Unveiling the role of soil water: Identifying primary sources of dissolved organic matter in surface waters

Dissolved organic matter (DOM) is crucial for various ecological processes, playing essential roles in carbon and nutrient cycling. In forested catchments, litter input contributes to soil organic matter, influencing DOM composition in surface waters. The transition of DOM from soil organic matter to the dissolved state significantly impacts ecological balance and highlights the role of specific soil horizons in the catchment for stream water. DOM fingerprints, reflecting variations and similarities, act as valuable indicators for identifying primary DOM sources. The increasing trend in dissolved organic carbon (DOC) concentrations in surface waters underscores the urgency to understand contributing sources comprehensively. This study aims to characterize DOM along the terrestrial-aquatic continuum, identifying sources in stream water. Soil, soil water, and stream water samples were collected biweekly for approximately two years at various depths in the Sosa drinking water reservoir catchment (Ore Mountains, Saxony, Germany). Two sub-catchments (one with a significant peatland component and one with predominantly mineral soils) were considered, each with two streams. The soil and soil water samples included four different soil types that characterize the entire catchment, i.e., intact and degraded peatland, cambisol, podzol. Pyrolysis gas chromatography-mass spectrometry (Py-GC-MS) was employed to characterize DOM in both solid and aqueous samples. Rstudio was used as a semiautomatic data processing aiming to achieve consistent compound identification. A principal component analysis (PCA) and cluster analysis were used to identify DOM sources in stream water. PCA results showed a clear distinction between solid and aqueous samples. Overlaps were observed among soil water and stream water samples, revealing shared organic compound sources and potential transfer pathways. Fluctuations and degradation patterns were noted across seasons, especially for soil water samples. Cluster analysis revealed that soil water DOM from peat horizons predominantly influenced upstream stream water DOM in peatland-dominated areas. Downstream, the DOM composition changed and was influenced by soil water from the cambisol and the podzol (mineral soils). Concerning the sub-catchment composed mainly of mineral soils, stream water reflected DOM of deep mineral horizons of cambisols and podzols. Forest floor soil water from cambisol had no to very little effect in both sub-catchments, however, soil water of the podzol forest floor slightly contributed to the streams located in the mineral soil-dominated sub-catchment. Thus, the results show that the primary source of DOM in the Sosa catchment came from soil water of deep mineral soil horizons and that stream proximity was a primary factor influencing the influx of allochthonous DOM into stream water. The research emphasized that DOM composition in the four streams closely resembled soil water DOM and analyzing the composition of the solid organic soil horizons did not help to identify the potential DOM source of the streams. Therefore, although recognizing intrinsic stream processes is important, successful source identification requires analysis of DOM in soil water from major catchment soil types. Proximity to stream water played a critical role as the predominant factor contributing to the introduction of allochthonous DOM into stream water.