Wetland Emissions Research Articles

Sulfate availability affect sulfate reduction pathways and methane consumption in freshwater wetland sediments

Methane (CH4) emissions from wetlands significantly contribute to global greenhouse gas fluxes, yet the mechanisms regulating CH4 production and oxidation in freshwater wetlands remain underexplored, particularly in the role of sulfate in the anaerobic oxidation of CH4. This study investigated the production, consumption, and release of CH4 in the sediments of the Qilihai wetland, focusing on the roles of hydrogenotrophic methanogenesis and sulfate dynamics across different seasons. CH4 concentrations ranged from 2.42 to 2290.52 μmol L⁻1, and δ13C–CH4 values became progressively more negative with depth, ranging from −87.37‰ to −57.18‰. Results indicate that hydrogenotrophic methanogenesis is the dominant pathway for CH4 production, particularly in sulfate-depleted environments, with CH4 concentrations in the sulfate-methane transition zone (SMTZ) strongly correlated with sulfate availability. Sulfate consumption through Sulfate Anaerobic Oxidation of Methane (SAOM) and Organoclastic Sulfate Reduction (OSR) demonstrated significant seasonal variation, with OSR accounting for up to 73% of sulfate consumption in the SMTZ. The SMTZ exhibited variations ranging from 2 to 40 cm in October, narrowing to 2–4 cm in July. These findings emphasize the complex interactions between sulfate availability, methanogenic pathways, and CH4 emissions in freshwater wetlands, highlighting the need for further research on sulfate dynamics and their implications for greenhouse gas emissions in the context of global climate change.

Impacts of plastic pollution on soil–plant properties and greenhouse gas emissions in wetlands: A meta-analysis

Plastic pollution in wetlands has recently emerged as an urgent environmental problem. However, the impacts of plastic contamination on soil-plant properties and greenhouse gas (GHG) emissions in wetlands remain unclear. Thus, this study conducted a meta-analysis based on 44 study sites to explore the influence of plastic pollution on soil physicochemical variables, soil microorganisms, enzyme activity, functional genes, plant characteristics, and GHG emissions (CO2, CH4, and N2O) in different wetland types. Based on the collected dataset, the plastic pollution significantly increased soil organic matter and organic carbon by on average 28.9 % and 34.2 %, respectively, while decreased inorganic nutrient elements, bacteria alpha diversity and enzyme activities by an average of 5.9 −14.2 %. The response of bacterial abundance to plastic pollution varied depending on phylum classes. Plant biomass and photosynthetic efficiency were decreased by an average of 12.8 % and 18.4 % due to plastic pollution. The concentration and exposure time of plastics play a key role in influencing the soil and plant properties in wetlands. Furthermore, plastic exposure notably increased the abundance of the functional genes related to C degradation and the ammonia oxidizing microorganisms, and the consequent CO2 and N2O emissions (with effect sizes of 2.10 and 1.94, respectively). We also found that plastic concentrations and exposure duration affected the wetland soil–plant system. Our results might be helpful to design further investigations on plastic effects and develop appropriate measures for mitigating plastic pollution in wetlands.

Assessment of the impact of observations at Nainital (India) and Comilla (Bangladesh) on the CH4 flux inversion

Methane emission in South Asia is poorly understood due to a lack of observations, despite being a major contributor to methane emissions globally. We present the first results of atmospheric CH4 inversions using air samples collected weekly at Nainital, India (NTL), and Comilla, Bangladesh (CLA), in addition to surface background flask measurements by NOAA, CSIRO and AGAGE using the MIROC4-ACTM. Our simulations span from 2000 to 2020 (considering the fixed “edge” effect), but the main analysis period is 2013–2020, when both the NTL and CLA datasets are available. An additional flux uncertainty reduction of up to 40% was obtained (mainly in the northern part of the Indian subcontinent), which enhanced our confidence in flux estimation and reaffirmed the significance of observations at the NTL and CLA sites. Our estimated regional flux was 64.0 ± 4.7 Tg-CH4 yr−1 in South Asia for the period 2013–2020. We considered two combinations of a priori fluxes that represented different approaches for CH4 emission from rice fields and wetlands. By the inversion, the difference in emissions between these combinations was notably reduced due to the adjustment of the CH4 emission from the agriculture, oil and gas, and waste sectors. At the same time, the discrepancy in wetland emissions, approximately 8 Tg-CH4 yr−1, remained unchanged. In addition to adjusting the annual totals, the inclusion of NTL/CLA observations in the inversion analysis modified the seasonal cycle of total fluxes, possibly due to the agricultural sector. While the a priori fluxes consisted of a single peak in August, the a posteriori values indicated double peaks in May and September. These peaks are highly likely associated with field preparation for summer crops and emissions from rice fields during the heading stage (panicle formation). The newly incorporated sites primarily exhibit sensitivity to the Indo-Gangetic Plain subregion, while coverage in southern India remains limited. Expanding the observation network is necessary, with careful analysis of potential locations using back-trajectory methods for footprint evaluation.

