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1. Rising plant‐mediated methane emissions from arctic wetlands

   https://doi.org/10.1111/gcb.13469  

   Andresen, Christian G. ; Lara, Mark J. ; Tweedie, Craig E. ; Lougheed, Vanessa L. ( September 2016 , Global Change Biology)

   Plant‐mediatedCH₄flux is an important pathway for land–atmosphereCH₄emissions, but the magnitude, timing, and environmental controls, spanning scales of space and time, remain poorly understood in arctic tundra wetlands, particularly under the long‐term effects of climate change.CH₄fluxes were measuredin situduring peak growing season for the dominant aquatic emergent plants in the Alaskan arctic coastal plain,Carex aquatilisandArctophila fulva, to assess the magnitude and species‐specific controls onCH₄flux. Plant biomass was a strong predictor ofA. fulvaCH₄flux while water depth and thaw depth were copredictors forC. aquatilisCH₄flux. We used plant and environmental data from 1971 to 1972 from the historic International Biological Program (IBP) research site near Barrow, Alaska, which we resampled in 2010–2013, to quantify changes in plant biomass and thaw depth, and used these to estimate species‐specific decadal‐scale changes inCH₄fluxes. A ~60% increase inCH₄flux was estimated from the observed plant biomass and thaw depth increases in tundra ponds over the past 40 years. Despite covering only ~5% of the landscape, we estimate that aquaticC. aquatilisandA. fulvaaccount for two‐thirds of the total regionalCH₄flux of the Barrow Peninsula. The regionally observed increases in plant biomass and active layer thickening over the past 40 years not only have major implications for energy and water balance, but also have significantly altered land–atmosphereCH₄emissions for this region, potentially acting as a positive feedback to climate warming.

    

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2. 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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3. Experimental Soil Warming and Permafrost Thaw Increase CH 4 Emissions in an Upland Tundra Ecosystem

   https://doi.org/10.1029/2021JG006376  

   Taylor, M. A. ; Celis, G. ; Ledman, J. D. ; Mauritz, M. ; Natali, S. M. ; Pegoraro, E. ‐F. ; Schädel, C. ; Schuur, E. A. G. ( November 2021 , Journal of Geophysical Research: Biogeosciences)

   Rapid Arctic warming is causing permafrost to thaw and exposing large quantities of soil organic carbon (C) to potential decomposition. In dry upland tundra systems, subsidence from thawing permafrost can increase surface soil moisture resulting in higher methane (CH₄) emissions from newly waterlogged soils. The proportion of C released as carbon dioxide (CO₂) and CH₄remains uncertain as previously dry landscapes transition to a thawed state, resulting in both wetter and drier microsites. To address how thaw and moisture interact to affect total C emissions, we measured CH₄and CO₂emissions from paired chambers across thaw and moisture gradients created by nine years of experimental soil warming in interior Alaska. Cumulative growing season (May–September) CH₄emissions were elevated at both wetter (216.1–1,099.4 mg CH₄‐C m⁻²) and drier (129.7–392.3 mg CH₄‐C m⁻²) deeply thawed microsites relative to shallow thaw (55.6–215.7 mg CH₄‐C m⁻²) and increased with higher deep soil temperatures and permafrost thaw depth. Interannual variability in CH₄emissions was driven by wet conditions in graminoid‐dominated plots that generated >70% of emissions in a wet year. Shoulder season emissions were equivalent to growing season CH₄emissions rates in the deeply thawed, warmed soils, highlighting the importance of non‐growing season CH₄emissions. Net C sink potential was reduced in deeply thawed wet plots by 4%–42%, and by 3.5%–8% in deeply thawed drier plots due to anaerobic respiration, suggesting that some dry upland tundra landscapes may transition into stronger CH₄sources in a warming Arctic.

