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

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.

Extracellular Quinones Affecting Methane Production and Methanogenic Community in Paddy Soil

“…Soil-derived humic substances (humics), accounting for at least 50 % of the organic carbon found in soil [24], can serve as electron acceptor for carbon mineralization in wetland soils [15,21,22,33]. Microbial reduction of humics coupled to organic matter oxidation was first demonstrated by Lovley et al [28], and a number of bacteria capable of utilizing humics as TEA were isolated from natural wetlands [10].…”

Review on iron availability in soil: interaction of Fe minerals, plants, and microbes

“…The chemical reduction of Fe III by HS is strongly pH dependent; as pH increases, HS are more frequently bound to metal cations and therefore have a decreased reducing ability. In soils with changing redox conditions, HS may act as redox buffers by accepting electrons from microbial respiration under anoxic conditions (Heitmann et al 2007) and, upon re-aeration, by donating electrons to oxygen. This process is very important in peatlands and bogs, where it is believed to significantly decrease methanogenesis.…”

Humic acids as electron acceptors in wetland decomposition

“…From a thermodynamic perspective, reduction of AQDS is more favorable than methanogenesis, which should lead to an increase in the CO 2 :CH 4 ratio in soils where AQDS-like humics are being used for microbial respiration; however, direct inhibition of methanogenesis by AQDS-like humics is also possible (Cervantes et al, 2000). Recent research in a Canadian peatland demonstrated that dissolved organic matter is an important electron acceptor, contributing directly (through humic reduction) or indirectly (by regenerating oxidized sulfur species for sulfate reduction) to high CO 2 :CH 4 ratios (Heitmann and Blodau, 2006;Heitmann et al, 2007). Although the total electron accepting capacity of dissolved humic substances was relatively small, it was hypothesized that the much larger pool of humic-like organic matter in the solid phase of wetland soils could be used in a similar manner (Heitmann and Blodau, 2006).…”

“…A number of researchers have hypothesized the utilization of humic substances as organic TEAs to explain the high CO 2 :CH 4 ratio observed in many wetland soils (Segers, 1998;Neubauer et al, 2005;Heitmann et al, 2007;Keller and Bridgham, 2007). Lovley et al (1996) first demonstrated that bacteria can use humic substances as organic electron acceptors, and bacteria capable of reducing the humic substance analog anthraquinone-2,6-disulfonate (AQDS) have been isolated from natural wetland environments (Coates et al, 1998).…”