Seasonal and interannual variations in methane emissions and carbon and hydrogen isotopes from an upland ombrotrophic peat bog

Despite their importance for global CH4 biogeochemistry, there is limited evidence on seasonal patterns in CH4 stable isotope composition from peatlands. We conducted a 2.5-year study investigating seasonal changes in CH4 fluxes and dual-stable isotopes (δ13C, δ2H) in an ombrotrophic peat bog in Northern England. We performed chamber flux measurements and incubation experiments to quantify CH4 and CO2 fluxes, CH4 isotopes, and CH4 production and oxidation potential. CH4 fluxes varied significantly across seasons and years, with exceptionally high emissions (>45,000 μmol m−2 h−1) recorded in summer 2016, contrasting with median fluxes of 10.6–91.8 μmol m−2 h−1. Fluxes correlated with soil temperature and soil solution DOC and color, but not with water table depth, which had limited variability. Laboratory CH4 production potentials peaked below the water table at 30–40 cm depths, while oxidation potential decreased with depth. Carbon and hydrogen isotope δ-values ranged from −88.4‰ to −73.2‰ and −345.3‰ to −275.6‰, respectively and showed opposing trends as ẟ13C increased with CH4 flux while ẟ2H decreased. This is consistent with increasing fluxes being driven by acetoclastic methanogenesis. Our results highlight that seasonal shifts in CH4 emissions are accompanied by changes in isotopic signatures, even without major changes in water table or CH4 oxidation. Incorporating temporal variability into isotope-based CH4 source attribution may therefore improve constraints on peatland emissions in global CH4 budgets.