotal role of extreme drought in altering hydrological regimes and CH₄ emission dynamics in floodplain wetlands, with important implications for predicting wetland responses under future climate change scenarios.

Abstract Mangroves are critical blue carbon ecosystems, essential for coastal preservation and carbon sequestration. Despite widespread attention to mangrove deforestation and reforestation driven by land-use/land-cover changes (LULCCs), integrated assessments of CO2 and CH4 fluxes across diverse transitions under the same spatiotemporal context remain scarce, hindering advances in restoration planning and land-use emission modeling. This study investigated five LULCC types on Qi’ao Island, China: mudflat, native Kandelia obovata, exotic Sonneratia apetala, S. apetala deforestation areas, and abandoned fish ponds. Combining one year of in situ carbon flux measurements with remote sensing data (1980–2020), we assessed changes in carbon sink dynamics and CH4 fluxes at both site and regional scales. At the site level, mudflats acted as weak carbon sinks (2.13 ± 1.70 t CO2 hm-2 a-1). By contrast, the exotic S. apetala exhibited a carbon sink 3-4 times stronger (60.55 ± 5.83 t CO2 hm-2 a-1) than the native K. obovata (16.79 ± 1.98 t CO2 hm-2 a-1). Abandoned fish ponds (7.73 ± 1.26 t CO2 hm–2 a–1) retained a moderate carbon sink due to abundant aquatic vegetation, whereas deforestation resulted in a carbon source (–1.09 ± 0.93 t CO2 hm–2 a–1). Notably, sites with higher CO2 uptake also emitted higher CH4 emissions, which indicated a climate mitigation trade-off. CH4 emissions offset ∼12% of the climate benefit from CO2 uptake. Regionally, LULCCs, driven largely by mangrove reforestation, shifted the region from a net carbon source to a net carbon sink around 1990. These findings highlight the impacts of LULCC in shaping carbon sinks, and they offer valuable insights for optimizing future restoration strategies with greater climate benefits.

Landscape patterns of land use serve as critical mediators of air pollution source convergence and directly shape the spatiotemporal distribution of population-weighted air pollutant concentration (PWP). However, the differential effects of anthropogenic (impervious surfaces) and natural (barren land) source landscapes on population-weighted exposure to multiple pollutants remain insufficiently quantified. Here, we calculated population-weighted exposure to PM2.5, PM10 and O3 for 337 prefecture-level cities in China from 2008 to 2023 and used random forest (RF) models with SHAP analysis to explain how six landscape metrics of impervious surfaces and barren land affect exposure. PM2.5 and PM10 exposure declined significantly over the period, whereas O3 exposure increased steadily, indicating worsening complex pollution. Nationally, impervious landscape indicators contributed more than barren indicators to all exposure metrics, with class area (CA) and patch proportion (PLAND) being the strongest predictors, while aggregation index (AI) and division index (DIVISION) had the weakest effects. Regionally, impervious surfaces dominated exposure in central and eastern China, whereas barren land dominated in western China. Nonlinear analysis revealed that pollution-source landscapes with large total area, high aggregation, low connectivity, and regular shape significantly increase exposure risk, with clear thresholds. These findings suggest that mitigating air pollution exposure also requires strategic landscape planning. Our study provides quantitative, landscape-based guidelines for integrating air quality management into land use planning.

Coastal salt marshes (CSMs) are vital blue carbon (BC) reservoirs, yet accurately quantifying their gross primary productivity (GPP) remains challenging due to limitations in terrestrial biosphere models (TBMs), which often overlook coastal-specific processes. Here, we present SAL-GPP, a process-based model that incorporates coastal-specific modules to capture the effects of salinity and temperature stress on photosynthesis, as well as light-use efficiency across salinity gradients in diverse CSM plant species. Model validation showed strong agreement with observations, with R2 of 0.82 and model efficiencies of 0.82 and 0.74 for daily and seasonal GPP, respectively. Driven with global inputs, SAL-GPP produced high-resolution global simulations, yielding a mean annual GPP of 66.89 ± 11.68 TgC yr-1 (2011-2020), with 64% concentrated in key hotspots across the southeastern United States, western Europe, southeastern China, and Australia. From 2011 to 2016, global CSM GPP increased by 1.56 TgC yr-1, then declined, rebounded after 2018, and peaked at 71.45 ± 12.02 TgC yr-1 in 2020. Model evaluation showed that SAL-GPP outperformed existing remote sensing-based GPP products and TBMs at both site and grid levels. By explicitly incorporating coastal ecosystem dynamics, SAL-GPP supports global BC accounting and climate mitigation strategies aligned with nature-based solutions for carbon neutrality.

