Electron transfer of dissolved organic matter and its potential significance for anaerobic respiration in a northern bog

We investigated electron transfer processes of dissolved organic matter (DOM) and their potential importance for anaerobic heterotrophic respiration in a northern peatland. Electron accepting and donating capacities (EAC, EDC) of DOM were quantified using dissolved H2S and ferric iron as reactants. Carbon turnover rates were obtained from porewater profiles (CO2, CH4) and inverse modeling. Carbon dioxide was released at rates of 0.2-5.9 mmol m(-2) day(-1) below the water table. Methane (CH4) formation contributed < 10%, and oxygen consumption 2% to 40%, leaving a major fraction of CO2 production unexplained. DOM oxidized H2S to thiosulfate and was reduced by dissolved ferric iron. Reduction with H2S increased the subsequently determined EDC compared to untreated controls, indicating a reversibility of the electron transfer. In situ redox capacities of DOM ranged from 0.2 to 6.1 mEq g(-1) C (EAC) and from 0.0 to 1.4 mEq g(-1) C (EDC), respectively. EAC generally decreased with depth and changed after a water table drawdown and rebound by 20 and -45 mEq m(-2), respectively. The change in EAC during the water table fluctuation was similar to CH4 formation rates. In peatlands, electron transfer of DOM may thus significantly contribute to the oxidation of reduced organic substrates by anaerobic heterotrophic respiration, or by maintaining the respiratory activity of sulfate reducers via provision of thiosulfate. Part of the anaerobic electron flow in peat soils is thus potentially diverted from methanogenesis, decreasing its contribution to the total carbon emitted to the atmosphere.