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1. Carbon uptake in Eurasian boreal forests dominates the high‐latitude net ecosystem carbon budget

   https://doi.org/10.1111/gcb.16553  

   Watts, Jennifer D. ; Farina, Mary ; Kimball, John S. ; Schiferl, Luke D. ; Liu, Zhihua ; Arndt, Kyle A. ; Zona, Donatella ; Ballantyne, Ashley ; Euskirchen, Eugénie S. ; Parmentier, Frans‐Jan W. ; et al ( January 2023 , Global Change Biology)

   Arctic‐boreal landscapes are experiencing profound warming, along with changes in ecosystem moisture status and disturbance from fire. This region is of global importance in terms of carbon feedbacks to climate, yet the sign (sink or source) and magnitude of the Arctic‐boreal carbon budget within recent years remains highly uncertain. Here, we provide new estimates of recent (2003–2015) vegetation gross primary productivity (GPP), ecosystem respiration (R_eco), net ecosystem CO₂exchange (NEE;R_eco − GPP), and terrestrial methane (CH₄) emissions for the Arctic‐boreal zone using a satellite data‐driven process‐model for northern ecosystems (TCFM‐Arctic), calibrated and evaluated using measurements from >60 tower eddy covariance (EC) sites. We used TCFM‐Arctic to obtain daily 1‐km²flux estimates and annual carbon budgets for the pan‐Arctic‐boreal region. Across the domain, the model indicated an overall average NEE sink of −850 Tg CO₂‐C year⁻¹. Eurasian boreal zones, especially those in Siberia, contributed to a majority of the net sink. In contrast, the tundra biome was relatively carbon neutral (ranging from small sink to source). Regional CH₄emissions from tundra and boreal wetlands (not accounting for aquatic CH₄) were estimated at 35 Tg CH₄‐C year⁻¹. Accounting for additional emissions from open water aquatic bodies and from fire, using available estimates from the literature, reduced the total regional NEE sink bymore »21% and shifted many far northern tundra landscapes, and some boreal forests, to a net carbon source. This assessment, based on in situ observations and models, improves our understanding of the high‐latitude carbon status and also indicates a continued need for integrated site‐to‐regional assessments to monitor the vulnerability of these ecosystems to climate change.

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2. CO ₂ emissions from an undrained tropical peatland: Interacting influences of temperature, shading and water table depth

   https://doi.org/10.1111/gcb.14702  

   Hoyt, Alison M. ; Gandois, Laure ; Eri, Jangarun ; Kai, Fuu Ming ; Harvey, Charles F. ; Cobb, Alexander R. ( July 2019 , Global Change Biology)

   Emission of CO₂from tropical peatlands is an important component of the global carbon budget. Over days to months, these fluxes are largely controlled by water table depth. However, the diurnal cycle is less well understood, in part, because most measurements have been collected daily at midday. We used an automated chamber system to make hourly measurements of peat surface CO₂emissions from chambers root‐cut to 30 cm. We then used these data to disentangle the relationship between temperature, water table and heterotrophic respiration (R_het). We made two central observations. First, we found strong diurnal cycles in CO₂flux and near‐surface peat temperature (<10 cm depth), both peaking at midday. The magnitude of diurnal oscillations was strongly influenced by shading and water table depth, highlighting the limitations of relying on daytime measurements and/or a single correction factor to remove daytime bias in flux measurements. Second, we found mean daily R_hethad a strong linear relationship to the depth of the water table, and under flooded conditions, R_hetwas small and constant. We used this relationship between R_hetand water table depth to estimate carbon export from both R_hetand dissolved organic carbon over the course of a year based on water table records. R_hetdominates annual carbon export, demonstratingmore »the potential for peatland drainage to increase regional CO₂emissions. Finally, we discuss an apparent incompatibility between hourly and daily average observations of CO₂flux, water table and temperature: water table and daily average flux data suggest that CO₂is produced across the entire unsaturated peat profile, whereas temperature and hourly flux data appear to suggest that CO₂fluxes are controlled by very near surface peat. We explore how temperature‐, moisture‐ and gas transport‐related mechanisms could cause mean CO₂emissions to increase linearly with water table depth and also have a large diurnal cycle.