Floodplains are important methane (CH4) sources, yet it remains a challenge to capture diel CH4 fluxes dynamics due to strong hydrological seasonalities as well as recently frequent droughts. Based on intensive diel observations (2022-2023) in the Poyang Lake, China's largest floodplain lake, we revealed the importance of hydrological seasonality to drive CH4 dynamics in wet and dry seasons, including an extreme drought event. During the wet season, the floodplain functioned as a net CH4 source with minor diel differences (daytime 1.40 and nighttime 1.38 mg CH4 m- 2 h- 1). While the dry season showed a significant diel asymmetry, with daytime emission (1.38 mg CH4 m- 2 h- 1) exceeding nighttime emission (0.38 mg CH4 m- 2 h- 1), due to large nocturnal CH4 uptake. The pattern was shaped by the combined effects of temperature, soil wetness, and net ecosystem CO2 exchange. However, the contrasts in CH4 emissions tend to diminish during extreme droughts, which weakened the nocturnal CH4 sink. If the seasonal influences on diel CH4 variability are not considered, the CH4 emission can be overestimated by up to 65% during dry season. Our results will inform researchers to include the new and overlooked mechanism into ecosystem models.

Macroalgae aquaculture has been increasingly recognized as a promising nature-based solution to enhance carbon sinks towards climate change mitigation. However, a limited understanding of the temporal patterns of air-sea carbon dioxide (CO2) fluxes and their environmental controls across time scales poses an enormous obstacle to the carbon sink potential assessment of macroalgae aquaculture. Here, we utilized the eddy covariance (EC) approach to acquire continuous and high-frequency measurements of net ecosystem exchange (NEE) of CO2 over the macroalgae aquaculture in a subtropical enclosed bay in southeast China, throughout one full year from April 2023 to March 2024. The results showed (a) this ecosystem acted as a CO2 source in most months with the strongest source and sink occurring at the beginning of autumn and winter, respectively; (b) annually this ecosystem emitted 58.9 g C m-2 of CO2 into the atmosphere with nighttime source contributing 84.7 %; (c) macroalgae aquaculture of Saccharina japonica and Gracilariopsis Lemaneiformis tended to reduce CO2 emission from this ecosystem, while the extent of the reduction varied with aquaculture types and growth stages; (d) temporal variability of NEE was most correlated with air temperature, while faster tidal currents tended to stimulate CO2 emission during both flood and ebb tides. The strong temporal variability of NEE highlights the importance of high-frequency EC measurements in improving the understanding of temporal patterns of air-sea CO2 fluxes over the macroalgae aquaculture ecosystems. This study suggests that macroalgae aquaculture has the potential to mitigate CO2 emission, although the ecosystem itself overall functions as a net CO2 source on an annual time scale.

Climate warming induces temporally varying atmospheric water vapor (WV), yet the spatial distribution of opposing trends across global land remains elusive. Here, we use the monthly European Centre for Medium-range Weather Forecasts Reanalysis v5 dataset to discern the responses of WV changes to the rising air temperature from 1982 to 2020. Simultaneous increases in both the WV and air temperature are observed over approximately three-quarters of global land, with a median of 0.21 mmK-1, particularly evident in the tropics. Strong positive responses are primarily influenced by increasing trends in evapotranspiration (ET) and low-elevation areas. About one-fifth of global land shows a decline in WV with a median of -0.62 mmK-1, predominantly in southeastern South America and southwestern North America. Negative responses are also driven by ET trends, where strong ET enhances these effects that are less pronounced in high-altitude regions. The prevalence of a positive response is highest during September-October-November (81%), while a negative response was observed most in December-January-February (35%). The spatial distribution of negative responses generally aligns with soil desiccation patterns; soil desiccation exacerbates negative responses in humid regions due to evaporative cooling but mitigates them in arid regions due to intensified warming. This study enhances our comprehension regarding the divergent responses of atmospheric WV toward global warming.

