Redox processses of organic matter in a northern peatland
Tobias Heitmann (01/2006)
Support: Christian Blodau
The anaerobic mineralization of organic matter in ombrotrophic peatlands is yet poorly understood. Sulfate reduction typically accounts for a large fraction of the anaerobic electron flow and may consequently suppress production and emission of methane. Regarding the small sulfate pool in these systems, a yet unidentified anaerobic recycling mechanism for sulfate is required to support the respiratory activity of sulfate reducing bacteria. The purpose of this work was to study the interactions of dissolved organic matter (DOM) with sulfide and the potential influence of organic electron acceptors on electron transfer processes in an ombrotrophic peatland. We investigated the reaction of a reference humic acid with hydrogen sulfide in batch experiments, analyzed inorganic and organic products, and determined concentration dependencies and kinetics of these reactions. Electron accepting and donating capacities (EAC,EDC)were calculated from production of oxidized inorganic sulfur species and reaction with ferric iron to ferrous iron. DOM abiotically oxidized sulfide to thiosulfate and EAC and EDC were significantly correlated with DOM concentration. Additionally, sulfur was incorporated into DOM, likely as aryl polysulfide, based on Fourier-transformed infrared (FTIR)spectroscopy. The overall kinetics of sulfide oxidation and incorporation were relatively fast, with half-lifes of sulfide on the order of several hours. The in situ redox capacities of DOM in an ombrotrophic peatland were investigated in a field study. We determined porewater profiles of dissolved carbon (C) compounds, sulfate, and chloride, quantified electron accepting and donating capacities, and obtained C turnover rates from porewater modeling. Below water table CO2 production was 0.2–2.2 mmol m−2d−1. Methane formation contributed only < 10 %, leaving a major fraction of CO2 production unexplained. In situ redox capacities of DOM were considerable, ranging from 0.2-6.1 mequiv g−1 C (EAC) and 0.0 to 1.4 mequiv g−1 C (EDC), respectively. EAC generally decreased with depth and increased after a water table drawdown and rebound by 20 mequiv m−2 (50 %) and -45 mequiv m−2, respectively. The studies showed that compared to other environments, the electron accepting capacity (EAC) of DOM in peatlands is substantial and changes on short time scales with fluctuations in water table. These changes were on the same order as methane formation rates. DOM anaerobically oxidizes sulfide to thiosulfate sufficiently fast to explain high respiratory activity of sulfate reducing bacteria. Therefore, DOM needs to be considered as an electron acceptor within anaerobic C mineralization in northern peatlands.