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1. Genomic evidence that microbial carbon degradation is dominated by iron redox metabolism in thawing permafrost

   https://doi.org/10.1038/s43705-023-00326-5  

   Romanowicz, Karl J. ; Crump, Byron C. ; Kling, George W. ( November 2023 , ISME Communications)

   Microorganisms drive many aspects of organic carbon cycling in thawing permafrost soils, but the compositional trajectory of the post-thaw microbiome and its metabolic activity remain uncertain, which limits our ability to predict permafrost–climate feedbacks in a warming world. Using quantitative metabarcoding and metagenomic sequencing, we determined relative and absolute changes in microbiome composition and functional gene abundance during thaw incubations of wet sedge tundra collected from northern Alaska, USA. Organic soils from the tundra active-layer (0–50 cm), transition-zone (50–70 cm), and permafrost (70+ cm) depths were incubated under reducing conditions at 4 °C for 30 days to mimic an extended thaw duration. Following extended thaw, we found that iron (Fe)-cycling Gammaproteobacteria, specifically the heterotrophic Fe(III)-reducing Rhodoferax sp. and chemoautotrophic Fe(II)-oxidizing Gallionella sp., increased by 3–5 orders of magnitude in absolute abundance within the transition-zone and permafrost microbiomes, accounting for 65% of community abundance. We also found that the abundance of genes for Fe(III) reduction (e.g., MtrE) and Fe(II) oxidation (e.g., Cyc1) increased concurrently with genes for benzoate degradation and pyruvate metabolism, in which pyruvate is used to generate acetate that can be oxidized, along with benzoate, to CO2 when coupled with Fe(III) reduction. Gene abundance for CH4 metabolism decreased following extended thaw, suggesting dissimilatory Fe(III) reduction suppresses acetoclastic methanogenesis under reducing conditions. Our genomic evidence indicates that microbial carbon degradation is dominated by iron redox metabolism via an increase in gene abundance associated with Fe(III) reduction and Fe(II) oxidation during initial permafrost thaw, likely increasing microbial respiration while suppressing methanogenesis in wet sedge tundra.

    

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2. Assessing the Potential for Mobilization of Old Soil Carbon After Permafrost Thaw: A Synthesis of 14 C Measurements From the Northern Permafrost Region

   https://doi.org/10.1029/2020GB006672  

   Estop‐Aragonés, Cristian ; Olefeldt, David ; Abbott, Benjamin W. ; Chanton, Jeffrey P. ; Czimczik, Claudia I. ; Dean, Joshua F. ; Egan, Jocelyn E. ; Gandois, Laure ; Garnett, Mark H. ; Hartley, Iain P. ; et al ( September 2020 , Global Biogeochemical Cycles)

   The magnitude of future emissions of greenhouse gases from the northern permafrost region depends crucially on the mineralization of soil organic carbon (SOC) that has accumulated over millennia in these perennially frozen soils. Many recent studies have used radiocarbon (¹⁴C) to quantify the release of this “old” SOC as CO₂or CH₄to the atmosphere or as dissolved and particulate organic carbon (DOC and POC) to surface waters. We compiled ~1,900¹⁴C measurements from 51 sites in the northern permafrost region to assess the vulnerability of thawing SOC in tundra, forest, peatland, lake, and river ecosystems. We found that growing season soil¹⁴C‐CO₂emissions generally had a modern (post‐1950s) signature, but that well‐drained, oxic soils had increased CO₂emissions derived from older sources following recent thaw. The age of CO₂and CH₄emitted from lakes depended primarily on the age and quantity of SOC in sediments and on the mode of emission, and indicated substantial losses of previously frozen SOC from actively expanding thermokarst lakes. Increased fluvial export of aged DOC and POC occurred from sites where permafrost thaw caused soil thermal erosion. There was limited evidence supporting release of previously frozen SOC as CO₂, CH₄, and DOC from thawing peatlands with anoxic soils. This synthesis thus suggests widespread but not universal release of permafrost SOC following thaw. We show that different definitions of “old” sources among studies hamper the comparison of vulnerability of permafrost SOC across ecosystems and disturbances. We also highlight opportunities for future¹⁴C studies in the permafrost region.

    

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3. Increasing Organic Carbon Biolability With Depth in Yedoma Permafrost: Ramifications for Future Climate Change

   https://doi.org/10.1029/2018JG004712  

   Heslop, J. K. ; Winkel, M. ; Walter Anthony, K. M. ; Spencer, R. G. M. ; Podgorski, D. C. ; Zito, P. ; Kholodov, A. ; Zhang, M. ; Liebner, S. ( July 2019 , Journal of Geophysical Research: Biogeosciences)

   Permafrost thaw subjects previously frozen organic carbon (OC) to microbial decomposition, generating the greenhouse gases (GHG) carbon dioxide (CO₂) and methane (CH₄) and fueling a positive climate feedback. Over one quarter of permafrost OC is stored in deep, ice‐rich Pleistocene‐aged yedoma permafrost deposits. We used a combination of anaerobic incubations, microbial sequencing, and ultrahigh‐resolution mass spectrometry to show yedoma OC biolability increases with depth along a 12‐m yedoma profile. In incubations at 3 °C and 13 °C, GHG production per unit OC at 12‐ versus 1.3‐m depth was 4.6 and 20.5 times greater, respectively. Bacterial diversity decreased with depth and we detected methanogens at all our sampled depths, suggesting that in situ microbial communities are equipped to metabolize thawed OC into CH₄. We concurrently observed an increase in the relative abundance of reduced, saturated OC compounds, which corresponded to high proportions of C mineralization and positively correlated with anaerobic GHG production potentials and higher proportions of OC being mineralized as CH₄. Taking into account the higher global warming potential (GWP) of CH₄compared to CO₂, thawed yedoma sediments in our study had 2 times higher GWP at 12‐ versus 9.0‐m depth at 3 °C and 15 times higher GWP at 13 °C. Considering that yedoma is vulnerable to processes that thaw deep OC, our findings imply that it is important to account for this increasing GHG production and GWP with depth to better understand the disproportionate impact of yedoma on the magnitude of the permafrost carbon feedback.