Interpreting the Seasonality of Atmospheric Methane

AbstractSurface and satellite observations of atmospheric methane show smooth seasonal behavior in the Southern Hemisphere driven by loss from the hydroxyl (OH) radical. However, observations in the Northern Hemisphere show a sharp mid‐summer increase that is asymmetric with the Southern Hemisphere and not captured by the default configuration of the GEOS‐Chem chemical transport model. Using an ensemble of 22 OH model estimates and 24 wetland emission inventories in GEOS‐Chem, we show that the magnitude, latitudinal distribution, and seasonality of Northern Hemisphere wetland emissions are critical for reproducing the observed seasonality of methane in that hemisphere, with the interhemispheric OH ratio playing a lesser role. Reproducing the observed seasonality requires a wetland emission inventory with ∼80 Tg a−1 poleward of 10°N including significant emissions in South Asia, and an August peak in boreal emissions persisting into autumn. In our 24‐member wetland emission ensemble, only the LPJ‐wsl MERRA‐2 inventory has these attributes.

Micro/nanoplastic pollution heterogeneously increased greenhouse gas emissions from wetlands: A multilevel meta-analysis

The ubiquitous presence of micro/nanoplastics (MNPs) in the environment poses high potential risks to living organisms and ecosystems. Wetlands are important sinks for MNPs, which can impact the ecological and environmental functions of wetland systems. However, the responses of greenhouse gas (GHG) emissions from wetlands to MNPs have rarely been evaluated. A multilevel meta-analysis was conducted to assess the effect of MNPs on GHG (CO2, CH4, and N2O) emissions from wetlands. The results showed that the presence of MNPs generally led to increases in GHG emissions from wetlands, especially N2O emissions. MNP type and exposure dose were the decisive factors influencing wetland GHG emissions, with CO2 and N2O emissions stimulated by biodegradable MNPs made of polylactic acid and by a high MNP dose of 1–2%, while polystyrene reduced CO2 and N2O emissions, and a low MNP dose of less than 0.5% increased CH4 emissions. CO2 emissions from the sediment of coastal wetlands were more sensitive to MNP pollution, while CH4 emissions from the sediment of freshwater wetlands were significantly increased by MNP pollution. Furthermore, microbial diversity and the number of carbon–nitrogen cycling functional genes associated with GHG emissions in wetlands were increased by MNPs. More research on the effect of MNPs on GHG emissions from wetlands over a longer time is needed, especially in real wetland ecosystems with aquatic organisms.

Unraveling the dynamics of atmospheric methane: the impact of anthropogenic and natural emissions

The reduction in methane concentration is crucial for achieving the goals of the Paris agreement. However, its annual growth rate is unstable, and understanding the reasons for changes in methane growth is essential for climate policy-making. Currently, there is considerable uncertainty regarding its attribution. Here, we utilize multi-source data and optimal fingerprinting methods to detect the contributions of several key drivers to the methane trend and interannual variability. We find that the methane growth trend is primarily influenced by anthropogenic emissions, while interannual variability is predominantly determined by wetland and biomass burning emissions. This result underscores the central role of anthropogenic emissions in methane dynamics, providing confidence in the effectiveness of human efforts to control methane atmospheric concentrations through emission reductions. It also helps alleviate concerns about the recent surge in atmospheric methane concentration, as it may be a short-term peak caused by increased wetland emissions rather than a long-term change.

