Seasonal and Interannual Variations in Methane Emissions and Carbon and Hydrogen Isotopes From an Upland Ombrotrophic Peat Bog

Abstract

Despite their importance for global CH ₄ biogeochemistry, there is limited evidence on seasonal patterns in CH ₄ stable isotope composition from peatlands. We conducted a 2.5‐year study investigating seasonal changes in CH ₄ fluxes and dual‐stable isotopes (δ ¹³ C, δ ² H) in an ombrotrophic peat bog in Northern England. We performed chamber flux measurements and incubation experiments to quantify CH ₄ and CO ₂ fluxes, CH ₄ isotopes, and CH ₄ production and oxidation potential. CH ₄ 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 CH ₄ 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 ẟ ¹³ C increased with CH ₄ flux while ẟ ² H decreased. This is consistent with increasing fluxes being driven by acetoclastic methanogenesis. Our results highlight that seasonal shifts in CH ₄ emissions are accompanied by changes in isotopic signatures, even without major changes in water table or CH ₄ oxidation. Incorporating temporal variability into isotope‐based CH ₄ source attribution may therefore improve constraints on peatland emissions in global CH ₄ budgets.