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  1. Abstract

    Northern peatlands are a globally significant source of methane (CH4), and emissions are projected to increase due to warming and permafrost loss. Understanding the microbial mechanisms behind patterns in CH4production in peatlands will be key to predicting annual emissions changes, with stable carbon isotopes (δ13C‐CH4) being a powerful tool for characterizing these drivers. Given that δ13C‐CH4is used in top‐down atmospheric inversion models to partition sources, our ability to model CH4production pathways and associated δ13C‐CH4values is critical. We sought to characterize the role of environmental conditions, including hydrologic and vegetation patterns associated with permafrost thaw, on δ13C‐CH4values from high‐latitude peatlands. We measured porewater and emitted CH4stable isotopes, pH, and vegetation composition from five boreal‐Arctic peatlands. Porewater δ13C‐CH4was strongly associated with peatland type, with δ13C enriched values obtained from more minerotrophic fens (−61.2 ± 9.1‰) compared to permafrost‐free bogs (−74.1 ± 9.4‰) and raised permafrost bogs (−81.6 ± 11.5‰). Variation in porewater δ13C‐CH4was best explained by sedge cover, CH4concentration, and the interactive effect of peatland type and pH (r2 = 0.50,p < 0.001). Emitted δ13C‐CH4varied greatly but was positively correlated with porewater δ13C‐CH4. We calculated a mixed atmospheric δ13C‐CH4value for northern peatlands of −65.3 ± 7‰ and show that this value is more sensitive to landscape drying than wetting under permafrost thaw scenarios. Our results suggest northern peatland δ13C‐CH4values are likely to shift in the future which has important implications for source partitioning in atmospheric inversion models.

     
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    Free, publicly-accessible full text available July 1, 2025
  2. Hernandez, Marcela (Ed.)
    ABSTRACT

    While wetlands are major sources of biogenic methane (CH4), our understanding of resident microbial metabolism is incomplete, which compromises the prediction of CH4emissions under ongoing climate change. Here, we employed genome-resolved multi-omics to expand our understanding of methanogenesis in the thawing permafrost peatland of Stordalen Mire in Arctic Sweden. In quadrupling the genomic representation of the site’s methanogens and examining their encoded metabolism, we revealed that nearly 20% of the metagenome-assembled genomes (MAGs) encoded the potential for methylotrophic methanogenesis. Further, 27% of the transcriptionally active methanogens expressed methylotrophic genes; forMethanosarcinalesandMethanobacterialesMAGs, these data indicated the use of methylated oxygen compounds (e.g., methanol), while forMethanomassiliicoccales, they primarily implicated methyl sulfides and methylamines. In addition to methanogenic methylotrophy, >1,700 bacterial MAGs across 19 phyla encoded anaerobic methylotrophic potential, with expression across 12 phyla. Metabolomic analyses revealed the presence of diverse methylated compounds in the Mire, including some known methylotrophic substrates. Active methylotrophy was observed across all stages of a permafrost thaw gradient in Stordalen, with the most frozen non-methanogenic palsa found to host bacterial methylotrophy and the partially thawed bog and fully thawed fen seen to house both methanogenic and bacterial methylotrophic activities. Methanogenesis across increasing permafrost thaw is thus revised from the sole dominance of hydrogenotrophic production and the appearance of acetoclastic at full thaw to consider the co-occurrence of methylotrophy throughout. Collectively, these findings indicate that methanogenic and bacterial methylotrophy may be an important and previously underappreciated component of carbon cycling and emissions in these rapidly changing wetland habitats.

