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1. Soil respiration strongly offsets carbon uptake in Alaska and Northwest Canada

   https://doi.org/10.1088/1748-9326/ac1222  

   Watts, Jennifer D. ; Natali, Susan M. ; Minions, Christina ; Risk, Dave ; Arndt, Kyle ; Zona, Donatella ; Euskirchen, Eugénie S. ; Rocha, Adrian V. ; Sonnentag, Oliver ; Helbig, Manuel ; et al ( August 2021 , Environmental Research Letters)

   Soil respiration (i.e. from soils and roots) provides one of the largest global fluxes of carbon dioxide (CO₂) to the atmosphere and is likely to increase with warming, yet the magnitude of soil respiration from rapidly thawing Arctic-boreal regions is not well understood. To address this knowledge gap, we first compiled a new CO₂flux database for permafrost-affected tundra and boreal ecosystems in Alaska and Northwest Canada. We then used the CO₂database, multi-sensor satellite imagery, and random forest models to assess the regional magnitude of soil respiration. The flux database includes a new Soil Respiration Station network of chamber-based fluxes, and fluxes from eddy covariance towers. Our site-level data, spanning September 2016 to August 2017, revealed that the largest soil respiration emissions occurred during the summer (June–August) and that summer fluxes were higher in boreal sites (1.87 ± 0.67 g CO₂–C m⁻²d⁻¹) relative to tundra (0.94 ± 0.4 g CO₂–C m⁻²d⁻¹). We also observed considerable emissions (boreal: 0.24 ± 0.2 g CO₂–C m⁻²d⁻¹; tundra: 0.18 ± 0.16 g CO₂–C m⁻²d⁻¹) from soils during the winter (November–March) despite frozen surface conditions. Our model estimates indicated an annual region-wide loss from soil respiration of 591 ± 120 Tg CO₂–C during the 2016–2017 period. Summer months contributed to 58% of the regional soil respiration, winter months contributed to 15%, and the shoulder months contributed to 27%. In total, soil respiration offset 54% of annual gross primary productivity (GPP) across the study domain. We also found that in tundra environments, transitional tundra/boreal ecotones, and in landscapes recently affected by fire, soil respiration often exceeded GPP, resulting in a net annual source of CO₂to the atmosphere. As this region continues to warm, soil respiration may increasingly offset GPP, further amplifying global climate change.

    

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2. The Net GHG Balance and Budget of the Permafrost Region (2000–2020) From Ecosystem Flux Upscaling

   https://doi.org/10.1029/2023GB007953  

   Ramage, Justine ; Kuhn, McKenzie ; Virkkala, Anna‐Maria ; Voigt, Carolina ; Marushchak, Maija E. ; Bastos, Ana ; Biasi, Christina ; Canadell, Josep G. ; Ciais, Philippe ; López‐Blanco, Efrèn ; et al ( April 2024 , Global Biogeochemical Cycles)

   The northern permafrost region has been projected to shift from a net sink to a net source of carbon under global warming. However, estimates of the contemporary net greenhouse gas (GHG) balance and budgets of the permafrost region remain highly uncertain. Here, we construct the first comprehensive bottom‐up budgets of CO₂, CH₄, and N₂O across the terrestrial permafrost region using databases of more than 1000 in situ flux measurements and a land cover‐based ecosystem flux upscaling approach for the period 2000–2020. Estimates indicate that the permafrost region emitted a mean annual flux of 12 (−606, 661) Tg CO₂–C yr⁻¹, 38 (22, 53) Tg CH₄–C yr⁻¹, and 0.67 (0.07, 1.3) Tg N₂O–N yr⁻¹to the atmosphere throughout the period. Thus, the region was a net source of CH₄and N₂O, while the CO₂balance was near neutral within its large uncertainties. Undisturbed terrestrial ecosystems had a CO₂sink of −340 (−836, 156) Tg CO₂–C yr⁻¹. Vertical emissions from fire disturbances and inland waters largely offset the sink in vegetated ecosystems. When including lateral fluxes for a complete GHG budget, the permafrost region was a net source of C and N, releasing 144 (−506, 826) Tg C yr⁻¹and 3 (2, 5) Tg N yr⁻¹. Large uncertainty ranges in these estimates point to a need for further expansion of monitoring networks, continued data synthesis efforts, and better integration of field observations, remote sensing data, and ecosystem models to constrain the contemporary net GHG budgets of the permafrost region and track their future trajectory.

