Stable carbon isotope fractionation during methanogenesis in three boreal peatland ecosystems

The degradation of organic matter to CH4 and CO2 was investigated in three different boreal peatland sys- tems in Finland, a mesotrophic fen (MES), an oligotrophic fen (OLI), and an ombrotrophic peat (OMB). MES had sim- ilar production rates of CO2 and CH4, but the two nutrient- poor peatlands (OLI and OMB) produced in general more CO2 than CH4. δ 13C analysis of CH4 and CO2 in the pres- ence and absence methyl fluoride (CH3F), an inhibitor of acetoclastic methanogenesis, showed that CH4 was predom- inantly produced by hydrogenotrophic methanogenesis and that acetoclastic methanogenesis only played an important role in MES. These results, together with our observations concerning the collective inhibition of CH4 and CO2 pro- duction rates by CH3F, indicate that organic matter was de- graded through different paths in the mesotrophic and the nutrient-poor peatlands. In the mesotrophic fen, the ma- jor process is canonical fermentation followed by aceto- clastic and hydrogenotrophic methanogenesis, while in the nutrient-poor peat, organic matter was apparently degraded to a large extent by a different path which finally involved hydrogenotrophic methanogenesis. Our data suggest that degradation of organic substances in the oligotrophic envi- ronments was incomplete and involved the use of organic compounds as oxidants.Peer reviewe