Isotopic evidence for vertical diversification of methane production pathways in freshwater sediments of Nielisz reservoir (Poland)

Abstract

The paper presents the research results on methane production (CH4) pathways in freshwater sediments of the Nielisz Reservoir located in south-eastern Poland. The research was performed in spring, summer and autumn 2018 at four research stations located along the reservoir and in three sediments layers: 0–5 cm, 5–10 cm and 10–15 cm. The values of δ13C-CH4 in pore water, the fractionation coefficient αCH4-CO2, and the δ13C isotope mass balance of pore water CH4 and CO2 were used to determine the temporal and spatial changes of CH4 production pathways. The obtained values of δ13C-CH4 in the pore water of the Nielisz Reservoir ranged from −68.40‰ to −50.87‰, the αCH4-CO2 values varied from 1.04 to 1.07, and the calculated contributions of hydrogenotrophic methanogenesis in the total production of CH4 (fCH4,h) ranged from 7% to 85%. These values varied depending on the research station and the sediment layer studied. During the entire study period, both acetate and hydrogenotrophic methanogenesis participated in CH4 and the contribution of hydrogenotrophic methanogenesis increases in the deeper layers of sediment and in the autumn season. The main controlling factor of CH4 production pathways in sediments is the qualitative composition of organic matter (content of labile organic matter and humic substances).