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1. Vegetation and hydrology stratification as proxies to estimate methane emission from tidal marshes

   https://doi.org/10.1007/s10533-021-00870-z  

   Derby, R. Kyle; Needelman, Brian A.; Roden, Ana A.; Megonigal, J. Patrick (December 2021, Biogeochemistry)

   Abstract Direct measurement of methane emissions is cost-prohibitive for greenhouse gas offset projects, necessitating the development of alternative accounting methods such as proxies. Salinity is a useful proxy for tidal marsh CH₄emissions when comparing across a wide range of salinity regimes but does not adequately explain variation in brackish and freshwater regimes, where variation in emissions is large. We sought to improve upon the salinity proxy in a marsh complex on Deal Island Peninsula, Maryland, USA by comparing emissions from four strata differing in hydrology and plant community composition. Mean CH₄chamber-collected emissions measured as mg CH₄m⁻² h⁻¹ranked asS. alterniflora(1.2 ± 0.3) ≫ High-elevationJ. roemerianus(0.4 ± 0.06) > Low-elevationJ. roemerianus(0.3 ± 0.07) = S. patens(0.1 ± 0.01). Sulfate depletion generally reflected the same pattern with significantly greater depletion in theS. alterniflorastratum (61 ± 4%) than in theS. patensstratum (1 ± 9%) with theJ. roemerianusstrata falling in between. We attribute the high CH₄emissions in theS. alterniflorastratum to sulfate depletion likely driven by limited connectivity to tidal waters. Low CH₄emissions in theS. patensstratum are attributed to lower water levels, higher levels of ferric iron, and shallow rooting depth. Moderate CH₄emissions from theJ. roemerianusstrata were likely due to plant traits that favor CH₄oxidation over CH₄production. Hydrology and plant community composition have significant potential as proxies to estimate CH₄emissions at the site scale. 

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

   https://doi.org/10.1029/2023AV000956  

   McNicol, Gavin; Fluet‐Chouinard, Etienne; Ouyang, Zutao; Knox, Sara; Zhang, Zhen; Aalto, Tuula; Bansal, Sheel; Chang, Kuang‐Yu; Chen, Min; Delwiche, Kyle; et al (October 2023, AGU Advances)

   Abstract Wetlands are responsible for 20%–31% of global methane (CH₄) emissions and account for a large source of uncertainty in the global CH₄budget. Data‐driven upscaling of CH₄fluxes from eddy covariance measurements can provide new and independent bottom‐up estimates of wetland CH₄emissions. Here, we develop a six‐predictor random forest upscaling model (UpCH4), trained on 119 site‐years of eddy covariance CH₄flux data from 43 freshwater wetland sites in the FLUXNET‐CH4 Community Product. Network patterns in site‐level annual means and mean seasonal cycles of CH₄fluxes were reproduced accurately in tundra, boreal, and temperate regions (Nash‐Sutcliffe Efficiency ∼0.52–0.63 and 0.53). UpCH4 estimated annual global wetland CH₄emissions of 146 ± 43 TgCH₄ y⁻¹for 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 CH₄fluxes has the potential to produce realistic extra‐tropical wetland CH₄emissions 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). 

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3. Freshwater faces a warmer and saltier future from headwaters to coasts: climate risks, saltwater intrusion, and biogeochemical chain reactions

   https://doi.org/10.1007/s10533-025-01219-6  

   Kaushal, Sujay_S; Shelton, Sydney_A; Mayer, Paul_M; Kellmayer, Bennett; Utz, Ryan_M; Reimer, Jenna_E; Baljunas, Jenna; Bhide, Shantanu_V; Mon, Ashley; Rodriguez-Cardona, Bianca_M; et al (March 2025, Biogeochemistry)

