Measurements of surface-atmosphere carbon dioxide (CO2) and methane (CH4) fluxes have been relatively sparse across the Arctic tundra and boreal biomes, causing significant uncertainties in carbon budget estimates from the region. While the availability of Arctic-boreal carbon flux data has increased substantially over the past decade, the data have remained spread across different repositories, scientific articles, and unpublished sources, making it difficult to leverage. Here we present a new dataset of monthly Arctic-boreal carbon fluxes (ABCFlux v2) across terrestrial (wetlands and uplands) and freshwater (lakes and rivers) ecosystems compiled from previous syntheses including the Arctic-boreal CO2 flux database (ABCFlux v1), the Boreal-Arctic Wetland and Lake Methane Dataset (BAWLD-CH4), and the Global River Methane Database (GRiMeDB). In addition, we consider data from general-purpose (e.g., Zenodo) and flux network repositories, literature, and site principal investigators. The dataset includes surface-atmosphere CO2 fluxes of gross primary production (GPP), ecosystem respiration (Reco), and net ecosystem exchange (NEE), alongside CH4 fluxes. For aquatic ecosystems, we split CH4 fluxes into diffusive and ebullitive flux pathways, and included potential emissions from transient storage in the water column (“storage fluxes”), alongside CO2 and CH4 concentrations dissolved in the surface water. Fluxes are measured through a variety of methods including chamber and eddy covariance techniques alongside bubble traps, ice-surveys, and concentration-based turbulence-driven modelling in aquatic ecosystems. The monthly flux data are reported together with supporting methodological and environmental metadata. The resulting ABCFlux v2 has 23 847 flux site-months, 8182 concentration site-months, and 199 seasonal observations from 1024 sites, and includes 56 139 reported fluxes (i.e. sum of GPP, Reco, NEE, and CH4 fluxes) from the years 1984 to 2024. The majority of monthly observations occurred after 1999. Wetlands had the highest number of site-month observations (8758), followed by boreal forest (6981), lotic ecosystems (6275), lentic ecosystems (3799) and upland tundra (3308). Measurements of CO2 dominated the dataset across most ecosystem types (25 222) except for lentic ecosystems, where CH4 flux site-months (3098) were more frequent than CO2 flux site-months (2915). Overall, ABCFlux v2 includes 160 % more site-months for terrestrial CO2 flux data compared to ABCFlux v1. Integrating and updating BAWLD-CH4 flux data from growing season averages to monthly fluxes resulted in 5671 site-months of chamber CH4 data compared to 762 site-years. This collaborative initiative, involving contributions from over 260 researchers, provides a comprehensive overview of the current state of the Arctic-boreal carbon flux network and its data, and serves as an important step in reducing uncertainties in Arctic-boreal carbon budgets and in enhancing our understanding of climate feedbacks. The data can be accessed at ORNL DAAC at https://doi.org/10.3334/ORNLDAAC/2448 (Virkkala et al., 2026).

Abstract. Permafrost thaw in subarctic peatlands alters ecosystem methane (CH4) fluxes. Collapsing permafrost palsas change hydrology, interstitial oxygen availability, and vegetation composition, and each of these factors contribute to net CH4 flux by influencing CH4 production, consumption and transport. However, changes in plant-mediated CH4 fluxes have mostly been estimated using aboveground characteristics, such as biomass and leaf area, leaving belowground parts (roots and rhizomes) understudied despite their direct contact to depth-dependent CH4 flux processes. Here, we explored the potential of using root and rhizome traits as proxies for plant-mediated CH4 cycling along a peatland permafrost thaw gradient in subarctic Sweden. We investigated changes in root and rhizome biomass, surface area (SA), diameter, tissue density (TD), and specific root length (SRL) along the permafrost thaw gradient, and how these traits relate to early-, middle-, peak- and season median CH4 fluxes. We utilized chamber CH4 flux and pore water CH4 concentration and isotopic measurements during the productive season. Shrub SRL, diameter and isotopic data suggested increased plant-mediated carbon substrates available for acetoclastic methanogenesis across the thaw gradient. Root TD, proxy for root porosity, decreased with thaw and had negative correlations with CH4 fluxes throughout the season. Simultaneously, herbaceous rhizome SA-CH4 flux correlations were positive and pore water CH4 concentrations were lowest in the fully thawed stage. These results indicated increasing herbaceous plant-mediated transport of acetoclastically-produced CH4 with thaw. Our study demonstrates that integrating plant belowground traits with environmental and biogeochemical data can help improve CH4 flux predictions in thawing landscapes and revealed key mechanistic insights regarding the interplay between substrate availability for methanogenesis and gas transport.

