Isotope analysis of pyrolysis products from Sphagnum peat and dissolved organic matter from bog water

Elemental analyses (H, N, C, O, S), bulk isotope analyses (13C, 15N) and 13C content of pyrolysis products from Sphagnum moss, underlying peat from a bog and the bog water (Hohlohsee, Black Forest, Germany) were performed to identify humification processes in the early diagenesis of peat formation and to determine the origin of dissolved organic matter (DOM) in the bog lake. Atomic ratios of bulk elemental analysis suggest a compositional shift from carbohydrate dominated structures to phenolic polymers. Observed variations of bulk δ15N and total nitrogen are likely due to microbial production. Combining isotopic and structural information using a coupled Py-GC/MS-IRMS system provides further information on the peat formation, e.g. biogeochemical processes of (1) biological degradation of source material, (2) selective preservation of individual compounds, and (3) formation of microbial biomass (e.g. trophic level effect). In particular, the stable isotope data helped to identify microbial processes as a source for carbohydrate pyrolysis products in deeper horizons. The comparison of pyrolysis products from peat and DOM from the bog lake indicates different sources. From the isotope ratios of pyrolysates it is suggested that DOM is formed in situ by microbial production and not simply dissolved from the peat profile. The uptake of 13C depleted carbon, e.g. respired CO2, CO2 from the oxidation of methane or acetic acid is proposed as an important factor in the formation of dissolved organic matter.