Soil respiration and its response to climate change and anthropogenic factors in a karst plateau wetland, southwest China.

It is important to investigate the responses of greenhouse gases to climate change (temperature, precipitation) and anthropogenic factors in plateau wetland. Based on the DNDC model, we used meteorological, soil, and land cover data to simulate the soil CO2 emission pattern and its responses to climate change and anthropogenic factors in Guizhou, China. The results showed that the mean soil CO2 emission flux in the Caohai Karst Plateau Wetland was 5.89 ± 0.17 t·C·ha-1·yr-1 from 2000 to 2019, and the annual variation showed an increasing trend with the rate of 23.02 kg·C·ha-1·yr-1. The soil total annual mean CO2 emissions were 70.62 ± 2.04 Gg·C·yr-1 (annual growth rate was 0.28 Gg·C·yr-1). Caohai wetland has great spatial heterogeneity. The emissions around Caohai Lake were high (the areas with high, middle, and low values accounted for 3.07%, 70.96%, and 25.97%, respectively), and the emission pattern was characterized by a decrease in radiation from Caohai Lake to the periphery. In addition, the cropland and forest areas exhibited high intensities (7.21 ± 0.15 t·C·ha-1·yr-1 and 6.73 ± 0.58 t·C·ha-1·yr-1, respectively) and high total emissions (54.97 ± 1.16 Gg·C·yr-1 and 10.24 ± 0.88 Gg·C·yr-1, respectively). Croplands and forests were the major land cover types controlling soil CO2 emissions in the Caohai wetland, while anthropogenic factors (cultivation) significantly increased soil CO2 emissions. Results showed that the soil CO2 emissions were positively correlated with temperature and precipitation; and the temperature change had a greater impact on soil respiration than the change in precipitation. Our results indicated that future climate change (increased temperature and precipitation) may promote an increase in soil CO2 emissions in karst plateau wetlands, and reasonable control measures (e.g. returning cropland to lakes and reducing anthropogenic factors) are the keys to controlling CO2 emissions.

Measuring and Modeling Methane Emissions in Wetlands

Scientists zero in on a Delaware salt marsh to study what shapes methane emissions in wetland environments.

Investigation of the renewed methane growth post-2007 with high-resolution 3-D variational inverse modeling and isotopic constraints

Abstract. We investigate the causes of the renewed growth of atmospheric methane (CH4) amount fractions after 2007 by using variational inverse modeling with a three-dimensional chemistry-transport model. Together with CH4 amount fraction data, we use the additional information provided by observations of CH4 isotopic compositions (13C : 12C and D : H) to better differentiate between the emission categories compared to the differentiation achieved by assimilating CH4 amount fractions alone. Our system allows us to optimize either the CH4 emissions only or both the emissions and the source isotopic signatures (δsource(13C,CH4) and δsource(D,CH4)) of five emission categories. Consequently, we also assess, for the first time, the influence of applying random errors to both emissions and source signatures in an inversion framework. As the computational cost of a single inversion is high at present, the methodology applied to prescribe source signature uncertainties is simple, so it can serve as a basis for future work. Here, we investigate the post-2007 increase in atmospheric CH4 using the differences between 2002–2007 and 2007–2014. When random uncertainties in source isotopic signatures are accounted for, our results suggest that the post-2007 increase (here defined using the two periods 2002–2007 and 2007–2014) in atmospheric CH4 was caused by increases in emissions from (1) fossil sources (51 % of the net increase in emissions) and (2) agriculture and waste sources (49 %), which were slightly compensated for by a small decrease in biofuel- and biomass-burning emissions. The conclusions are very similar when assimilating CH4 amount fractions alone, suggesting either that random uncertainties in source signatures are too large at present to impose any additional constraint on the inversion problem or that we overestimate these uncertainties in our setups. On the other hand, if the source isotopic signatures are considered to be perfectly known (i.e., ignoring their uncertainties), the relative contributions of the different emission categories are significantly changed. Compared to the inversion where random uncertainties are accounted for, fossil emissions and biofuel- and biomass-burning emissions are increased by 24 % and 41 %, respectively, on average over 2002–2014. Wetland emissions and agricultural and waste emissions are decreased by 14 % and 7 %, respectively. Also, in this case, our results suggest that the increase in CH4 amount fractions after 2007 (despite a large decrease in biofuel- and biomass-burning emissions) was caused by increases in emissions from (1) fossil fuels (46 %), (2) agriculture and waste (37 %), and (3) wetlands (17 %). Additionally, some other sensitivity tests have been performed. While the prescribed interannual variability in OH can have a large impact on the results, assimilating δ(D,CH4) observations in addition to the other constraints has only a minor influence. Using all the information derived from these tests, the net increase in emissions is still primarily attributed to fossil sources (50 ± 3 %) and agriculture and waste sources (47 ± 5 %). Although our methods have room for improvement, these results illustrate the full capacity of our inversion framework, which can be used to consistently account for random uncertainties in both emissions and source signatures.