IntroductionPeatlands store up to a third of global soil carbon, and in high latitudes their litter inputs are increasing and changing in composition under climate change. Although litter significantly influences peatland carbon and nutrient dynamics by changing the overall lability of peatland organic matter, the physicochemical mechanisms of this impact—and thus its full scope—remain poorly understood.MethodsWe applied multimodal metabolomics (UPLC-HRMS, 1H NMR) paired with 13C Stable Isotope-Assisted Metabolomics (SIAM) to track litter carbon and its potential priming effects on both existing soil organic matter and carbon gas emissions. Through this approach, we achieved molecule-specific tracking of carbon transformations at unprecedented detail.ResultsOur analysis revealed several key findings about carbon dynamics in palsa peat. Microbes responded rapidly to litter addition, producing a short-term increase in CO2 emissions, fueled nearly exclusively by transformations of litter carbon. Litter inputs significantly contributed to the organic nitrogen pool through amino acids and peptide derivatives, which served as readily accessible nutrient sources for microbial communities. We traced the fate of plant-derived polyphenols including flavonoids like rutin, finding evidence of their degradation through heterocyclic C-ring fission, while accumulation of some polyphenols suggested their role in limiting overall decomposition. The SIAM approach detected subtle molecular changes indicating minimal and transient priming activity that was undetectable through conventional gas measurements alone. This transient response was characterized by brief microbial stimulation followed by rapid return to baseline metabolism. Pre-existing peat organic matter remained relatively stable; significant priming of its consumption was not observed, nor was its structural alteration.DiscussionThis suggests that while litter inputs temporarily increase CO2 emissions, they don’t sustain long-term acceleration of stored carbon decomposition or substantially decrease peat’s carbon store capacity. Our findings demonstrate how technological advancements in analytical tools can provide a more detailed view of carbon cycling processes in complex soil systems.

Microbes drive the biogeochemical cycles of earth systems, yet the long-standing goal of linking emerging genomic information, microbial traits, mechanistic ecosystem models, and projections under climate change has remained elusive despite a wealth of emerging genomic information. Here we developed a general genome-to-ecosystem (G2E) framework for integrating genome-inferred microbial kinetic traits into mechanistic models of terrestrial ecosystems and applied it at a well-studied Arctic wetland by benchmarking predictions against observed greenhouse gas emissions. We found variation in genome-inferred microbial kinetic traits resulted in large differences in simulated annual methane emissions, quantitatively demonstrating that the genomically observable variations in microbial capacity are consequential for ecosystem functioning. Applying microbial community-aggregated traits via genome relative-abundance-weighting gave better methane emissions predictions (i.e., up to 54% decrease in bias) compared to ignoring the observed abundances, highlighting the value of combined trait inferences and abundances. This work provides an example of integrating microbial functional trait-based genomics, mechanistic and pragmatic trait parameterizations of diverse microbial metabolisms, and mechanistic ecosystem modeling. The generalizable G2E framework will enable the use of abundant microbial metagenomics data to improve predictions of microbial interactions in many complex systems, including oceanic microbiomes.

Microbiome responses to warming may amplify or ameliorate terrestrial carbon loss and thus are a critical unknown in predicting climate outcomes. We quantified microbial warming response over seven years in a thawing permafrost peatland, using large-scale multi-omics data. We integrated analyses of organisms (via taxonomy), functions (via metabolic pathways and proteins), and community organization (via network structure and ecological assembly) to deeply characterize response mechanisms. This revealed unexpected and comprehensive microbiome stability. This resistance to change appeared bolstered by habitat-specific dispersal and community-level functional redundancy, particularly via versatile carbon generalists. Our findings challenge widespread assumptions of significant soil microbiome alteration with climate change, and reveal key genome-inferred processes that underlie microbiome stability. This suggests that future research should reorient towards critical habitat transitions. ### Competing Interest Statement The authors have declared no competing interest.

