The deeper, the more distinct: Dissolved organic matter composition differs between soil types and diverges with depths

Livia Vieira Carlini Charamba^1*Tobias Houska^1,2Klaus Kaiser³Klaus-Holger Knorr⁴Tobias Krause¹Huan Chen⁵Pavel Kram^6,7Jakub Hruška^6,7Ingo Müller⁸Karsten Kalbitz¹

• ¹Institute of Soil Science and Site Ecology, Department of Forest Sciences, Technische Universitat Dresden, Dresden, Germany
• ²Department of Landscape Ecology and Resource Management, Justus-Liebig-Universitat Giessen, Giessen, Germany
• ³Soil Science and Soil Protection, Martin-Luther-Universitat Halle-Wittenberg, Halle (Saale), Germany
• ⁴Institute for Landscape Ecology, Ecohydrology and Biogeochemistry Group, Universitat Munster, Münster, Germany
• ⁵Department of Environmental Engineering and Earth Science, Clemson University, Clemson, United States
• ⁶Czech Geological Survey, Prague, Czechia
• ⁷Global Change Research Institute of the Czech Academy of Sciences, Brno, Czechia
• ⁸Agriculture and Geology, Saxon State Office for Environmental, Freiberg, Germany

Dissolved organic matter (DOM) is a key component in the carbon and energy cycling of soil and aquatic ecosystems. Tracking DOM composition through soil profiles provides valuable insight into the processes driving its transport and transformation. However, there is a lack of studies investigating whether DOM composition in deeper mineral soil is largely driven by topsoil inputs, or if processes taking place during soil passage cause a rather uniform quality of DOM irrespective of the source quality. Understanding the extent of topsoil influence on subsoil DOM, as well as depth-dependent transformation patterns, is crucial for the transfer to and its fate within aquatic ecosystems. To address this knowledge gap, we examined the compositional and chemical features of DOM sampled in situ along depth profiles of four contrasting soil types (Peat, peaty Gleysol, Cambisol, Podzol) in a mountainous catchment (Ore Mountains, Germany). A combination of pyrolysis-gas chromatography/mass spectrometry and UV and fluorescence spectroscopy was used to characterize the molecular properties of DOM. Results revealed site-specific decreases in similarity with depth, driven by soil processes that progressively alter DOM composition. In Peat, composition remained rather similar across depth, likely due to constantly anoxic conditions that inhibit oxidative degradation and transformation of DOM. In the peaty Gleysol, moderate transformations were observed, likely driven by alternating redox conditions and sorptive interactions. The strongest compositional changes occurred in the Cambisol, suggesting microbial processing in conjunction with sorptive interactions with the mineral phase. In the Podzol, the formation of organo-metal complexes promoted selective preservation of aromatic structures. The site-specific processes led to decreases in both the number and abundance of identified shared compounds with depth, contrasting the assumption of DOM similarity across different soil types. Despite the changes with depth, subsoil DOM composition in Peat, peaty Gleysol, and Podzol still retained some imprint of topsoil sources. This study highlights how site-specific biotic and abiotic processing generates unique DOM composition that shape organic matter cycling in soils and its fate in aquatic systems.