Assessing compositional variability of dissolved organic matter across different soil types and depths

Dissolved organic matter (DOM) plays a crucial role in terrestrial and aquatic ecosystems through its carbon, nutrient, and contaminant transport involvement. Its transfer from soil to surface waters is influenced by soil interactions, which alter both its quantity and composition through various physical, biological, and biochemical processes before reaching surface waters.  This study aims to characterize the DOM composition across different sites and soil depths and assess how organic surface layers (peats and forest floor) affect the DOM composition in deeper mineral horizons, representing the major source of DOM in streams of mountainous catchments. We hypothesize that while organic surface layers show greater DOM compositional variability due to different primary plant sources (e.g., leaves, roots) and different stages of microbial processing, deeper mineral subsoils will contain a more uniform set of non-sorptive and persistent compounds. Despite becoming more uniform in deeper mineral horizons, we expect DOM to maintain some characteristics from the overlying organic layers.

Soil water samples were collected from four sites representing potential terrestrial sources of stream DOM within the catchment area of the Sosa drinking water reservoir located in the Ore Mountains (Germany). Each site was characterized by a different type of soil: Peat, peaty Gleysol, Cambisol, and Podzol. Soil water was sampled from three depths (D1, underneath the organic surface layer; D2, uppermost mineral horizon; and D3, deeper mineral horizon). DOM was characterized using fluorescence spectroscopy and pyrolysis gas chromatography/mass spectrometry (Py-GC-MS), with subsequent Bray-Curtis dissimilarity analysis.

The DOM characterization revealed that across sites with mineral subsoils, the number of identified compounds (i.e., variability) decreased from the organic surface layers to the deeper mineral subsoils, while for the Peat soil, the variability slightly increased. The number of common compounds and the dissimilarity analysis indicated that the organic surface layer of the peaty Gleysol influenced the DOM composition of the underlying mineral horizons more strongly than the organic surface layers of the Cambisol and the Podzol. This stronger influence likely results from the higher water content and reduced mineral interaction in the peaty Gleysol, allowing for greater vertical transport of organic compounds. Pairwise comparisons of the number of shared compounds revealed that the DOM of the Podzol was more similar to the DOM of the peaty Gleysol than to that of the Cambisol at D1, which may be explained by comparable pH conditions and comparable microbial communities adapted to acidic, organic-rich environments. The similarity of DOM composition along the depth of the sites mostly decreased, except at the Peat, where the similarity slightly increased. In contrast to our hypothesis, we found no indications of DOM becoming increasingly uniform during the passage through the mineral subsoil. In the soil with the strongest DOM adsorption in the mineral soil (i.e. the Cambisol), DOM composition showed the largest changes with increasing depth, likely because of transformative processes adding to the changes due to sorptive fractionation.