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1. Permafrost Mapping with Electrical Resistivity Tomography: A Case Study in Two Wetland Systems in Interior Alaska

   https://doi.org/10.2113/JEEG19-091  

   Conaway, Christopher H. ; Johnson, Cordell D. ; Lorenson, Thomas D. ; Turetsky, Merritt ; Euskirchen, Eugénie ; Waldrop, Mark P. ; Swarzenski, Peter W. ( June 2020 , Journal of Environmental and Engineering Geophysics)

   null (Ed.)

   Surface-based 2D electrical resistivity tomography (ERT) surveys were used to characterize permafrost distribution at wetland sites on the alluvial plain north of the Tanana River, 20 km southwest of Fairbanks, Alaska, in June and September 2014. The sites were part of an ecologically-sensitive research area characterizing biogeochemical response of this region to warming and permafrost thaw, and the site contained landscape features characteristic of interior Alaska, including thermokarst bog, forested permafrost plateau, and a rich fen. The results show how vegetation reflects shallow (0–10 m depth) permafrost distribution. Additionally, we saw shallow (0–3 m depth) low resistivity areas in forested permafrost plateau potentially indicating the presence of increased unfrozen water content as a precursor to ground instability and thaw. Time-lapse study from June to September suggested a depth of seasonal influence extending several meters below the active layer, potentially as a result of changes in unfrozen water content. A comparison of several electrode geometries (dipole-dipole, extended dipole-dipole, Wenner-Schlumberger) showed that for depths of interest to our study (0–10 m) results were similar, but data acquisition time with dipole-dipole was the shortest, making it our preferred geometry. The results show the utility of ERT surveys to characterize permafrost distribution at these sites, and how vegetation reflects shallow permafrost distribution. These results are valuable information for ecologically sensitive areas where ground-truthing can cause excessive disturbance. ERT data can be used to characterize the exact subsurface geometry of permafrost such that over time an understanding of changing permafrost conditions can be made in great detail. Characterizing the depth of thaw and thermal influence from the surface in these areas also provides important information as an indication of the depth to which carbon storage and microbially-mediated carbon processing may be affected. 

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2. Methane production controls in a young thermokarst lake formed by abrupt permafrost thaw

   https://doi.org/10.1111/gcb.16151  

   Pellerin, André ; Lotem, Noam ; Walter Anthony, Katey ; Eliani Russak, Efrat ; Hasson, Nicholas ; Røy, Hans ; Chanton, Jeffrey P. ; Sivan, Orit ( March 2022 , Global Change Biology)

   Methane (CH₄) release to the atmosphere from thawing permafrost contributes significantly to global CH₄emissions. However, constraining the effects of thaw that control the production and emission of CH₄is needed to anticipate future Arctic emissions. Here are presented robust rate measurements of CH₄production and cycling in a region of rapidly degrading permafrost. Big Trail Lake, located in central Alaska, is a young, actively expanding thermokarst lake. The lake was investigated by taking two 1 m cores of sediment from different regions. Two independent methods of measuring microbial CH₄ production, long term (CH₄accumulation) and short term (¹⁴C tracer), produced similar average rates of 11 ± 3.5 and 9 ± 3.6 nmol cm⁻³ d⁻¹, respectively. The rates had small variations between the different lithological units, indicating homogeneous CH₄production despite heterogeneous lithology in the surface ~1 m of sediment. To estimate the total CH₄production, the CH₄production rates were multiplied through the 10–15 m deep talik (thaw bulb). This estimate suggests that CH₄ production is higher than emission by a maximum factor of ~2, which is less than previous estimates. Stable and radioactive carbon isotope measurements showed that 50% of dissolved CH₄in the first meter was produced further below. Interestingly, labeled¹⁴C incubations with 2‐¹⁴C acetate and¹⁴C CO₂indicate that variations in the pathway used by microbes to produce CH₄depends on the age and type of organic matter in the sediment, but did not appear to influence the rates at which CH₄ was produced. This study demonstrates that at least half of the CH₄produced by microbial breakdown of organic matter in actively expanding thermokarst is emitted to the atmosphere, and that the majority of this CH₄is produced in the deep sediment.

    

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3. Arctic riparian shrub expansion indicates a shift from streams gaining water to those that lose flow

   https://doi.org/10.1038/s43247-020-00050-1  

   Liljedahl, Anna K. ; Timling, Ina ; Frost, Gerald V. ; Daanen, Ronald P. ( November 2020 , Communications Earth & Environment)

   Shrub expansion has been observed across the Arctic in recent decades along with warming air temperatures, but tundra shrub expansion has been most pronounced in protected landscape positions such as floodplains, streambanks, water tracks, and gullies. Here we show through field measurements and laboratory analyses how stream hydrology, permafrost, and soil microbial communities differed between streams in late summer with and without tall shrubs. Our goal was to assess the causes and consequences of tall shrub expansion in Arctic riparian ecosystems. Our results from Toolik Alaska, show greater canopy height and density, and distinctive plant and soil microbial communities along stream sections that lose water into unfrozen ground (talik) compared to gaining sections underlain by shallow permafrost. Leaf Area Index is linearly related to the change in streamflow per unit stream length, with the densest canopies coinciding with increasingly losing stream sections. Considering climate change and the circumpolar scale of riparian shrub expansion, we suggest that permafrost thaw and the resulting talik formation and shift in streamflow regime are occurring across the Low Arctic.

    

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4. 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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5. Deep roots of Carex aquatilis have greater ammonium uptake capacity than shallow roots in peatlands following permafrost thaw

   https://doi.org/10.1007/s11104-021-04978-x  

   Albano, Lucas J. ; Turetsky, Merritt R. ; Mack, Michelle C. ; Kane, Evan S. ( May 2021 , Plant and Soil)

   null (Ed.)

   Aims Climate warming in northern ecosystems is triggering widespread permafrost thaw, during which deep soil nutrients, such as nitrogen, could become available for biological uptake. Permafrost thaw shift frozen organic matter to a saturated state, which could impede nutrient uptake. We assessed whether soil nitrogen can be accessed by the deep roots of vascular plants in thermokarst bogs, potentially allowing for increases in primary productivity. Methods We conducted an ammonium uptake experiment on Carex aquatilis Wahlenb. roots excavated from thermokarst bogs in interior Alaska. Ammonium uptake capacity was compared between deep and shallow roots. We also quantified differences in root ammonium uptake capacity and plant size characteristics (plant aboveground and belowground biomass, maximum shoot height, and maximum root length) between the actively-thawing margin and the centre of each thermokarst bog as a proxy for time-following-thaw. Results Deep roots had greater ammonium uptake capacity than shallow roots, while rooting depth, but not belowground biomass, was positively correlated with aboveground biomass. Although there were no differences in aboveground biomass between the margin and centre, our findings suggest that plants can benefit from investing in the acquisition of resources near the vertical thaw front. Conclusions Our results suggest that deep roots of C. aquatilis can contribute to plant nitrogen uptake and are therefore able to tolerate anoxic conditions in saturated thermokarst bogs. This work furthers our understanding of how subarctic and wetland plants respond to warming and how enhanced plant biomass production might help offset ecosystem carbon release with future permafrost thaw. 

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