Controls on Stable Methane Isotope Values in Northern Peatlands and Potential Shifts in Values under Permafrost Thaw Scenarios

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.