    

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4. Biogeochemical and source characteristics of preferential groundwater discharge in the Farmington River watershed (Connecticut and Massachusetts, 2017 - 2021)

   https://doi.org/10.5066/P941XKST  

   Moore, Eric M ; Haynes, Adam B ; Jackson, Kevin E ; Bisson, Alaina M ; Barclay, Janey R ; Liu, Fiona ; Briggs, Martin A ; Helton, Ashley M ( January 2023 , U.S. Geological Survey)

   We used spatial data from previously mapped preferential groundwater discharges throughout the Farmington River watershed in Connecticut and Massachusetts (https://doi.org/10.5066/P915E8JY) to guide water sample collection at known locations of groundwater discharging to surface water. In 2017 and 2019 - 2021, samples were collected during general river baseflow conditions (July ? November, less than 30.9 cms mean daily discharge (USGS gage 01189995, statistics 2010-2022) when the riverbank discharge points were exposed. We collected a suite of dissolved constituents and stable isotopes of water directly in the shallow saturated sediments of active points of discharge, and coincident stream chemical samples were also collected adjacent to locations of groundwater discharge. Data collected includes nutrients (NO3, NH4, Cl, SO4, PO4, dissolved organic carbon (DOC), and total nitrogen (TN)), greenhouse gases (CO2, CH4, and N2O), dissolved gases (N2, dissolved oxygen (DO)), conductivity, sediment characteristics, temperature, and spatial information. This dataset includes 2 main files: 1) Farmington_Chemistry_2017_2021.csv contains attribute information for each biogeochemical constituent collected at preferential groundwater discharges along the Farmington River network. 2)Farmington_Temporal_Cl_Rn_Iso_2020.csv contain attribute information for source characteristic data (Chloride, Radon, Isotope) collected at locations of repeat sampling at 5 groundwater seep faces along the Farmington River (Alsop and Rainbow Island). 

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5. Geochemistry of Coastal Permafrost and Erosion-Driven Organic Matter Fluxes to the Beaufort Sea Near Drew Point, Alaska

   https://doi.org/10.3389/feart.2020.598933  

   Bristol, Emily M. ; Connolly, Craig T. ; Lorenson, Thomas D. ; Richmond, Bruce M. ; Ilgen, Anastasia G. ; Choens, R. Charles ; Bull, Diana L. ; Kanevskiy, Mikhail ; Iwahana, Go ; Jones, Benjamin M. ; et al ( January 2021 , Frontiers in Earth Science)

   null (Ed.)

   Accelerating erosion of the Alaska Beaufort Sea coast is increasing inputs of organic matter from land to the Arctic Ocean, and improved estimates of organic matter stocks in eroding coastal permafrost are needed to assess their mobilization rates under contemporary conditions. We collected three permafrost cores (4.5–7.5 m long) along a geomorphic gradient near Drew Point, Alaska, where recent erosion rates average 17.2 m year −1 . Down-core patterns indicate that organic-rich soils and lacustrine sediments (12–45% total organic carbon; TOC) in the active layer and upper permafrost accumulated during the Holocene. Deeper permafrost (below 3 m elevation) mainly consists of Late Pleistocene marine sediments with lower organic matter content (∼1% TOC), lower C:N ratios, and higher δ 13 C values. Radiocarbon-based estimates of organic carbon accumulation rates were 11.3 ± 3.6 g TOC m −2  year −1 during the Holocene and 0.5 ± 0.1 g TOC m −2  year −1 during the Late Pleistocene (12–38 kyr BP). Within relict marine sediments, porewater salinities increased with depth. Elevated salinity near sea level (∼20–37 in thawed samples) inhibited freezing despite year-round temperatures below 0°C. We used organic matter stock estimates from the cores in combination with remote sensing time-series data to estimate carbon fluxes for a 9 km stretch of coastline near Drew Point. Erosional fluxes of TOC averaged 1,369 kg C m −1  year −1 during the 21st century (2002–2018), nearly doubling the average flux of the previous half-century (1955–2002). Our estimate of the 21st century erosional TOC flux year −1 from this 9 km coastline (12,318 metric tons C year −1 ) is similar to the annual TOC flux from the Kuparuk River, which drains a 8,107 km 2 area east of Drew Point and ranks as the third largest river on the North Slope of Alaska. Total nitrogen fluxes via coastal erosion at Drew Point were also quantified, and were similar to those from the Kuparuk River. This study emphasizes that coastal erosion represents a significant pathway for carbon and nitrogen trapped in permafrost to enter modern biogeochemical cycles, where it may fuel food webs and greenhouse gas emissions in the marine environment. 

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