Wetland emissions on the rise

Wetland emissions on the rise

Underestimated Dry Season Methane Emissions from Wetlands in the Pantanal.

Tropical wetlands contribute ∼30% of the global methane (CH4) budget. Limited observational constraints on tropical wetland CH4 emissions lead to large uncertainties and disparities in representing emissions. In this work, we combine remote sensing observations with atmospheric and wetland models to investigate dry season wetland CH4 emissions from the Pantanal region of South America. We incorporate inundation maps generated from the Cyclone Global Navigation Satellite System (CYGNSS) satellite constellation together with traditional inundation maps to generate an ensemble of wetland CH4 emission realizations. We challenge these realizations with daily satellite observations for May-July when wetland CH4 emission predictions diverge. We find that the CYGNSS inundation products predict larger emissions in May, in better agreement with observations. We use the model ensemble to generate an empirical observational constraint on CH4 emissions independent of choice of inundation map, finding large dry season wetland CH4 emissions (31.7 ± 13.6 and 32.0 ± 20.2 mg CH4/m2/day in May and June/July during 2018/2019, respectively). These May/June/July emissions are 2-3 times higher than current models, suggesting that annual wetland emissions may be higher than traditionally simulated. Observed trends in the early dry season indicate that dynamics during this period are of importance in representing tropical wetland CH4 behaviors.

Zonal variability of methane trends derived from satellite data

Abstract. The Tropospheric Monitoring Instrument (TROPOMI) on board the Sentinel-5 Precursor (S5P) satellite is part of the latest generation of trace gas monitoring satellites and provides a new level of spatio-temporal information with daily global coverage, which enables the calculation of daily globally averaged CH4 concentrations. To investigate changes in atmospheric methane, the background CH4 level (i.e. the CH4 concentration without seasonal and short-term variations) has to be determined. CH4 growth rates vary in a complex manner and high-latitude zonal averages may have gaps in the time series, and thus simple fitting methods do not produce reliable results. In this paper we present an approach based on fitting an ensemble of dynamic linear models (DLMs) to TROPOMI data, from which the best model is chosen with the help of cross-validation to prevent overfitting. This method is computationally fast and is not dependent on additional inputs, allowing for fast and continuous analysis of the most recent time series data. We present results of global annual methane increases (AMIs) for the first 4.5 years of S5P/TROPOMI data, which show good agreement with AMIs from other sources. Additionally, we investigated what information can be derived from zonal bands. Due to the fast meridional mixing within hemispheres, we use zonal growth rates instead of AMIs, since they provide a higher temporal resolution. Clear differences can be observed between Northern Hemisphere and Southern Hemisphere growth rates, especially during 2019 and 2022. The growth rates show similar patterns within the hemispheres and show no short-term variations during the years, indicating that air masses within a hemisphere are well-mixed during a year. Additionally, the growth rates derived from S5P/TROPOMI data are largely consistent with growth rates derived from Copernicus Atmospheric Monitoring Service (CAMS) global-inversion-optimized (CAMS/INV) data, which use surface observations. In 2019 a reduction in growth rates can be observed for the Southern Hemisphere, while growth rates in the Northern Hemisphere stay stable or increase. During 2020 a strong increase in Southern Hemisphere growth rates can be observed, which is in accordance with recently reported increases in Southern Hemisphere wetland emissions. In 2022 the reduction in the global AMI can be attributed to decreased growth rates in the Northern Hemisphere, while growth rates in the Southern Hemisphere remain high. Investigations of fluxes from CAMS/INV data support these observations and suggest that the Northern Hemisphere decrease is mainly due to the decrease in anthropogenic fluxes, while in the Southern Hemisphere, wetland fluxes continued to rise. While the continued increase in Southern Hemisphere wetland fluxes agrees with existing studies about the causes of observed methane trends, the difference between Northern Hemisphere and Southern Hemisphere methane increases in 2022 has not been discussed before and calls for further research.

