Extraction approach influences soil water-soluble organic matter: Insights from absorbance, fluorescence, and PARAFAC analysis

Water-soluble organic matter (WSOM) plays a key role in soil and aquatic biogeochemical processes. As a mobile fraction of soil organic matter (OM), WSOM is commonly studied to understand OM dynamics, yet its chemical composition is strongly influenced by the extraction method employed. Here, we evaluated two WSOM extraction techniques—distilled water and 0.5M K₂SO₄—across 217 soil samples from 83 profiles spanning four central European regions. We applied absorbance and fluorescence spectroscopy combined with Parallel Factor Analysis (PARAFAC) to assess dissolved organic carbon (DOC) concentrations and composition, approaches increasingly used to trace soil OM transformation. DOC concentrations generally decreased with depth. K₂SO₄ extracts yielded consistently higher DOC levels, dominated by humic-like fluorescence components, whereas water extracts showed greater variability, with stronger protein-like signatures and more pronounced depth-related trends, suggesting enrichment of microbially-derived DOM in deeper layers. These differences highlight the role of extraction chemistry: water-based methods preferentially recover reactive, microbially-produced WSOM that may reflect short-term inputs to aquatic systems, while salt-based extractions emphasize more stable, less bioavailable pools, indicative of long-term terrestrial OM reservoirs. Selecting the appropriate extraction approach is therefore critical for addressing specific ecological or biogeochemical questions.