Chemical and spectroscopic investigation of porewater and aqueous extracts of corresponding peat samples throughout a bog core (Jura Mountains, Switzerland)

Fluorescence and UV-visible spectroscopies are simple but useful methods to characterize organic matter in the aqueous phase according to its aromatic nature and humification degree. Although there are several studies about porewater and water-extractable organic matter (WEOM) from peat, at present, no comparative investigations are available in the literature. Thus, the aim of the present study was to identify and compare chemical and spectroscopic features of porewaters and corresponding WEOM samples along a 105-cm undisturbed peat profile. The peat core was collected in June 2005 from a Swiss ombrotrophic bog and divided into slices of 1 ± 0.15-cm thickness. Porewater samples (n = 91) were extracted and analyzed for pH; from these, 30 samples were filtered (0.45 μm), analyzed for dissolved organic carbon (DOC) concentration, and characterized by means of UV-vis (E4/E6 ratio) and fluorescence spectroscopies. The same analyses were also carried out on the WEOM of the corresponding peat samples. The results show several differences between porewaters and WEOM throughout the bog profile. In particular, and with the exception of the first ca. 20 cm depth, spectroscopic data clearly underline a more “humic-like” character of DOC of WEOM compared to porewater samples. Furthermore, the trends in E4/E6 ratios of both porewaters and WEOM samples indicate that the organic matter is characterized by decreasing aromaticity, molecular weights, and degree of polycondensation throughout the profile. The high correlation among the bulk density of the peat and the DOC concentrations in both porewaters and WEOM samples demonstrates a clear stratification of the organic matter which suggests that the vertical migration of DOC is rather limited. The first ca. 20 cm of the profile shows also opposite physicochemical and spectroscopic features compared to the deeper horizons probably because this is the predominantly oxic zone with far greater physiological activity of plants and microorganisms. Assessing porewater and WEOM features throughout peat, soil, and sediment profiles is important to better understand water–sediment interface dynamics and translocation processes of organic and inorganic pollutants, especially considering that ombrotrophic bogs are often used as archives of atmospheric depositions.