Solvent-extractable lipids of Sphagnum, Carex, Bryales and Carex-Bryales peats: content and compositional features vs peat humification

Abstract

The effect of humification on the content and composition of the lipids in Sphagnum, Carex, Bryales and Carex-Bryales peats has been studied by capillary GC and GC-MS. The content of lipidic matter and those of its principal parts, i.e. lipid monomers (30–50% of total lipids), non-volatile matter (30–50%) and other volatiles (10–20%) mainly increased with increasing humification in all peat types. The extract content increased in the order Bryales, Carex-Bryales Sphagnum Carex, ranging from the average 4–5% on dry weight basis in Bryales and Carex-Bryales peats up to 11% in highly humified Carex peat. Fatty and ω-hydroxy acids comprised 65–90% of the lipid monomer fraction; normal 1-alkanols and sterols together comprised up to 25%. Sphagnum peat was rich in sterols, Carex peat contained high amounts of fatty and ω-hydroxy acids, and Bryales peat showed a comprehensive 1-alkanol content. In Carex-Bryales peat the characteristics of both Bryales and Carex peats were found. Peat lipids originate in mosses and higher plants. Under oxygen-poor or anaerobic conditions they are, however, subjected to microbial changes, e.g. to hydrogenation and degradation, as reflected by the stanol/stenol ratios and changes to the CPI values of n-alkanes. The one-carbon loss in the degradation of n-alkanoic acids was also found to occur under aerobic conditions in the low-decomposed Sphagnum peat samples. The long-chain ω-hydroxyalkanoic acids with other longer-chain (⩾C 22) lipid monomers accumulate in polymerized lipids, resulting in increased decay resistance of the various peat-forming plants. Polymerized polyhydroxyalkanoic (9,16- and 10,16-dihydroxyhexadecanoic and 9,10,18-trihydroxyoctadecanoic) acids are thought to be responsible for the especially high decay resistance of Sphagnum mosses. It also seems likely that the primary reaction type changes from degradation to polymerization at the low-decomposed (H3H4 on the von Post scale) stage of peat.