Methane cycle in deep layers of boreal acidic peatlands: prevalence of acetoclastic methanogenesis and importance of anaerobic methane oxidation

Since temperatures in high latitudes have increased 2-3 times faster than the global mean over the past century, boreal peatlands as carbon reservoirs could be extremely vulnerable to future global warming. Although peat layers deeper than 0.5 m (referred as catotelm) store most of the carbon in boreal bogs, little is known about biogeochemical processes in these layers and their environmental controls. Methane production is a final stage of anaerobic organic matter degradation, crucial for understanding carbon cycling in peatlands. Here, we investigated metabolic pathways of methane production in the catotelm of boreal bogs. We collected catotelm peat samples in three typical Sphagnum-dominated raised bogs in West Siberian taiga for incubation experiments with pathway-specific inhibitor. In one of them, the Mukhrino bog, we also collected acrotelm peat samples and determined in situ depth patterns of stable isotopes in methane and its precursors, methane and short-chain organic acids concentrations and microbial community structure using 16S rRNA gene amplicon sequencing. Surprisingly, acetotrophic methanogenesis was higher in the catotelm than in the acrotelm contributing on average 81%of total methane produced deeper than 0.5 m. Enrichment of both methane carbon and hydrogen isotope compositions in unamended incubation experiments compared to theoretically expected from methanogenic pathway contributions indicated that anaerobic methane oxidation is quantitatively important process in catotelm peats. Calculated fractions of oxidized methane were 0.42 in the catotelm peat but were negligible in the acrotelm. The presence of methylotrophic methanogens indicated that methanol is an important intermediate of organic matter degradation in deep peat layers. We thus conclude that all three methanogenic pathways as well as anaerobic methane oxidation play an important role in the methane cycle of bog catotelm peats and should be accounted in biogeochemical models of wetlands.