Characterization of dissolved organic matter composition along the terrestrial-aquatic continuum in the Oren Mountains, Germany

In forested catchments, soil organic matter leaching leads to the transport of dissolved organic matter (DOM) into deeper soil horizons and surface waters, directly affecting the composition and functions of surface water DOM undergoing microbial decomposition and photochemical reactions. The knowledge of the amounts and composition of DOM in the soil is, thus, fundamental for characterizing its role in aquatic ecosystems. Differences in DOM composition might be used to identify major DOM sources in streams, e.g. to distinguish the influence of peatland and upland mineral soils, considering that each source has its own typical DOM fingerprint. Furthermore, a deeper understanding of the complex interactions at the terrestrial-aquatic interface could shed a light on the role of DOM composition in nutrient cycling in surface water, which is so far not well described in the literature. Thus, the aim of this study is to present a preliminary analysis of DOM composition along the terrestrial-aquatic continuum as the basis for source identification in streams characterized by long-term increasing dissolved organic carbon concentrations.

Soil water samples from different depths and stream water samples were taken biweekly for roughly a year in the catchment area of the Sosa drinking water reservoir located in the Ore Mountains (Saxony, Germany). Four different sub-catchments were analyzed. Two of them consisted mainly of peatland and degraded peatland soils (P1 and P2, respectively) and the remaining two of mineral soils such as Podzols and Cambisols (M1 and M2). The aqueous samples (soil and surface water) and soil solid phase samples (forest floor and topsoil peat horizons) from the four sub-catchments were analyzed by pyrolysis gas chromatography mass spectrometry (Py-GC-MS) and the results were further processed in Rstudio to facilitate and standardize the chemical identification of the pyrolysates. A principal component analysis (PCA) was applied to the Rstudio results.

With the PCA results we can clearly differentiate soil samples from soil water and stream water samples in principal component 1, i.e. PC1 (25.6%). Even different layers within the soil can be separated, with the deepest layer being most similar to the stream water. Lignin-derived compounds were mainly responsible to distinguish the different sample types (i.e. soil, soil water and stream water), e.g. vanillin lactoside, guaiacol, creosol, apocynin, trans-m-propenylguaiacol, and 4-ethylguaiacol. From those, only guaiacol was found to be present in stream water of P1 and P2 and in soil water of P1, with decreasing concentration along the terrestrial-aquatic continuum (i.e. from soil to stream water). PC2 (7.2%) accounted for differences i) in soil type, i.e., peatland vs. mineral soil and ii) between soil and stream water, including the different sampling sites.

In conclusion, our research indicates that DOM composition clearly changes along the terrestrial-aquatic continuum. These differences can be used to separate two important potential DOM sources, i.e. DOM from peatlands and DOM from the forest floor horizon of mineral soils. Py-GC-MS coupled with a semiautomatic data processing routine and PCA is a very promising tool for identifying DOM from different sources in stream water samples.