Direct and Indirect Effects of Water‐Table Levels on Redox‐Active Organic Matter Reduction in an Alaskan Rich Fen

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 −1 dw peat, p = 0.003) and 30–40 cm (18.0 ± 0.5 vs. 3.6 ± 1.2 μmol e − g −1 dw 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.