Driving and limiting factors of CH4 and CO2 emissions from coastal brackish-water wetlands in temperate regions

Abstract. Coastal wetlands play a fundamental role in mitigating climate change thanks to their ability to store large amounts of organic carbon in the soil. However, degraded freshwater wetlands are also known to be the first natural emitter of methane (CH4). Salinity is known to inhibit CH4 production, but its effect in brackish ecosystems is still poorly understood. This study provides a contribution to understanding how environmental variables may affect greenhouse gas (GHG) emissions in coastal temperate wetlands. We present the results of over 1 year of measurements performed in four wetlands located along a salinity gradient on the northeast Adriatic coast near Ravenna, Italy. Soil properties were determined by coring soil samples, while carbon dioxide (CO2) and CH4 fluxes from soils and standing waters were monitored monthly by a portable gas flux meter. Additionally, water levels and surface and groundwater physical–chemical parameters (temperature, pH, electrical conductivity, and sulfate concentrations of water) were monitored monthly by multiparametric probes. We observed a substantial reduction in CH4 emissions when water depth exceeded the critical threshold of 50 cm. Regardless of the water salinity value, the mean CH4 flux was 5.04 gm-2d-1 in freshwater systems and 12.27 gm-2d-1 in brackish ones. In contrast, when water depth was shallower than 50 cm, CH4 fluxes reached an average of 196.98 gm-2d-1 in freshwater systems, while non-significant results are available for brackish/saline waters. Results obtained for CO2 fluxes showed the same behavior described for CH4 fluxes, even though they were statistically non-significant. Temperature and irradiance strongly influenced CH4 emissions from water and soil, resulting in higher rates during summer and spring.

Role of n-DAMO in Mitigating Methane Emissions from Intertidal Wetlands Is Regulated by Saltmarsh Vegetations.

Coastal wetlands are hotspots for methane (CH4) production, reducing their potential for global warming mitigation. Nitrite/nitrate-dependent anaerobic methane oxidation (n-DAMO) plays a crucial role in bridging carbon and nitrogen cycles, contributing significantly to CH4 consumption. However, the role of n-DAMO in reducing CH4 emissions in coastal wetlands is poorly understood. Here, the ecological functions of the n-DAMO process in different saltmarsh vegetation habitats as well as bare mudflats were quantified, and the underlying microbial mechanisms were explored. Results showed that n-DAMO rates were significantly higher in vegetated habitats (Scirpus mariqueter and Spartina alterniflora) than those in bare mudflats (P < 0.05), leading to an enhanced contribution to CH4 consumption. Compared with other habitats, the contribution of n-DAMO to the total anaerobic CH4 oxidation was significantly lower in the Phragmites australis wetland (15.0%), where the anaerobic CH4 oxidation was primarily driven by ferric iron (Fe3+). Genetic and statistical analyses suggested that the different roles of n-DAMO in various saltmarsh wetlands may be related to divergent n-DAMO microbial communities as well as environmental parameters such as sediment pH and total organic carbon. This study provides an important scientific basis for a more accurate estimation of the role of coastal wetlands in mitigating climate change.