Global atmospheric methane concentrations are rapidly rising and becoming isotopically more depleted, implying an unresolved microbial contribution. Rising Arctic temperatures are variably altering soil methane cycling, causing consequential uncertainty in the atmospheric methane budget. We demonstrated in an Arctic wetland that below-ground microbiota and methane-cycling features parallelled above-ground plant communities. To upscale emissions, we applied machine learning to remote sensing data to identify habitats, which were assigned average emissions. To upscale dynamically, we incorporated climate data, remotely-sensed water table variation, and habitat classes into a temporally-resolved biogeochemical model, to predict methane flux and isotope dynamics. This accurately estimated more depleted 13C-methane than previously used for Arctic habitats in global source partitioning. Remote-sensing of these rapidly changing inaccessible landscapes can thus help constrain the role of the Arctic in ongoing changes in global methane emissions. ### Competing Interest Statement The authors have declared no competing interest.

Sphagnum-dominated bogs are climatically impactful systems that exhibit two puzzling characteristics: CO2:CH4 ratios are greater than those predicted by electron balance models and C decomposition rates are enigmatically slow. We hypothesized that Maillard reactions partially explain both phenomena by increasing apparent CO2 production via eliminative decarboxylation and sequestering bioavailable nitrogen (N). We tested this hypothesis using incubations of sterilized Maillard reactants, and live and sterilized bog peat. Consistent with our hypotheses, CO2 production in the sterilized peat was equivalent to 8-13% of CO2 production in unsterilized peat, and the increased formation of aromatic N compounds decreased N-availability. Numerous sterility assessments rule out biological contamination or extracellular enzyme activity as significant sources of this CO2. These findings suggest a need for a reevaluation of the fixed CO2:CH4 production ratios commonly used in wetland biogeochemical models, which could be improved by incorporating abiotic sources of CO2 production and N sequestration.

Methane emissions from the boreal-Arctic region are likely to increase due to warming and permafrost thaw, but the magnitude of increase is unconstrained. Here we show that distinguishing several wetland and lake classes improves our understanding of current and future methane emissions. Our estimate of net annual methane emission (1988-2019) was 34 (95% CI: 25-43) Tg CH4 yr(-1), dominated by five wetland (26 Tg CH4 yr(-1)) and seven lake (5.7 Tg CH4 yr(-1)) classes. Our estimate was lower than previous estimates due to explicit characterization of low methane-emitting wetland and lake classes, for example, permafrost bogs, bogs, large lakes and glacial lakes. To reduce uncertainty further, improved wetland maps and further measurements of wetland winter emissions and lake ebullition are needed. Methane emissions were estimated to increase by similar to 31% under a moderate warming scenario (SSP2-4.5 by 2100), driven primarily by warming rather than permafrost thaw.

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 CH4 production in peatlands will be key to predicting annual emissions changes, with stable carbon isotopes (delta C-13-CH4) being a powerful tool for characterizing these drivers. Given that delta C-13-CH4 is used in top-down atmospheric inversion models to partition sources, our ability to model CH4 production pathways and associated delta C-13-CH4 values is critical. We sought to characterize the role of environmental conditions, including hydrologic and vegetation patterns associated with permafrost thaw, on delta C-13-CH4 values from high-latitude peatlands. We measured porewater and emitted CH4 stable isotopes, pH, and vegetation composition from five boreal-Arctic peatlands. Porewater delta C-13-CH4 was strongly associated with peatland type, with delta C-13 enriched values obtained from more minerotrophic fens (-61.2 +/- 9.1 parts per thousand) compared to permafrost-free bogs (-74.1 +/- 9.4 parts per thousand) and raised permafrost bogs (-81.6 +/- 11.5 parts per thousand). Variation in porewater delta C-13-CH4 was best explained by sedge cover, CH4 concentration, and the interactive effect of peatland type and pH (r(2) = 0.50, p < 0.001). Emitted delta C-13-CH4 varied greatly but was positively correlated with porewater delta C-13-CH4. We calculated a mixed atmospheric delta C-13-CH4 value for northern peatlands of -65.3 +/- 7 parts per thousand and show that this value is more sensitive to landscape drying than wetting under permafrost thaw scenarios. Our results suggest northern peatland delta C-13-CH4 values are likely to shift in the future which has important implications for source partitioning in atmospheric inversion models. Plain Language Summary Peatlands are abundant across the boreal-Arctic landscape and are important sources of methane, a powerful greenhouse gas. The amount of methane emitted into the atmosphere depends on multiple factors including the organic material being decomposed and the microbial processes ("pathways") that produce methane. The different pathways of methane production leave distinct fingerprints ("stable carbon isotopes") on methane that provide information on how that methane was formed and help to trace methane in the atmosphere back to its ground source. We looked at how stable carbon isotopes change across five northern peatland locations and wanted to know what controls those changes. We found that stable carbon isotopes differ between bogs and fens and are further controlled by soil pH and the abundance of sedge vegetation present. We used this information to test how stable carbon isotopes from northern peatlands might change as the landscape becomes wetter or drier due to the impacts of climate change. We found that peatland stable carbon isotopes are most sensitive to potential landscape drying opposed to wetting which has important implications for improving methane emission models.