Carbon dynamics in floodplain lakes are critical to gaining a full understanding of the global carbon budget. Here, we constructed a spatially explicit carbon dioxide (CO2) flux data set covering 2003-2022 for China's largest floodplain lake (R2 = 0.86, RMSE = 0.49 gC m-2 d-1). The annual fluxes varied from 52.57 ± 4.71 gC m-2 in 2010 to -186.36 ± 7.27 gC m-2 in 2011. Temporal variations in CO2 flux were primarily driven by changes in the hydrological regime and wetland vegetation conditions. Specifically, water rise onset and recession onset emerged as the two most influential factors. A 10-day delay in lake water rise enhanced CO2 uptake by 19.20 gC m-2, whereas a 10-day advance in lake water recession increased uptake by 11.63 gC m-2. However, the enhancement of the CO2 sink can be impaired in the case of excessively early or rapid lake water level decline. For example, the extreme drought in 2022 reduced CO2 uptake by over 20% compared to moderate drought years due to plant water stress and increased ecosystem respiration. The findings offer insights into fully evaluating the ecological consequences of lake and water resource management from the perspective of carbon neutrality.

Carbon dynamics in floodplain lakes are critical to gaining a full understanding of the global carbon budget. Here, we constructed a spatially explicit carbon dioxide (CO2) flux data set covering 2003-2022 for China's largest floodplain lake (R2 = 0.86, RMSE = 0.49 gC m-2 d-1). The annual fluxes varied from 52.57 ± 4.71 gC m-2 in 2010 to -186.36 ± 7.27 gC m-2 in 2011. Temporal variations in CO2 flux were primarily driven by changes in the hydrological regime and wetland vegetation conditions. Specifically, water rise onset and recession onset emerged as the two most influential factors. A 10-day delay in lake water rise enhanced CO2 uptake by 19.20 gC m-2, whereas a 10-day advance in lake water recession increased uptake by 11.63 gC m-2. However, the enhancement of the CO2 sink can be impaired in the case of excessively early or rapid lake water level decline. For example, the extreme drought in 2022 reduced CO2 uptake by over 20% compared to moderate drought years due to plant water stress and increased ecosystem respiration. The findings offer insights into fully evaluating the ecological consequences of lake and water resource management from the perspective of carbon neutrality.

Cooling is an important ecosystem service provided by lakes, yet how its strength changes over time remains unclear, particularly for lakes sensitive to climate change. We assessed the cooling effects of China's largest freshwater lake from 1980 to 2023. On average, the lake can cool the basin land within 60 km, reducing the respective annual mean air temperature (Ta) and apparent temperature (AT) by 0.37 degrees C and 0.86 degrees C. The daily maximum temperatures can be lowered by up to 3 degrees C. A regime shift has been observed since 2003, with the lake's cooling effect weakening by 23 % for Ta and 29 % for AT, mainly due to shrinking water surfaces and reduced water storage. As a result, the capacity of heat storage and heat release has been impaired by 17% and 37%, respectively, weakening the lake's thermal buffering role. Extreme events exert divergent impacts: the cooling effect persisted during drought years, but a warming effect occurred during flood years due to cloud cover suppressing evaporation. Our findings highlight how hydrological changes affect lake cooling and emphasize the value of lakes in reducing extreme heat in a warming climate.

The climate benefit of blue carbon sequestered by mangrove forests can be partially offset by CH4 emission, but this offset is rarely assessed using multi‐year high‐frequency measurements. Here, four‐year eddy covariance measurements were used to examine temporal patterns of CH4 flux and its blue carbon offset (i.e., reduced climate benefit) in a subtropical estuarine mangrove in China. We found both diel and seasonal CH4 fluxes were mainly driven by soil temperature and tidal activities, showing greater nighttime emission. On average, one‐tenth of CO2 uptake was offset by CH4 emission using the sustained‐flux global warming potential metric at a 20‐year time horizon, while this offset could vary over an order of magnitude due to asynchronous fluxes of CH4 and CO2 across diel and seasonal cycles. These results highlight the significant contribution of nighttime emission to mangrove CH4 budget and the importance of asynchronous flux variations in assessing mangrove's climate benefit.