Methane Gas Ebullition Dynamics From Different Subtropical Wetland Vegetation Communities in Big Cypress National Preserve, Florida Are Revealed Using a Multi‐Method, Multi‐Scale Approach

AbstractMethane (CH4) emissions from peat soils are highly variable in space and time and are influenced by changes in biogeochemical controls and other environmental factors. Areas or times with disproportionally high CH4 emissions in wetlands may develop where conditions are especially conducive for microbial processes like methanogenesis. Currently, eddy covariance methods are employed to quantify CH4 exchanges over several extensive subtropical forested wetland communities in the Big Cypress National Preserve, Florida. In this work, we investigate the importance of multi‐scale measurements to characterize CH4 ebullition dynamics from subtropical wetlands. Our approach uses a combination of gas traps, time‐lapse photography, and capacitance probes to characterize ebullition dynamics from two different wetland vegetation communities for comparison to eddy covariance CH4 measurements at the site. Ground‐penetrating radar surveys and soil sampling are used to assess differences in subsurface properties between sites that influence ebullition. Our results show that the mean measurement bias between fluxes measured in this study and the eddy covariance measurements over the same period was 10–14 times larger during the wet season when ebullition rates were greatest, than the dry season, when ebullition rates were smallest. This suggests that eddy covariance measurements may underestimate the CH4 contribution of ebullition across heterogeneous wetland vegetation communities and that the comparability of CH4 fluxes from methods varying in spatio‐temporal scale changes in response to subtropical Florida seasonality. Our work suggests that these methods can be used to complement eddy covariance measurements and improve the characterization of ebullition dynamics in subtropical wetlands.

Effects of salinity on methane emissions and methanogenic archaeal communities in different habitat of saline-alkali wetlands.

The increase in temperature caused by global climate change has promoted the salinization of wetlands. Inland saline-alkaline wetlands have an environment of over-humidity and shallow water and are hot spots for CH4 emissions. However, there are few reports on the effect of salinity on CH4 emissions in inland saline-alkaline wetlands. This study conducted simulation experiments of increased salinity to investigate the impact of salinity, habitat, and their interactions on CH4 emissions, as well as to examine the response of methanogenic archaea to salinity. Overall, salinity inhibited CH4 emissions. But there were different responses in the three habitat soils. Salinity decreased the relative abundance of methanogenic archaea and changed the community structure. In addition, salinity changed soil pH and dissolved organic carbon (DOC) and ammonium (NH4+) concentrations, which were significantly correlated with methanogenic archaea. Our study showed that salinity changed the soil physicochemical properties and characteristics of the methanogenic archaeal community, affecting CH4 emissions.

Trends in atmospheric methane concentrations since 1990 were driven and modified by anthropogenic emissions

The atmospheric methane trend is not fully understood. Here we investigate the role of the main sink, the main natural source, and anthropogenic emissions on the methane growth rate over the last three decades using numerical models and emission inventories. We find that the long-term trend is driven by increased anthropogenic methane emissions, while wetland emissions show large variability and can modify the trend. The anthropogenic influence on hydroxyl radical, through nitrogen oxides and carbon monoxide emissions, has modified the trend over the last decades and contributed to the atmospheric methane stabilization from 2000 to 2007. The hydroxyl radical increase prior to this stabilization period might have contributed to the decline in the isotopic ratio after 2007 due to the time dependent isotopic response of hydroxyl radical. Emission reductions due to COVID-19 restrictions via the influence on hydroxyl radical, possibly contributed to approximately two thirds of the increase in methane growth from 2019 to 2020.

Upscaling Wetland Methane Emissions From the FLUXNET‐CH4 Eddy Covariance Network (UpCH4 v1.0): Model Development, Network Assessment, and Budget Comparison

