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Abstract Redox‐active organic matter (RAOM) reduction is an important control on methane production in northern peatlands, but it is unclear how global climate change will affect RAOM reduction. We investigated the effects of water‐table levels on RAOM reduction by leveraging a long‐term water‐table manipulation experiment in an Alaskan fen, which includes Lowered and Raised treatment plots relative to a Control. Common substrate peat was incubated in each plot during one summer of experimental manipulation and another summer of site‐wide flooding. During experimental manipulation, common substrate RAOM was more reduced in the Raised plot than the Lowered plot at both 10–20 cm (19.1 ± 0.8 vs. 0.7 ± 0.3 μmol e⁻g⁻¹dw peat,p = 0.003) and 30–40 cm (18.0 ± 0.5 vs. 3.6 ± 1.2 μmol e⁻g⁻¹dw peat,p = 0.011). During site‐wide flooding, differences in common substrate RAOM persisted with greater RAOM reduction in the Raised plot than both Control and Lowered plots (p < 0.05) and greater methane production from Raised plot common substrate. A comparison of the chemical composition of Raised and Control peat during an anaerobic laboratory incubation showed that the compounds removed during microbial processing differed between plots with a higher double bond equivalence to carbon ratio for the Raised plot (0.54 ± 0.13) compared to the Control plot (0.44 ± 0.17). Together, these field and laboratory results suggest that long‐term increases in water‐table levels can have complex effects on RAOM beyond oxygen availability with the potential to impact methane production from northern peatlands. 

Abstract Peatlands play a critical role in the global carbon (C) cycle, encompassing ∼30% of the 1,500 Pg of C stored in soils worldwide. However, this C is vulnerable to climate and land‐use change. Ecosystem models predict the impact of perturbation on C fluxes based on soil C pools, yet responses could vary markedly depending on soil organic matter (SOM) chemistry. Here, we show that one SOM functional group responds strongly to environmental factors and predicts the risk of carbon dioxide (CO₂) release from peatlands. The molecular composition of SOM in 125 peatlands differed markedly at the global scale due to variation in temperature, land‐use, vegetation, and nutrient status. Despite this variation, incubation of peat from a subset of 11 sites revealed thatO‐alkyl C (i.e., carbohydrates) was the strongest predictor of aerobic CO₂production. This explicit link provides a simple parameter that can improve models of peatland CO₂fluxes.