    IMPORTANCE

    Wetlands are the biggest natural source of atmospheric methane (CH4) emissions, yet we have an incomplete understanding of the suite of microbial metabolism that results in CH4formation. Specifically, methanogenesis from methylated compounds is excluded from all ecosystem models used to predict wetland contributions to the global CH4budget. Though recent studies have shown methylotrophic methanogenesis to be active across wetlands, the broad climatic importance of the metabolism remains critically understudied. Further, some methylotrophic bacteria are known to produce methanogenic by-products like acetate, increasing the complexity of the microbial methylotrophic metabolic network. Prior studies of Stordalen Mire have suggested that methylotrophic methanogenesis is irrelevantin situand have not emphasized the bacterial capacity for metabolism, both of which we countered in this study. The importance of our findings lies in the significant advancement toward unraveling the broader impact of methylotrophs in wetland methanogenesis and, consequently, their contribution to the terrestrial global carbon cycle.

     
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  3. Riaz, Muhammad (Ed.)
    The mechanisms controlling the extraordinarily slow carbon (C) mineralization rates characteristic of Sphagnum -rich peatlands (“bogs”) are not fully understood, despite decades of research on this topic. Soluble phenolic compounds have been invoked as potentially significant contributors to bog peat recalcitrance due to their affinity to slow microbial metabolism and cell growth. Despite this potentially significant role, the effects of soluble phenolic compounds on bog peat C mineralization remain unclear. We analyzed this effect by manipulating the concentration of free soluble phenolics in anaerobic bog and fen peat incubations using water-soluble polyvinylpyrrolidone (“PVP”), a compound that binds with and inactivates phenolics, preventing phenolic-enzyme interactions. CO 2 and CH 4 production rates (end-products of anaerobic C mineralization) generally correlated positively with PVP concentration following Michaelis-Menten (M.M.) saturation functions. Using M.M. parameters, we estimated that the extent to which phenolics inhibit anaerobic CO 2 production was significantly higher in the bog—62 ± 16%—than the fen—14 ± 4%. This difference was found to be more substantial with regards to methane production—wherein phenolic inhibition for the bog was estimated at 54 ± 19%, while the fen demonstrated no apparent inhibition. Consistent with this habitat difference, we observed significantly higher soluble phenolic content in bog vs. fen pore-water. Together, these findings suggest that soluble phenolics could contribute to bogs’ extraordinary recalcitrance and high (relative to other peatland habitats) CO 2 :CH 4 production ratios. 
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  4. Permafrost thaw increases active layer thickness, changes landscape hydrology and influences vegetation species composition. These changes alter belowground microbial and geochemical processes, affecting production, consumption and net emission rates of climate forcing trace gases. Net carbon dioxide (CO 2 ) and methane (CH 4 ) fluxes determine the radiative forcing contribution from these climate-sensitive ecosystems. Permafrost peatlands may be a mosaic of dry frozen hummocks, semi-thawed or perched sphagnum dominated areas, wet permafrost-free sedge dominated sites and open water ponds. We revisited estimates of climate forcing made for 1970 and 2000 for Stordalen Mire in northern Sweden and found the trend of increasing forcing continued into 2014. The Mire continued to transition from dry permafrost to sedge and open water areas, increasing by 100% and 35%, respectively, over the 45-year period, causing the net radiative forcing of Stordalen Mire to shift from negative to positive. This trend is driven by transitioning vegetation community composition, improved estimates of annual CO 2 and CH 4 exchange and a 22% increase in the IPCC's 100-year global warming potential (GWP_100) value for CH 4 . These results indicate that discontinuous permafrost ecosystems, while still remaining a net overall sink of C, can become a positive feedback to climate change on decadal timescales. This article is part of a discussion meeting issue ‘Rising methane: is warming feeding warming? (part 2)’. 
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  5. Lewis, David B. (Ed.)
    Peatlands account for 15 to 30% of the world’s soil carbon (C) stock and are important controls over global nitrogen (N) cycles. However, C and N concentrations are known to vary among peatlands contributing to the uncertainty of global C inventories, but there are few global studies that relate peatland classification to peat chemistry. We analyzed 436 peat cores sampled in 24 countries across six continents and measured C, N, and organic matter (OM) content at three depths down to 70 cm. Sites were distinguished between northern (387) and tropical (49) peatlands and assigned to one of six distinct broadly recognized peatland categories that vary primarily along a pH gradient. Peat C and N concentrations, OM content, and C:N ratios differed significantly among peatland categories, but few differences in chemistry with depth were found within each category. Across all peatlands C and N concentrations in the 10–20 cm layer, were 440 ± 85.1 g kg -1 and 13.9 ± 7.4 g kg -1 , with an average C:N ratio of 30.1 ± 20.8. Among peatland categories, median C concentrations were highest in bogs, poor fens and tropical swamps (446–532 g kg -1 ) and lowest in intermediate and extremely rich fens (375–414 g kg -1 ). The C:OM ratio in peat was similar across most peatland categories, except in deeper samples from ombrotrophic tropical peat swamps that were higher than other peatlands categories. Peat N concentrations and C:N ratios varied approximately two-fold among peatland categories and N concentrations tended to be higher (and C:N lower) in intermediate fens compared with other peatland types. This study reports on a unique data set and demonstrates that differences in peat C and OM concentrations among broadly classified peatland categories are predictable, which can aid future studies that use land cover assessments to refine global peatland C and N stocks. 
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  6. Abstract