Quantifying the temperature sensitivity of methane (CH _4 ) production is crucial for predicting how wetland ecosystems will respond to climate warming. Typically, the temperature sensitivity (often quantified as a Q _10 value) is derived from laboratory incubation studies and then used in biogeochemical models. However, studies report wide variation in incubation-inferred Q _10 values, with a large portion of this variation remaining unexplained. Here we applied observations in a thawing permafrost peatland (Stordalen Mire) and a well-tested process-rich model ( ecosys ) to interpret incubation observations and investigate controls on inferred CH _4 production temperature sensitivity. We developed a field-storage-incubation modeling approach to mimic the full incubation sequence, including field sampling at a particular time in the growing season, refrigerated storage, and laboratory incubation, followed by model evaluation. We found that CH _4 production rates during incubation are regulated by substrate availability and active microbial biomass of key microbial functional groups, which are affected by soil storage duration and temperature. Seasonal variation in substrate availability and active microbial biomass of key microbial functional groups led to strong time-of-sampling impacts on CH _4 production. CH _4 production is higher with less perturbation post-sampling, i.e. shorter storage duration and lower storage temperature. We found a wide range of inferred Q _10 values (1.2–3.5), which we attribute to incubation temperatures, incubation duration, storage duration, and sampling time. We also show that Q _10 values of CH _4 production are controlled by interacting biological, biochemical, and physical processes, which cause the inferred Q _10 values to differ substantially from those of the component processes. Terrestrial ecosystem models that use a constant Q _10 value to represent temperature responses may therefore predict biased soil carbon cycling under future climate scenarios.

ABSTRACTWhile wetlands are major sources of biogenic methane (CH4), our understanding of resident microbial metabolism is incomplete, which compromises the prediction of CH4 emissions 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; for Methanosarcinales and Methanobacteriales MAGs, these data indicated the use of methylated oxygen compounds (e.g., methanol), while for Methanomassiliicoccales, 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.IMPORTANCEWetlands 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 CH4 formation. Specifically, methanogenesis from methylated compounds is excluded from all ecosystem models used to predict wetland contributions to the global CH4 budget. 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 irrelevant in situ and 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.

&lt;p&gt;High latitude peatlands are a significant source of atmospheric methane. Production, consumption and emission rates are spatially and temporally heterogeneous, resulting in a wide range of global estimates for the atmospheric budget of methane. Increasing temperatures in Arctic regions cause degradation of underlying permafrost, changing hydrology, vegetation and microbial communities, but the consequences of this for methane cycling, including stable methane isotopes, are poorly understood. We provide evidence of direct linkages between below ground methanogen communities and above ground plant communities that can be remotely sensed and used in model simulations to effectively predict methane and isotopic fluxes across the landscape. Combining remote sensing with biogeochemical modeling can be used to predict methane dynamics, including the fraction derived from hydrogenotrophic versus acetoclastic microbial methanogenesis. Applying this approach across heterogeneous discontinuous permafrost peatlands enables us to accurately predict isotopic emissions, which will help constrain the global role of Arctic methane emissions.&lt;/p&gt;

A new special collection invites review papers to mark the 20th anniversary of the Journal of Geophysical Research: Biogeosciences with a focus on what we have learned and what is still unknown.

