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1. Resolving the Carbon‐Climate Feedback Potential of Wetland CO 2 and CH 4 Fluxes in Alaska

   https://doi.org/10.1029/2022GB007524  

   Ma, Shuang ; Bloom, A. Anthony ; Watts, Jennifer D. ; Quetin, Gregory R. ; Donatella, Zona ; Euskirchen, Eugénie S. ; Norton, Alexander J. ; Yin, Yi ; Levine, Paul A. ; Braghiere, Renato K. ; et al ( September 2023 , Global Biogeochemical Cycles)

   Boreal‐Arctic regions are key stores of organic carbon (C) and play a major role in the greenhouse gas balance of high‐latitude ecosystems. The carbon‐climate (C‐climate) feedback potential of northern high‐latitude ecosystems remains poorly understood due to uncertainty in temperature and precipitation controls on carbon dioxide (CO₂) uptake and the decomposition of soil C into CO₂and methane (CH₄) fluxes. While CH₄fluxes account for a smaller component of the C balance, the climatic impact of CH₄outweighs CO₂(28–34 times larger global warming potential on a 100‐year scale), highlighting the need to jointly resolve the climatic sensitivities of both CO₂and CH₄. Here, we jointly constrain a terrestrial biosphere model with in situ CO₂and CH₄flux observations at seven eddy covariance sites using a data‐model integration approach to resolve the integrated environmental controls on land‐atmosphere CO₂and CH₄exchanges in Alaska. Based on the combined CO₂and CH₄flux responses to climate variables, we find that 1970‐present climate trends will induce positive C‐climate feedback at all tundra sites, and negative C‐climate feedback at the boreal and shrub fen sites. The positive C‐climate feedback at the tundra sites is predominantly driven by increased CH₄emissions while the negative C‐climate feedback at the boreal site is predominantly driven by increased CO₂uptake (80% from decreased heterotrophic respiration, and 20% from increased photosynthesis). Our study demonstrates the need for joint observational constraints on CO₂and CH₄biogeochemical processes—and their associated climatic sensitivities—for resolving the sign and magnitude of high‐latitude ecosystem C‐climate feedback in the coming decades.

    

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2. Permafrost Carbon: Progress on Understanding Stocks and Fluxes Across Northern Terrestrial Ecosystems

   https://doi.org/10.1029/2023JG007638  

   Treat, Claire C. ; Virkkala, Anna‐Maria ; Burke, Eleanor ; Bruhwiler, Lori ; Chatterjee, Abhishek ; Fisher, Joshua B. ; Hashemi, Josh ; Parmentier, Frans‐Jan W. ; Rogers, Brendan M. ; Westermann, Sebastian ; et al ( February 2024 , Journal of Geophysical Research: Biogeosciences)

   Significant progress in permafrost carbon science made over the past decades include the identification of vast permafrost carbon stocks, the development of new pan‐Arctic permafrost maps, an increase in terrestrial measurement sites for CO₂and methane fluxes, and important factors affecting carbon cycling, including vegetation changes, periods of soil freezing and thawing, wildfire, and other disturbance events. Process‐based modeling studies now include key elements of permafrost carbon cycling and advances in statistical modeling and inverse modeling enhance understanding of permafrost region C budgets. By combining existing data syntheses and model outputs, the permafrost region is likely a wetland methane source and small terrestrial ecosystem CO₂sink with lower net CO₂uptake toward higher latitudes, excluding wildfire emissions. For 2002–2014, the strongest CO₂sink was located in western Canada (median: −52 g C m⁻² y⁻¹) and smallest sinks in Alaska, Canadian tundra, and Siberian tundra (medians: −5 to −9 g C m⁻² y⁻¹). Eurasian regions had the largest median wetland methane fluxes (16–18 g CH₄m⁻² y⁻¹). Quantifying the regional scale carbon balance remains challenging because of high spatial and temporal variability and relatively low density of observations. More accurate permafrost region carbon fluxes require: (a) the development of better maps characterizing wetlands and dynamics of vegetation and disturbances, including abrupt permafrost thaw; (b) the establishment of new year‐round CO₂and methane flux sites in underrepresented areas; and (c) improved models that better represent important permafrost carbon cycle dynamics, including non‐growing season emissions and disturbance effects.

    

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3. Groundwater‐Driven Methane Export Reduces Salt Marsh Blue Carbon Potential

   https://doi.org/10.1029/2020GB006587  

   Schutte, C. A. ; Moore, W. S. ; Wilson, A. M. ; Joye, S. B. ( October 2020 , Global Biogeochemical Cycles)