    Water table depth and vegetation are key controls of methane (CH4) emissions from peatlands. Microtopography integrates these factors into features called microforms. Microforms often differ in CH4emissions, but microform‐dependent patterns of belowground CH4cycling remain less clearly resolved. To investigate the impact of microtopography on belowground CH4cycling, we characterized depth profiles of the community composition and activity of CH4‐cycling microbes using 16S rRNA amplicon sequencing, incubations, and measurements of porewater CH4concentration and isotopic composition from hummocks and lawns at Sallie's Fen in NH, USA. Geochemical proxies of methanogenesis and methanotrophy indicated that microforms differ in dominant microbial CH4cycling processes. Hummocks, where water table depth is lower, had higher porewater redox potential (Eh) and higher porewater δ13C‐CH4values in the upper 30 cm than lawns, where water table depth is closer to the peat surface. Porewater δ13C‐CH4and δD‐CH3D values were highest at the surface of hummocks where the ratio of methanotrophs to methanogens was also greatest. These results suggest that belowground CH4cycling in hummocks is more strongly regulated by methanotrophy, while in lawns methanogenesis is more dominant. We also investigated controls of porewater CH4chemistry. The ratio of the relative abundance of methanotrophs to methanogens was the strongest predictor of porewater CH4concentration and δ13C‐CH4, while vegetation composition had minimal influence. As microbial community composition was strongly influenced by redox conditions but not vegetation, we conclude that water table depth is a stronger control of belowground CH4cycling across microforms than vegetation.

     
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  7. Abstract

    Permafrost peatlands are a significant source of methane (CH4) emissions to the atmosphere and could emit more CH4with continued permafrost thaw. Aerobic methane‐oxidizing bacteria may attenuate a substantial fraction of CH4emissions in thawing permafrost peatlands; however, the impact of permafrost thaw on CH4oxidation is uncertain. We measured potential CH4oxidation rates (hereafter, CH4oxidation) and their predictors using laboratory incubations and in situ porewater redox chemistry across a permafrost thaw gradient of eight thaw stages at Stordalen Mire, a permafrost peatland complex in northernmost Sweden. Methane oxidation rates increased across a gradient of permafrost thaw and differed in transitional thaw stages relative to end‐member stages. Oxidation was consistently higher in submerged fens than in bogs or palsas across a range of CH4concentrations. We also observed that CH4oxidation increased with decreasing in situ redox potential and was highest in sites with lower redox potential (Eh < 10 mV) and high water table. Our results suggest that redox potential can be used as an important predictor of CH4oxidation, especially in thawed permafrost peatlands. Our results also highlight the importance of considering transitional thaw stages when characterizing landscape‐scale CH4dynamics, because these transitional areas have different rates and controls of CH4oxidation relative to intact or completely thawed permafrost areas. As permafrost thaw increases the total area of semiwet and wet thaw stages in permafrost peatlands, CH4oxidation represents an important control on CH4emissions to the atmosphere.

     
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