No abstract

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 H 2 S and ferric iron as reactants. Carbon turnover rates were obtained from porewater profiles (CO 2 , CH 4 ) and inverse modeling. Carbon dioxide was released at rates of 0.2-5.9 mmol m À2 day À1 below the water table. Methane (CH 4 ) formation contributed o10%, and oxygen consumption 2% to 40%, leaving a major fraction of CO 2 production unexplained. DOM oxidized H 2 S to thiosulfate and was reduced by dissolved ferric iron. Reduction with H 2 S 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 À1 C (EAC) and from 0.0 to 1.4 mEq g À1 C (EDC), respectively. EAC generally decreased with depth and changed after a water table drawdown and rebound by 20 and À45 mEq m À2 , respectively. The change in EAC during the water table fluctuation was similar to CH 4 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.

Information about electron-transfer reactions of dissolved organic matter (DOM) is lacking. We determined electron acceptor and donor capacities (EAC and EDC) of a peat humic acid and an untreated peat DOM by electrochemical reduction and reduction with metallic Zn and H2S (EAC), and by oxidation with complexed ferric iron (EDC) at pH 6.5. DOC concentrations (10-100 mg L(-1)) and pH values (4.5-8) were varied in selected experiments. EAC reached up to 6.2 mequiv x (g C)(-1) and EDC reached up to 1.52 mequiv-(g C)(-1). EDC decreased with pH and conversion of chelated to colloidal iron, and the electron-transfer capacity (ETC) was controlled by the redox potential Eh of the reactant (ETC = 1.016x Eh - 0.138; R(2) = 0.87; p = 0.05). The kinetics could be adequately described by pseudo first-order rate laws, one or two DOM pools, and time constants ranging from 2.1 x 10(-3) d-1 to 1.9 x 10(-2) d(-1) for the fast pool. Reactions were completed after 24-160 h depending on the redox couple applied. The results indicate that DOM may act as a redox buffer over electrochemical potentials ranging from -0.9 to +1.0 V.

[1] To examine belowground carbon (C) turnover in peatlands, we measured fluxes of carbon dioxide (CO 2 ) and methane (CH 4 ) by chamber measurements, estimated respiration by in situ incubations of peat, and in situ production of dissolved carbon (CO 2 ; CH 4 ; and dissolved organic carbon, DOC) by pore water modeling at an ombrotrophic temperate bog. Ecosystem respiration (ER) averaged 205 mmol m À2 d À1 in summer and was related to temperature, but not water table position, and in situ rates of heterotrophic respiration in the unsaturated zone were also temperature-dependent, with Q 10 = 5.0 À 6.4. In the saturated zone, concentrations of 0.1 À 2.5 mmol L À1 (CO 2 ), 0 to 0.6 mmol L À1 (CH 4 ), and <10 À 120 mg L À1 (DOC) were recorded. Turnover was dominated by DOC unrelated to respiration, which ranged from <0.5 to 7 mmol m À2 d À1 and amounted on average to < 1% of ER. Peat decomposition constants k d were 0.060 yr À1 to 0.034 yr À1 in the unsaturated and <0.002 yr À1 in the saturated zone. Monthly averages of CH 4 fluxes ranged from 0 to 1.6 mmol m À2 d À1 and were higher than modeled diffusive fluxes when threshold concentrations for CH 4 ebullition were recorded closer to the peatland surface. Our results suggest that the saturated zone is of little relevance to ER in this dry temperate bog and that mobilization of DOC is a potentially more relevant process. Temperature is a more important control on ER than water table position because most of the ER is generated close to the peatland surface. Concurrent, moderate increases in temperature and soil moisture are thus likely to increase losses of CO 2 from ER and of CH 4 from this type of peatland.