AbstractWetlands are responsible for 20%–31% of global methane (CH4) emissions and account for a large source of uncertainty in the global CH4 budget. Data‐driven upscaling of CH4 fluxes from eddy covariance measurements can provide new and independent bottom‐up estimates of wetland CH4 emissions. Here, we develop a six‐predictor random forest upscaling model (UpCH4), trained on 119 site‐years of eddy covariance CH4 flux data from 43 freshwater wetland sites in the FLUXNET‐CH4 Community Product. Network patterns in site‐level annual means and mean seasonal cycles of CH4 fluxes were reproduced accurately in tundra, boreal, and temperate regions (Nash‐Sutcliffe Efficiency ∼0.52–0.63 and 0.53). UpCH4 estimated annual global wetland CH4 emissions of 146 ± 43 TgCH4 y−1 for 2001–2018 which agrees closely with current bottom‐up land surface models (102–181 TgCH4 y−1) and overlaps with top‐down atmospheric inversion models (155–200 TgCH4 y−1). However, UpCH4 diverged from both types of models in the spatial pattern and seasonal dynamics of tropical wetland emissions. We conclude that upscaling of eddy covariance CH4 fluxes has the potential to produce realistic extra‐tropical wetland CH4 emissions estimates which will improve with more flux data. To reduce uncertainty in upscaled estimates, researchers could prioritize new wetland flux sites along humid‐to‐arid tropical climate gradients, from major rainforest basins (Congo, Amazon, and SE Asia), into monsoon (Bangladesh and India) and savannah regions (African Sahel) and be paired with improved knowledge of wetland extent seasonal dynamics in these regions. The monthly wetland methane products gridded at 0.25° from UpCH4 are available via ORNL DAAC (https://doi.org/10.3334/ORNLDAAC/2253).

Assessing Methane Emissions From Tropical Wetlands: Uncertainties From Natural Variability and Drivers at the Global Scale

AbstractMethane (CH4) emissions from tropical wetlands represent half of the global wetland emissions, but uncertainties remain concerning the extent of tropical methane sources. One limitation is to conceive tropical wetlands as a single ecosystem, especially in global land surface models. We estimate CH4 emissions and assess their environmental and anthropogenic drivers. We created a data set with 101 studies involving 328‐point measurements, classified the sites into four wetland types, and included relevant biological and environmental information. We estimate the global CH4 emission rate from tropical wetlands as 35 (5–160) mg CH4 m−2 d−1 (median, first and third quartile) and an annual global rate of 94 (56, 158) Tg y−1. Fluxes differed among the wetland types, but except for anthropogenic factors, significant environmental drivers at the global scale could not be quantitatively identified because of high flux variability, even within wetland types. Coastal wetlands generate median emissions of 12 (5–23) Tg y−1. Inland deep‐water wetlands emit 53 (32–114) Tg y−1, with highly variable areal extent. Emissions from inland shallow‐water wetlands are 52 (33–78) Tg y−1 with variation due to seasonal changes in water table level. Human‐made wetlands emit 17 (10–24) Tg y−1. Pollution and N inputs from agriculture are significant anthropogenic drivers of emissions from tropical wetlands. Specific drivers of change need to be considered according to wetland type when estimating global emissions as well as their specific vulnerability to global change. Additionally, these differences should be considered when implementing wetland management practices aimed at decreasing methane emissions.

Time-lag effects of flood stimulation on methane emissions in the Dongting Lake floodplain, China

Natural wetlands are the primary sources of CH4 emissions in the natural environment. However, the understanding of CH4 fluxes in floodplain wetlands remains limited. This study employed eddy covariance methods to observe CH4 fluxes over a three-year period in a subtropical wetland floodplain, specifically the Miscanthus sacchariflorus (M. sacchariflorus) ecosystem in Dongting Lake wetland. Our analysis focused on exploring the impact of flooding frequency on CH4 emissions, flood stimulation effect, time-lag effects, and the environmental factors influencing CH4 fluxes. The M. sacchariflorus ecosystem exhibited an annual CH4 emission rate of 14.54 g CH4C m−2 y−1. During the flood period, the average daily CH4 emissions reached 0.155 g CH4C m−2 d−1, contrasting with the pre-flood period's average of 0.014 g CH4C m−2 d−1. Moreover, the time-lag effect of flooding on CH4 emissions was found to be 10 days, representing the period between inundation and a substantial increase in CH4 emissions. Comparatively, in 2021, following three fluctuations in floodwaters, the average CH4 emission intensity during the flood period decreased by 46.2% and 48.9% when compared to the years 2019 and 2020 which both following one fluctuation, respectively. CH4 emissions during flooding are predominantly influenced by water depth (WD), wherein shallow WD corresponds to higher CH4 emissions. This correlation can be attributed to factors such as vegetation type, water-column pressure, and soil oxygen content. Therefore, increasing frequency of inundation and a higher WD hold promise as effective measures for mitigating CH4 emissions in floodplain wetlands.