Organic sulfur and organic matter redox processes contribute to electron flow in anoxic incubations of peat
Zhi-Guo Yu A , Jörg Göttlicher B , Ralph Steininger B and Klaus-Holger Knorr A C
+ Author Affiliations
- Author Affiliations
A Ecohydrology and Biogeochemistry Group, University of Münster, Heisenbergstrasse 2, D-48149 Münster, Germany.
B ANKA Synchrotron Radiation Facility, Karlsruher Institut für Technologie (KIT), D-76344 Eggenstein-Leopoldshafen, Germany.
C Corresponding author. Email: kh.knorr@uni-muenster.de
Environmental Chemistry 13(5) 816-825 https://doi.org/10.1071/EN15091
Submitted: 7 May 2015 Accepted: 16 February 2016 Published: 7 April 2016
Environmental context. The extent to which organic matter decomposition generates carbon dioxide or methane in anaerobic ecosystems is determined by the presence or absence of particular electron acceptors. Evaluating carbon dioxide and methane production in anaerobic incubation of peat, we found that organic matter predominated as an electron acceptor over considered inorganic electron acceptors. We also observed changes in organic sulfur speciation suggesting a contribution of organic sulfur species to the electron-accepting capacity of organic matter.
Abstract. An often observed excess of CO2 production over CH4 production in freshwater ecosystems presumably results from a direct or indirect role of organic matter (OM) as electron acceptor, possibly supported by a cycling of oxidised and reduced sulfur species. To confirm the role of OM electron-accepting capacities (EACOM) in anaerobic microbial respiration and to elucidate internal sulfur cycling, peat soil virtually devoid of inorganic electron acceptors was incubated under anaerobic conditions. Thereby, production of CO2 and CH4 at a cumulative ratio of 3.2 : 1 was observed. From excess CO2 production and assuming a nominal oxidation state of carbon in OM of zero, we calculated a net consumption rate of EACOM of 2.36 µmol electron (e–) cm–3 day–1. Addition of sulfate (SO42–) increased CO2 and suppressed CH4 production. Moreover, subtracting the EAC provided though SO42–, net consumption rates of EACOM had increased to 3.88–4.85 µmol e– cm–3 day–1, presumably owing to a re-oxidation of sulfide by OM at sites otherwise not accessible for microbial reduction. As evaluated by sulfur K-edge X-ray absorption near-edge structure spectroscopy, bacterial sulfate reduction presumably involved not only a recycling of inorganic sulfur species, but also a sulfurisation of OM, yielding reduced organic sulfur, and changes in oxidised organic sulfur species. Organic matter thus contributes to anaerobic respiration: (i) directly by EAC of redox-active functional groups; (ii) directly by oxidised organic sulfur; and (iii) indirectly by re-oxidation of sulfide to maintain bacterial sulfate reduction.
Additional keywords: electron transfer, freshwater systems, methanogenesis, XANES spectroscopy.
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