Compound drought-heatwaves (CDHWs) accelerate the warming and drying of soils, triggering soil compound drought-heatwaves (SCDHWs) that jeopardize the health of soil ecosystems. Nevertheless, the behavior of these events worldwide and their responses to climatic warming are underexplored. Here, we show a global escalation in the frequency, duration, peak intensity, and severity of SCDHWs, as well as an increase in affected land area, from 1980 to 2023. The increasing trends, which are particularly prominent since the early 2000 s, and projected to persist throughout this century, are dominated by summertime SCDHWs and enhanced by El Ni & ntilde;o. Intensive soil warming as well as climatologically lower soil temperatures compared to air temperatures lead to localized hotspots of escalating SCDHW severity in northern high latitudes, while prolonged duration causes such hotspots in northern South America. Transformation of natural ecosystems, particularly forests and wetlands, to cropland as well as forest degradation substantially enhance the strength of SCDHWs. Global SCDHWs consistently exhibit higher frequencies, longer durations, greater severities, and faster growth rates than CDHWs in all aspects from 1980 to 2023. They are undergoing a critical transition, with droughts replacing heatwaves as the primary constraint. We emphasize the significant intensification of SCDHWs in northern high latitudes as well as the prolonged duration of SCDHWs in the Southern Hemisphere, posing an underrated threat to achieving carbon neutrality and food security goals.

Wetland ecosystems play a pivotal role in terrestrial carbon and water cycles, thereby possessing great potential to regulate terrestrial water use efficiency (WUE), which is calculated as the ratio of gross primary productivity (GPP) to evapotranspiration (ET). However, it remains unclear the possible changes in wetland WUE under present and future climate conditions. In this research, WUE variations in a Phragmites australis-dominated freshwater wetland were determined by the eddy covariance method during 2020-2023. Further, we projected future GPP, ET, and WUE under four Representative Concentration Pathway (RCP) scenarios based on five Earth System Models. The 3-year average field observation suggested that the P. australis marsh exhibited high GPP (1149 g C m(-2)), but consumed large amounts of water through ET (611 mm H2O), resulting in relatively low WUE (1.89 g C mm(-1) H2O). During wet years, the studied marsh consumed much more water through evaporation than through transpiration, thus exhibiting lower WUE. Contrarily, large amounts of water were utilized to maintain high primary productivity through transpiration in 2021-2022 dry year, leading to higher WUE. In future scenarios, GPP in the P. australis marsh shows consistently faster uptrends compared to ET from 2020 to 2100, consequently yielding persistent growths of WUE. Future growths of WUE indicate that P. australis marsh tends consume more water for maintaining productivity levels rather than loss via evaporation under future climate conditions, thereby intensifying the carbon-water interaction. Driven by the most rapid growth of environmental drivers, the RCP8.5 scenario shows the fastest increasing trends in GPP, ET, and WUE among four RCP scenarios, whereas the reverse ocurres under RCP2.6 scenario. Overall, our study advocates for more comprehensive research encompassing field observation and model simulation to address the current knowledge gap regarding the response of wetland WUE to contemporary and future climate change.

Abstract Mangrove ecosystems are becoming increasingly important in global climate mitigation. However, large gaps still exist in evaluating mangroves' gross primary productivity (GPP) due to reasons such as the specific influences, for example, temperature and salt stresses are poorly described in Earth System Models (ESMs). This study developed a process‐based biogeochemical model (Mango‐GPP) to improve the GPP simulation in natural and restored mangroves. The model integrates mangrove‐specific physiological processes, including the response to salt and temperature stresses, as well as the light‐use efficiency at different growing stages. Eddy covariance flux measurements at two natural sites and one restored site in China were used to calibrate and validate Mango‐GPP. The model was calibrated by inverse analysis approach based on two cases and independently validated against the other cases. The validation results showed that it was generally capable of simulating the seasonal and interannual GPP variations at different sites. The simulated daily and annual GPPs agreed well with the observations and yielded R2 of 0.67 and 0.96, with model efficiency of 0.64 and 0.93, respectively. In comparison, Mango‐GPP showed better performances than many current satellite‐based GPP products and ESMs. The model was more sensitive to solar radiation, carbon dioxide concentration, and leaf traits. Future improvements should focus on enhancing Mango‐GPP's descriptive power of key processes, and further simulating other carbon fluxes at regional scales. This work provides a model foundation for further simulating carbon exchanges between the atmosphere, mangrove, and ocean for studying the coastal wetland restoration on regional carbon neutrality.