   The burial of “blue carbon” in coastal marsh soils is partially offset by marsh‐atmosphere methane (CH₄) fluxes, but this offset may be greater if other pathways of CH₄export exist. Here we report that salt marshes also export dissolved CH₄via submarine groundwater discharge (SGD). The volumetric fluxes of salt marsh groundwater into adjacent tidal creeks were calculated from mass balances of the conservative tracer²²⁶Ra at four study sites in coastal Georgia, USA. Over the 2‐year study period, volumetric groundwater fluxes across all sites ranged between 1,700 and 105,000 m³ day⁻¹. Dissolved CH₄fluxes of 27–1,200 μmol CH₄m⁻² day⁻¹were calculated by multiplying the volumetric groundwater flux by the groundwater CH₄concentration and normalizing to the intertidal salt marsh area estimated from satellite images. On a mass basis, the cross‐site range in CH₄fluxes was 1.3–5.5 g CH₄ m⁻² year⁻¹with a cross‐site mean of 2.8 g CH₄ m⁻² year⁻¹. This is equivalent to 125 (56–245) g CO₂ m⁻² year⁻¹assuming that CH₄is 45 times more potent than CO₂as a greenhouse gas over a 100‐year time frame. This sustained‐flux global warming potential is similar to the 138 (1.1–260) g CO₂ m⁻² year⁻¹average calculated across other studies of the direct marsh soil to atmosphere CH₄flux. Therefore, SGD drives an effective doubling of salt marsh CH₄export that offsets a combined total of ~30% of the global cooling potential derived from soil carbon sequestration.

    

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4. Seasonality and Budgets of Soil Greenhouse Gas Emissions From a Tropical Dry Forest Successional Gradient in Costa Rica

   https://doi.org/10.1029/2020JG005647  

   Calvo‐Rodriguez, Sofia ; Kiese, Ralf ; Sánchez‐Azofeifa, G. Arturo ( September 2020 , Journal of Geophysical Research: Biogeosciences)

   Limited information on greenhouse gas emissions from tropical dry forest soils still hinders the assessment of the sources/sinks from this ecosystem and their contribution at global scales. Particularly, rewetting events after the dry season can have a significant effect on soil biogeochemical processes and associated exchange of greenhouse gases. This study evaluated the temporal variation and annual fluxes of CO₂, N₂O, and CH₄from soils in a tropical dry forest successional gradient. After a prolonged drought of 5 months, large emissions pulses of CO₂and N₂O were observed at all sites following first rain events, caused by the “Birch effect,” with a significant effect on the net ecosystem exchange and the annual emissions budget. Annual CO₂emissions were greatest for the young forest (8,556 kg C ha⁻¹yr⁻¹) followed by the older forest (7,420 kg C ha⁻¹yr⁻¹) and the abandoned pasture (7,224 kg C ha⁻¹yr⁻¹). Annual emissions of N₂O were greatest for the forest sites (0.39 and 0.43 kg N ha⁻¹yr⁻¹) and least in the abandoned pasture (0.09 kg N ha⁻¹yr⁻¹). CH₄uptake was greatest in the older forest (−2.61 kg C ha⁻¹yr⁻¹) followed by the abandoned pasture (−0.69 kg C ha⁻¹yr⁻¹) and the young forest (−0.58 kg C ha⁻¹yr⁻¹). Fluxes were mainly influenced by soil moisture, microbial biomass, and soil nitrate and ammonium concentrations. Annual CO₂and N₂O soil fluxes of tropical dry forests in this study and others from the literature were much lower than the annual fluxes in wetter tropical forests. Conversely, tropical dry forests and abandoned pastures are on average stronger sinks for CH₄than wetter tropical forests.

    

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5. Radiative forcing of methane fluxes offsets net carbon dioxide uptake for a tropical flooded forest

   https://doi.org/10.1111/gcb.14615  

   Dalmagro, Higo J. ; Zanella de Arruda, Paulo H. ; Vourlitis, George L. ; Lathuillière, Michael J. ; de S. Nogueira, José ; Couto, Eduardo G. ; Johnson, Mark S. ( April 2019 , Global Change Biology)

   Wetlands are important sources of methane (CH₄) and sinks of carbon dioxide (CO₂). However, little is known about CH₄and CO₂fluxes and dynamics of seasonally flooded tropical forests of South America in relation to local carbon (C) balances and atmospheric exchange. We measured net ecosystem fluxes of CH₄and CO₂in the Pantanal over 2014–2017 using tower‐based eddy covariance along with C measurements in soil, biomass and water. Our data indicate that seasonally flooded tropical forests are potentially large sinks for CO₂but strong sources of CH₄, particularly during inundation when reducing conditions in soils increase CH₄production and limit CO₂release. During inundation when soils were anaerobic, the flooded forest emitted 0.11 ± 0.002 g CH₄‐C m⁻² d⁻¹and absorbed 1.6 ± 0.2 g CO₂‐C m⁻² d⁻¹(mean ± 95% confidence interval for the entire study period). Following the recession of floodwaters, soils rapidly became aerobic and CH₄emissions decreased significantly (0.002 ± 0.001 g CH₄‐C m⁻² d⁻¹) but remained a net source, while the net CO₂flux flipped from being a net sink during anaerobic periods to acting as a source during aerobic periods. CH₄fluxes were 50 times higher in the wet season; DOC was a minor component in the net ecosystem carbon balance. Daily fluxes of CO₂and CH₄were similar in all years for each season, but annual net fluxes varied primarily in relation to flood duration. While the ecosystem was a net C sink on an annual basis (absorbing 218 g C m⁻²(as CH₄‐C + CO₂‐C) in anaerobic phases and emitting 76 g C m⁻²in aerobic phases), high CH₄effluxes during the anaerobic flooded phase and modest CH₄effluxes during the aerobic phase indicate that seasonally flooded tropical forests can be a net source of radiative forcings on an annual basis, thus acting as an amplifying feedback on global warming.

    

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