    

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4. Response of CO 2 and CH 4 emissions from Arctic tundra soils to a multifactorial manipulation of water table, temperature and thaw depth

   https://doi.org/10.1088/2752-664X/ad089d  

   Best, K. ; Zona, D. ; Briant, E. ; Lai, Chun-Ta ; Lipson, D. A. ; McEwing, K. R. ; Davidson, S. J. ; Oechel, W. C. ( November 2023 , Environmental Research: Ecology)

   Significant uncertainties persist concerning how Arctic soil tundra carbon emission responds to environmental changes. In this study, 24 cores were sampled from drier (high centre polygons and rims) and wetter (low centre polygons and troughs) permafrost tundra ecosystems. We examined how soil CO₂and CH₄fluxes responded to laboratory-based manipulations of soil temperature (and associated thaw depth) and water table depth, representing current and projected conditions in the Arctic. Similar soil CO₂respiration rates occurred in both the drier and the wetter sites, suggesting that a significant proportion of soil CO₂emission occurs via anaerobic respiration under water-saturated conditions in these Arctic tundra ecosystems. In the absence of vegetation, soil CO₂respiration rates decreased sharply within the first 7 weeks of the experiment, while CH₄emissions remained stable for the entire 26 weeks of the experiment. These patterns suggest that soil CO₂emission is more related to plant input than CH₄production and emission. The stable and substantial CH₄emission observed over the entire course of the experiment suggests that temperature limitations, rather than labile carbon limitations, play a predominant role in CH₄production in deeper soil layers. This is likely due to the presence of a substantial source of labile carbon in these carbon-rich soils. The small soil temperature difference (a median difference of 1 °C) and a more substantial thaw depth difference (a median difference of 6 cm) between the high and low temperature treatments resulted in a non-significant difference between soil CO₂and CH₄emissions. Although hydrology continued to be the primary factor influencing CH₄emissions, these emissions remained low in the drier ecosystem, even with a water table at the surface. This result suggests the potential absence of a methanogenic microbial community in high-centre polygon and rim ecosystems. Overall, our results suggest that the temperature increases reported for these Arctic regions are not responsible for increases in carbon losses. Instead, it is the changes in hydrology that exert significant control over soil CO₂and CH₄emissions.

    

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5. Persistent net release of carbon dioxide and methane from an Alaskan lowland boreal peatland complex

   https://doi.org/10.1111/gcb.17139  

   Euskirchen, Eugénie S. ; Edgar, Colin W. ; Kane, Evan S. ; Waldrop, Mark P. ; Neumann, Rebecca B. ; Manies, Kristen L. ; Douglas, Thomas A. ; Dieleman, Catherine ; Jones, Miriam C. ; Turetsky, Merritt R. ( January 2024 , Global Change Biology)

   Permafrost degradation in peatlands is altering vegetation and soil properties and impacting net carbon storage. We studied four adjacent sites in Alaska with varied permafrost regimes, including a black spruce forest on a peat plateau with permafrost, two collapse scar bogs of different ages formed following thermokarst, and a rich fen without permafrost. Measurements included year‐round eddy covariance estimates of net carbon dioxide (CO₂), mid‐April to October methane (CH₄) emissions, and environmental variables. From 2011 to 2022, annual rainfall was above the historical average, snow water equivalent increased, and snow‐season duration shortened due to later snow return. Seasonally thawed active layer depths also increased. During this period, all ecosystems acted as slight annual sources of CO₂(13–59 g C m⁻² year⁻¹) and stronger sources of CH₄(11–14 g CH₄ m⁻²from ~April to October). The interannual variability of net ecosystem exchange was high, approximately ±100 g C m⁻² year⁻¹, or twice what has been previously reported across other boreal sites. Net CO₂release was positively related to increased summer rainfall and winter snow water equivalent and later snow return. Controls over CH₄emissions were related to increased soil moisture and inundation status. The dominant emitter of carbon was the rich fen, which, in addition to being a source of CO₂, was also the largest CH₄emitter. These results suggest that the future carbon‐source strength of boreal lowlands in Interior Alaska may be determined by the area occupied by minerotrophic fens, which are expected to become more abundant as permafrost thaw increases hydrologic connectivity. Since our measurements occur within close proximity of each other (≤1 km²), this study also has implications for the spatial scale and data used in benchmarking carbon cycle models and emphasizes the necessity of long‐term measurements to identify carbon cycle process changes in a warming climate.

    

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