Abstract Aquatic networks contribute greenhouse gases and lateral carbon (C) export from catchments. The magnitudes of these fluxes exceed the global land C sink but are uncertain. Resolving this uncertainty is important for understanding climate feedbacks. We quantified vertical methane (CH4) and carbon dioxide (CO2) emissions from lakes and streams, and lateral export of dissolved inorganic and organic carbon from a hemiboreal catchment for 3 yr. Lateral C fluxes dominated the total aquatic C flux. All aquatic C fluxes were disproportionately contributed from spatially restricted areas and/or short‐term events. Hence, consideration of local and episodic variability is vital. Temperature and runoff were the main temporal drivers for lake and stream C emissions, respectively. Whole‐catchment aquatic C emissions scaled linearly with these drivers within timeframes of stable land‐cover. Hence, temperature and runoff increase across Northern Hemisphere humid areas from climate change may yield proportional increases in aquatic C fluxes.

Permafrost thaw is a major potential feedback source to climate change as it can drive the increased release of greenhouse gases carbon dioxide (CO2 ) and methane (CH4 ). This carbon release from the decomposition of thawing soil organic material can be mitigated by increased net primary productivity (NPP) caused by warming, increasing atmospheric CO2 , and plant community transition. However, the net effect on C storage also depends on how these plant community changes alter plant litter quantity, quality, and decomposition rates. Predicting decomposition rates based on litter quality remains challenging, but a promising new way forward is to incorporate measures of the energetic favorability to soil microbes of plant biomass decomposition. We asked how the variation in one such measure, the nominal oxidation state of carbon (NOSC), interacts with changing quantities of plant material inputs to influence the net C balance of a thawing permafrost peatland. We found: (1) Plant productivity (NPP) increased post-thaw, but instead of contributing to increased standing biomass, it increased plant biomass turnover via increased litter inputs to soil; (2) Plant litter thermodynamic favorability (NOSC) and decomposition rate both increased post-thaw, despite limited changes in bulk C:N ratios; (3) these increases caused the higher NPP to cycle more rapidly through both plants and soil, contributing to higher CO2 and CH4 fluxes from decomposition. Thus, the increased C-storage expected from higher productivity was limited and the high global warming potential of CH4 contributed a net positive warming effect. Although post-thaw peatlands are currently C sinks due to high NPP offsetting high CO2 release, this status is very sensitive to the plant community's litter input rate and quality. Integration of novel bioavailability metrics based on litter chemistry, including NOSC, into studies of ecosystem dynamics, is needed to improve the understanding of controls on arctic C stocks under continued ecosystem transition.

Rising temperatures and permafrost thaw are causing significant shifts in peatland carbon (C) cycling dynamics.Given the enormous C stores contained in permafrost peatlands-~30% of current atmospheric stores-predicting future shifts will be crucial to producing accurate global climate models.To address this need, we quantitatively constrained temperature vs. CO 2 and CH 4 production relationships in Swedish bog peat overlying permafrost using temperature-controlled incubations.Combining these results with seven years of field CH 4 flux measurements (2012-2018), we then assessed the extent to which soil temperature, alongside basic field data (soil density, water table, and active layer depth), allowed accurate prediction of measured CH 4 emissions.Since the relationship between CH 4 production vs. depth was not well constrained, two assumptions were applied: (a) methane production rate is controlled by temperature from below the water table to the depth of the permafrost; (b) under frozen conditions methane production is assumed to be 0.In incubations, CO 2 and CH 4 production rates increased exponentially with temperature from 1-26°C.Measured field fluxes were distinctly higher for two of the seven years of analysis (years 2016 and 2017)-a trend that was not reflected by modeled values.Excluding these years, our production-based estimates accounted for 64-81% of observed variance in daily, weekly, and monthly fluxes.Collectively these findings indicate that (1) three variables undergoing significant changetemperature, water table, and active layer depth-significantly regulate CH 4 fluxes in permafrost peatlands; and (2) temperature-controlled incubations, combined with basic field data, are viable tools for predicting CH 4 fluxes.Additional research aimed at better constraining CH 4 production vs. depth trends with temperature will facilitate assessment of the two assumptions applied (a and b) while research regarding parameters not considered herein-i.e.porewater geochemistry and microbial community composition-is necessary for explaining the distinctly elevated methane fluxes observed in 2016 and 2017.