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3. Carbon Dioxide Fluxes to the Atmosphere From Waters Within Flooded Forests in the Amazon Basin

   https://doi.org/10.1029/2019JG005293  

   Amaral, Joao Henrique Fernandes ; Melack, John M. ; Barbosa, Pedro Maia ; MacIntyre, Sally ; Kasper, Daniele ; Cortés, Alicia ; Silva, Thiago Sanna Freire ; Nunes de Sousa, Rodrigo ; Forsberg, Bruce R. ( February 2020 , Journal of Geophysical Research: Biogeosciences)

   Inundated tropical forests are underrepresented in analyses of the global carbon cycle and constitute 80% of the surface area of aquatic environments in the lowland Amazon basin. Diel variations in CO₂concentrations and exchanges with the atmosphere were investigated from August 2014 to September 2016 in two flooded forests sites with different wind exposure within the central Amazon floodplain (3°23′S, 60°18′W). CO₂profiles and estimates of air–water gas exchange were combined with ancillary environmental measurements. Surface CO₂concentrations ranged from 19 to 329 μM, CO₂fluxes ranged from −0.8 to 55 mmol m⁻² hr⁻¹and gas transfer velocities ranged from 0.2 to 17 cm hr⁻¹. CO₂concentrations and fluxes were highest during the high water period. CO₂fluxes were three times higher at a site with more wind exposure (WE) compared to one with less exposure (WP). Emissions were higher at the WP site during the day, whereas they were higher at night at the WE site due to vertical mixing. CO₂concentrations and fluxes were lower at the W P site following an extended period of exceptionally low water. The CO₂flux from the water in the flooded forest was about half of the net primary production of the forest estimated from the literature. Mean daily fluxes measured in our study (182 ± 247 mmol m⁻²d⁻¹) aremore »higher than or similar to the few other measurements in waters within tropical and subtropical flooded forests and highlight the importance of flooded forests in carbon budgets.

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4. 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 floodmore »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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5. Trace gas fluxes from tidal salt marsh soils: implications for carbon–sulfur biogeochemistry

   https://doi.org/10.5194/bg-19-4655-2022  

   Capooci, Margaret ; Vargas, Rodrigo ( January 2022 , Biogeosciences)

   Abstract. Tidal salt marsh soils can be a dynamic source of greenhouse gases such ascarbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O),as well as sulfur-based trace gases such as carbon disulfide (CS2) anddimethylsulfide (DMS) which play roles in global climate and carbon–sulfurbiogeochemistry. Due to the difficulty in measuring trace gases in coastalecosystems (e.g., flooding, salinity), our current understanding is based onsnapshot instantaneous measurements (e.g., performed during daytime lowtide) which complicates our ability to assess the role of these ecosystemsfor natural climate solutions. We performed continuous, automatedmeasurements of soil trace gas fluxes throughout the growing season toobtain high-temporal frequency data and to provide insights into magnitudesand temporal variability across rapidly changing conditions such as tidalcycles. We found that soil CO2 fluxes did not show a consistent dielpattern, CH4, N2O, and CS2 fluxes were highly variable withfrequent pulse emissions (> 2500 %, > 10 000 %,and > 4500 % change, respectively), and DMS fluxes onlyoccurred midday with changes > 185 000 %. When we comparedcontinuous measurements with discrete temporal measurements (during daytime,at low tide), discrete measurements of soil CO2 fluxes were comparablewith those from continuous measurements but misrepresent the temporalvariability and magnitudes of CH4, N2O, DMS, and CS2.Discrepancies between the continuous and discrete measurement data result indifferences for calculating the sustainedmore »global warming potential (SGWP),mainly by an overestimation of CH4 fluxes when using discretemeasurements. The high temporal variability of trace gas fluxes complicatesthe accurate calculation of budgets for use in blue carbon accounting andearth system models.« less