Abstract The effects of dams on carbon dioxide (CO2) fluxes in downstream lakes remain elusive. Here we combined eddy covariance observations and random forest models to examine multi‐decadal variations in CO2 fluxes in the Poyang Lake, the largest freshwater lake in China, and quantified the contribution of the Three Gorges Dam (TGD), the world's largest hydraulic project. We found the lake fluctuated between CO2 source and sink in 1961–2016, and tended to be CO2 sink in the post‐TGD period (2003–2016) when vegetation expanded early and spatially due to declining water level. TGD can explain approximately 6% of the total differences in annual CO2 fluxes, with major contributions in the impoundment period (up to 22% in middle September to October). The results show a positive side of operational major hydraulic projects on lake carbon sink, and probably caution the negative side of carbon release after dam removal.

Mangrove ecosystems can be both significant sources and sinks of greenhouse gases. The restoration of mangrove forests is increasingly used as a natural climate solution tool to mitigate climate change. However, the estimates of carbon exchanges remain unclear, especially from restored mangroves. In this study, we observed the temporal variations in carbon dioxide (CO2) and methane (CH4) fluxes and their biophysical controls for 4 years, based on a closed-path eddy covariance (EC) system. The measurements were conducted in a mangrove wetland park with 14-year-old restored mangroves surrounded by open waters in Guangdong Province, China. The EC measurements showed that the mangrove ecosystem acted as a CO2 source with a net CO2 ecosystem exchange (NEE) of 305 g C m−2 from January 2019 to May 2020 by the 5-m tower measurement. After the tower was adjusted to 10 m, the mangrove showed a CO2 sink with an NEE of −345 g C m−2 from June 2020 to December 2022. The change in tower height influenced the interpretation of interannual trends on NEE. There were no significant interannual trends in the gross primary productivity (GPP) and the ecosystem respiration (Re) values. The change from CO2 source to sink may be attributed to the decrease in land surface proportion by the tower replacement, which reduces the proportion of the mangrove canopy respiration and, therefore, captures lower CO2 fluxes from open waters. The restored mangroves indicated strong CH4 sources of 23.2–26.3 g C m−2 a−1. According to the random forest analysis, the land surface proportion, radiation, and relative humidity were the three most important predictors of NEE, while the CH4 flux was most sensitive to air temperature. Compared to the natural and long-term restored mangroves, this 14-year-old restored mangrove had not yet achieved a maximum carbon sequestration capability. Our study highlights the need for the careful design of long-term observations from restored mangroves and proposes future needs in the context of carbon neutrality.

The Soil Moisture Active Passive (SMAP) mission provides state-of-the-art global soil moisture (SM) datasets. However, seasonal SM biases and their contributing factors have not be systematically reviewed. This study evaluated the biases of SMAP V6 dual channel algorithm (DCA), single channel algorithm H-pol (SCA-H) and V-pol (SCA-V) SM products based on core validation sites data. All algorithms perform better under clear- than cloudy-sky, and in cloudless daytime than nighttime. Consecutive clear-sky benefits SM retrieval, progressively lowering the uncertainty of SM retrievals while at the cost of dry biases. Cloudy-sky deteriorates the quality of SM retrievals, and wet biases increase with the duration time of cloudy-sky. The modified V7 DCA has a major improvement, owning the potential to provide accurate retrievals under cloudy-sky. SMAP SM biases are co-determined by vegetation index, soil temperature and their biases, and a single factor only explains at most 54% variance in SM biases. Generally, SMAP SM bias is negatively correlated with soil temperature and positively correlated with its bias. SM bias correlates positively with vegetation index and its bias for single channel algorithms, and the underestimation of SM increases with vegetation density for DCA. To get a complete picture of SMAP SM biases, a total differential of radiative transfer equation is recommended for decomposing SMAP SM biases. To this end, more validation sites are required covering diverse land cover types and providing continuous data records.