   Abstract Alongside global climate change, many freshwater ecosystems are experiencing substantial shifts in the concentrations and compositions of salt ions coming from both land and sea. We synthesize a risk framework for anticipating how climate change and increasing salt pollution coming from both land and saltwater intrusion will trigger chain reactions extending from headwaters to tidal waters. Salt ions trigger ‘chain reactions,’ where chemical products from one biogeochemical reaction influence subsequent reactions and ecosystem responses. Different chain reactions impact drinking water quality, ecosystems, infrastructure, and energy and food production. Risk factors for chain reactions include shifts in salinity sources due to global climate change and amplification of salinity pulses due to the interaction of precipitation variability and human activities. Depending on climate and other factors, salt retention can range from 2 to 90% across watersheds globally. Salt retained in ecosystems interacts with many global biogeochemical cycles along flowpaths and contributes to ‘fast’ and ‘slow’ chain reactions associated with temporary acidification and long-term alkalinization of freshwaters, impacts on nutrient cycling, CO₂, CH₄, N₂O, and greenhouse gases, corrosion, fouling, and scaling of infrastructure, deoxygenation, and contaminant mobilization along the freshwater-marine continuum. Salt also impacts the carbon cycle and the quantity and quality of organic matter transported from headwaters to coasts. We identify the double impact of salt pollution from land and saltwater intrusion on a wide range of ecosystem services. Our salinization risk framework is based on analyses of: (1) increasing temporal trends in salinization of tributaries and tidal freshwaters of the Chesapeake Bay and freshening of the Chesapeake Bay mainstem over 40 years due to changes in streamflow, sea level rise, and watershed salt pollution; (2) increasing long-term trends in concentrations and loads of major ions in rivers along the Eastern U.S. and increased riverine exports of major ions to coastal waters sometimes over 100-fold greater than forest reference conditions; (3) varying salt ion concentration-discharge relationships at U.S. Geological Survey (USGS) sites across the U.S.; (4) empirical relationships between specific conductance and Na⁺, Cl⁻, SO₄²⁻, Ca²⁺, Mg²⁺, K⁺, and N at USGS sites across the U.S.; (5) changes in relationships between concentrations of dissolved organic carbon (DOC) and different salt ions at USGS sites across the U.S.; and (6) original salinization experiments demonstrating changes in organic matter composition, mobilization of nutrients and metals, acidification and alkalinization, changes in oxidation–reduction potentials, and deoxygenation in non-tidal and tidal waters. The interaction of human activities and climate change is altering sources, transport, storage, and reactivity of salt ions and chain reactions along the entire freshwater-marine continuum. Our salinization risk framework helps anticipate, prevent, and manage the growing double impact of salt ions from both land and sea on drinking water, human health, ecosystems, aquatic life, infrastructure, agriculture, and energy production. 

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4. Large Methane Emission Fluxes Observed From Tropical Wetlands in Zambia

   https://doi.org/10.1029/2021GB007261  

   Shaw, Jacob T.; Allen, Grant; Barker, Patrick; Pitt, Joseph R.; Pasternak, Dominika; Bauguitte, Stéphane J. ‐B.; Lee, James; Bower, Keith N.; Daly, Michael C.; Lunt, Mark F.; et al (June 2022, Global Biogeochemical Cycles)

   Abstract Methane (CH₄) is a potent greenhouse gas with a warming potential 84 times that of carbon dioxide (CO₂) over a 20‐year period. Atmospheric CH₄concentrations have been rising since the nineteenth century but the cause of large increases post‐2007 is disputed. Tropical wetlands are thought to account for ∼20% of global CH₄emissions, but African tropical wetlands are understudied and their contribution is uncertain. In this work, we use the first airborne measurements of CH₄sampled over three wetland areas in Zambia to derive emission fluxes. Three independent approaches to flux quantification from airborne measurements were used: Airborne mass balance, airborne eddy‐covariance, and an atmospheric inversion. Measured emissions (ranging from 5 to 28 mg m⁻² hr⁻¹) were found to be an order of magnitude greater than those simulated by land surface models (ranging from 0.6 to 3.9 mg m⁻²hr⁻¹), suggesting much greater emissions from tropical wetlands than currently accounted for. The prevalence of such underestimated CH₄sources may necessitate additional reductions in anthropogenic greenhouse gas emissions to keep global warming below a threshold of 2°C above preindustrial levels. 

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5. Eddy Covariance Data Reveal That a Small Freshwater Reservoir Emits a Substantial Amount of Carbon Dioxide and Methane

   https://doi.org/10.1029/2022JG007091  

   Hounshell, Alexandria G.; D’Acunha, Brenda M.; Breef‐Pilz, Adrienne; Johnson, Mark S.; Thomas, R. Quinn; Carey, Cayelan C. (March 2023, Journal of Geophysical Research: Biogeosciences)

   Abstract Small freshwater reservoirs are ubiquitous and likely play an important role in global greenhouse gas (GHG) budgets relative to their limited water surface area. However, constraining annual GHG fluxes in small freshwater reservoirs is challenging given their footprint area and spatially and temporally variable emissions. To quantify the GHG budget of a small (0.1 km²) reservoir, we deployed an Eddy covariance (EC) system in a small reservoir located in southwestern Virginia, USA over 2 years to measure carbon dioxide (CO₂) and methane (CH₄) fluxes near‐continuously. Fluxes were coupled with in situ sensors measuring multiple environmental parameters. Over both years, we found the reservoir to be a large source of CO₂(633–731 g CO₂‐C m⁻² yr⁻¹) and CH₄(1.02–1.29 g CH₄‐C m⁻² yr⁻¹) to the atmosphere, with substantial sub‐daily, daily, weekly, and seasonal timescales of variability. For example, fluxes were substantially greater during the summer thermally stratified season as compared to the winter. In addition, we observed significantly greater GHG fluxes during winter intermittent ice‐on conditions as compared to continuous ice‐on conditions, suggesting GHG emissions from lakes and reservoirs may increase with predicted decreases in winter ice‐cover. Finally, we identified several key environmental variables that may be driving reservoir GHG fluxes at multiple timescales, including, surface water temperature and thermocline depth followed by fluorescent dissolved organic matter. Overall, our novel year‐round EC data from a small reservoir indicate that these freshwater ecosystems likely contribute a substantial amount of CO₂and CH₄to global GHG budgets, relative to their surface area. 

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