Permafrost thaw in northern peatlands is likely to create a positive feedback to climate change, as microbes transform soil carbon (C) into carbon dioxide (CO2) or methane (CH4). While the microbiome's encoded C -processing potential changes with thaw, the impact on substrate utilization and gas emissions is less well characterized. We therefore examined microbial C-cycling dynamics from a partially thawed Sphagnum -domi-nated bog to a fully thawed sedge-dominated fen in Stordalen Mire (68.35 degrees N, 19.05 degrees E), Sweden. We profiled C substrate utilization diversity and extent by Biolog EcoplatesTM, then tested substrate-specific hypotheses by targeted additions (of glucose, the short chain fatty acids (SCFAs) acetate and butyrate, and the organic acids galacturonic acid and p-hydroxybenzoic acid, all at field-relevant concentrations) under anaerobic conditions at 15 degrees C. In parallel we characterized microbiomes (via 16S rRNA amplicon sequencing and quantitative poly-merase chain reaction) and C gas emissions. The fen exhibited a higher substrate use diversity and faster rate of overall substrate utilization than in the bog, based on Biolog EcoplateTM incubations. Simple glucose additions (akin to a positive control) to peat microcosms fueled fermentation as expected (reflected in enriched fermenter lineages, their inferred metabolisms, and CO2 production), but also showed potential priming of anaerobic phenol degradation in the bog. Addition of SCFAs to bog and fen produced the least change in lineages and in CO2, and modest suppression of CH4 primarily in the fen, attributed to inhibition. Addition of both organic acids greatly increased the CO2:CH4 ratio in the deep peats but had distinct individual gas dynamics and impacts on microbiota. Both organic acids appeared to act as both C source and as a microbial inhibitor, with galacturonic acid also likely playing a role in electron transfer or acceptance. Collectively, these results support the impor-tance of aboveground-belowground linkages -and in particular the role of Sphagnum spp.-in supplying substrates and inhibitors that drive microbiome assembly and C processing in these dynamically changing systems. In addition, they highlight an important temporal dynamic: responses on the short time scale of incubations (which would reflect transition conditions in the field) differ from those evident at the longer scales of habitat transition, in ways consequential to C gas emissions. In the short term, substrate addition response reflected microbiome legacy (e.g., bog communities were slower to process C and better tolerated inhibitors than fen communities) but led to little overall increase in C gas production (and a high skew to CO2). At the longer time scale of bog and fen thaw stages (which are used to represent these systems in models) the concomitant shifts in plants, hydrology and microbiota attenuate microbiome legacy impacts on substrate processing and C gas emissions over time. As habitat transition areas expand under accelerating change, we hypothesize an increased role of microbiome legacy in the landscape overall, leading to a lag in the increase of CH4 emissions expected from fen expansion.

Stordalen Mire is a peatland in the discontinuous permafrost zone in arctic Sweden that exhibits a habitat gradient from permafrost palsa, to Sphagnum bog underlain by permafrost, to Eriophorum ‐dominated fully thawed fen. We used three independent approaches to evaluate the annual, multi‐decadal, and millennial apparent carbon accumulation rates (aCAR) across this gradient: seven years of direct semi‐continuous measurement of CO 2 and CH 4 exchange, and 21 core profiles for 210 Pb and 14 C peat dating. Year‐round chamber measurements indicated net carbon balance of −13 ± 8, −49 ± 15, and −91 ± 43 g C m −2 y −1 for the years 2012–2018 in palsa, bog, and fen, respectively. Methane emission offset 2%, 7%, and 17% of the CO 2 uptake rate across this gradient. Recent aCAR indicates higher C accumulation rates in surface peats in the palsa and bog compared to current CO 2 fluxes, but these assessments are more similar in the fen. aCAR increased from low millennial‐scale levels (17–29 g C m −2 y −1 ) to moderate aCAR of the past century (72–81 g C m −2 y −1 ) to higher recent aCAR of 90–147 g C m −2 y −1 . Recent permafrost collapse, greater inundation and vegetation response has made the landscape a stronger CO 2 sink, but this CO 2 sink is increasingly offset by rising CH 4 emissions, dominated by modern carbon as determined by 14 C. The higher CH 4 emissions result in higher net CO 2‐equivalent emissions, indicating that radiative forcing of this mire and similar permafrost ecosystems will exert a warming influence on future climate.