Sea spray, trace elements, and decomposition patterns as possible constraints on the evolution of CH4 and CO2 concentrations and isotopic signatures in oceanic ombrotrophic bogs
Ombrotrophic, oceanic bogs in southern Patagonia have not yet been studied with respect to ongoing belowground organic matter decomposition. To obtain such information we analyzed three sites differing in precipitation and sea spray input and quantified concentration patterns and 12/13C isotopic composition of CO2 and CH4 and iron, sulfur and trace metal contents that can influence decomposition. Concentrations of CO2 and CH4 increased with depth and reached 4,000–6,000 µmol L−1 of CO2 and 500–1,400 µmol L−1 of CH4. Chamber surface fluxes ranged from 40 to 62 mmol m−2 day−1 for CO2 and were not detectable for CH4 (<0.2 mmol m−2 day−1). Lowest gaseous C concentrations and fluxes occurred at the driest site under high sea spray input, which was accompanied by a higher degree of decomposition. Isotope fractionation factors αc ranged from 1.047 to 1.077 and suggested a predominance of hydrogenotrophic methanogenesis. The lower CH4 concentrations at one particular site may have been caused by a number of processes but isotope mass balances indicated a preferential loss of CH4 at all sites, especially at the site of lowest CH4 concentrations. Low CH4 concentrations were found under a high sea spray input and higher sulfate and reduced inorganic sulfur contents, suggesting a potential for attenuation of methanogenesis by sulfate reduction. All sites were characterized by very low Nickel concentrations of mostly <15 nmol L−1 and low concentrations of other essential trace elements that may further inhibit methanogenesis but also methanotrophy. The Patagonian sites fell within the reported range of CO2 and CH4 concentrations and depth patterns, and isotopic composition of the gases at northern sites despite different vegetation composition and seemingly lower surface fluxes. Fairly high sulfate and low trace element concentrations due to differences in atmospheric deposition may locally modify the decomposition patterns.
KeywordsCarbon mineralization Peatlands Methanogenesis Decomposition Patagonia Stable isotopes Micronutrients
This study was in part supported by Deutsche Forschungsgemeinschaft (DFG) grants to H. Biester and C. Blodau, and a travel grant of the Deutsche Gesellschaft für Limnologie (DGL) to KH. Knorr. Stable Isotope measurements were conducted at the BayCEER Laboratory of Isotope-Biogeochemistry, headed by G. Gebauer. The assistance of Silke Hammer, Martina Rohr, Karin Söllner, Heidi Zier, Yvonne Hermanns, and Ina Reisen is greatly acknowledged. We thank Mr. and Mrs. Friedli and Mr. Fernandez for providing access to sites.
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