    

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3. Greenhouse gas balance in global pasturelands and rangelands

   https://doi.org/10.1088/1748-9326/abaa79  

   Dangal, Shree R. S. ; Tian, Hanqin ; Pan, Shufen ; Zhang, Lei ; Xu, Rongting ( September 2020 , Environmental Research Letters)

   Grassland ecosystems play an essential role in climate regulation through carbon (C) storage in plant and soil. But, anthropogenic practices such as livestock grazing, grazing related excreta nitrogen (N) deposition, and manure/fertilizer N application have the potential to reduce the effectiveness of grassland C sink through increased nitrous oxide (N₂O) and methane (CH₄) emissions. Although the effect of anthropogenic activities on net greenhouse gas (GHG) fluxes in grassland ecosystems have been investigated at local to regional scales, estimates of net GHG balance at the global scale remains uncertain. With the data-model framework integrating empirical estimates of livestock CH₄emissions with process-based modeling estimates of land CO₂, N₂O and CH₄fluxes, we examined the overall global warming potential (GWP) of grassland ecosystems during 1961–2010. We then quantified the grassland-specific and regional variations to identify hotspots of GHG fluxes. Our results show that, over a 100-year time horizon, grassland ecosystems sequestered a cumulative total of 113.9 Pg CO₂-eq in plant and soil, but then released 91.9 Pg CO₂-eq to the atmosphere, offsetting 81% of the net CO₂sink. We also found large grassland-specific variations in net GHG fluxes, withpasturelandsacting as a small GHG source of 1.52 ± 143 Tg CO₂-eq yr⁻¹(mean ± 1.0 s.d.) andrangelandsa strong GHG sink (−442 ± 266 Tg CO₂-eq yr⁻¹) during 1961–2010. Regionally, Europe acted as a GHG source of 23 ± 10 Tg CO₂-eq yr⁻¹, while other regions (i.e. Africa, Southern Asia) were strong GHG sinks during 2001–2010. Our study highlights the importance of considering regional and grassland-specific differences in GHG fluxes for guiding future management and climate mitigation strategies in global grasslands.

    

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4. Emission of Greenhouse Gases From Water Tracks Draining Arctic Hillslopes

   https://doi.org/10.1029/2020JG005889  

   Harms, Tamara K. ; Rocher‐Ros, Gerard ; Godsey, Sarah E. ( December 2020 , Journal of Geophysical Research: Biogeosciences)

   Experimental and ambient warming of Arctic tundra results in emissions of greenhouse gases to the atmosphere, contributing to a positive feedback to climate warming. Estimates of gas emissions from lakes and terrestrial tundra confirm the significance of aquatic fluxes in greenhouse gas budgets, whereas few estimates describe emissions from fluvial networks. We measured dissolved gas concentrations and estimated emissions of carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O) from water tracks, vegetated depressions that hydrologically connect hillslope soils to lakes and streams. Concentrations of trace gases generally increased as ground thaw deepened through the growing season, indicating active production of greenhouse gases in thawed soils. Wet antecedent conditions were correlated with a decline in CO₂and CH₄concentrations. Dissolved N₂O in excess of atmospheric equilibrium occurred in drier water tracks, but on average water tracks took up N₂O from the atmosphere at low rates. Estimated CO₂emission rates for water tracks were among the highest observed for Arctic aquatic ecosystems, whereas CH₄emissions were of similar magnitude to streams. Despite occupying less than 1% of total catchment area, surface waters within water tracks were an estimated source of up to 53–85% of total CH₄emissions from their catchments and offset the terrestrial C sink by 5–9% during the growing season. Water tracks are abundant features of tundra landscapes that contain warmer soils and incur deeper thaw than adjacent terrestrial ecosystems and as such might contribute to ongoing and accelerating release of greenhouse gases from permafrost soils to the atmosphere.

    

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5. Methane and Carbon Dioxide Fluxes in a Temperate Tidal Salt Marsh: Comparisons Between Plot and Ecosystem Measurements

   https://doi.org/10.1029/2022JG006943  

   Hill, Andrew C. ; Vargas, Rodrigo ( July 2022 , Journal of Geophysical Research: Biogeosciences)

   Tidal wetlands are comprised of complex interdependent pathways where measurements of carbon exchange are often scale dependent. Common data collection methods (i.e., chambers and eddy covariance) are inherently constrained to different spatial and temporal scales which could generate biased information for applications of carbon accounting, identifying functional relationships and predicting future responses to climate change. Consequently, it is needed to systematically evaluate measurements derived from multiple approaches to identify differences and how techniques complement each other to reconcile interpretations. To accomplish this, we tested ecosystem‐scale eddy covariance with plot‐scale chamber measurements within a temperate salt marsh. We found good agreement (R² = 0.71–0.95) when comparing measurements of CH₄emissions and CO₂exchange but this agreement was dependent upon canopy phenology with discrepancies mainly arising during senescence and dormancy phenophases. The environmental drivers for CH₄and CO₂fluxes were mostly preserved across different measurement techniques, but the number of drivers increases while their individual strength decreases at the ecosystem scale. Empirical upscaling models parameterized with chamber measurements overestimated annual net ecosystem exchange (NEE; 108%) and gross primary production (GPP; 12%) while underestimating ecosystem respiration (Reco; 14%) and CH₄emissions (69%) compared to eddy covariance measurements. Our results suggest that the environmental complexity of CH₄and CO₂fluxes in salt marshes may be underestimated by chamber‐based measurements, and highlights how different techniques are complementary while considering limitations at each level of measurement.

    

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