Extraction method shapes soil water-soluble organic matter composition as revealed by absorbance, fluorescence, and parallel factor analysis (PARAFAC)

Organic matter (OM) is central to biogeochemical processes in both soils and aquatic systems. Water-soluble organic matter (WSOM), leached from soil, is widely analyzed as a proxy for the mobile OM fraction, yet the chemical composition of extracts depends strongly on the extraction method used. We compared two WSOM extraction protocols-distilled water and 0.5 M K₂SO₄-across 217 soil samples from 83 depth profiles spanning four central European regions. Absorbance and fluorescence spectroscopy with PARAFAC modeling were used to characterize dissolved organic carbon (DOC) concentration and composition-approaches increasingly applied in soil science to trace soil organic matter dynamics. DOC generally declined with profile depth. K₂SO₄ extracts consistently yielded higher DOC concentrations, dominated by humic-like fluorescence. Water extracts were more variable, with stronger protein-like signals-showing clearer depth-related trends, with deeper layers enriched in microbially-derived DOM. This higher variability likely reflects the dynamic nature of labile WSOM fractions. We highlight the importance of extraction chemistry: water-based methods capture reactive, microbially-produced WSOM-likely indicators of immediate inputs to aquatic systems, whereas salt-based methods emphasize more stable pools-acting as indicators of less bio-available, long-term terrestrial reservoirs. Extraction methodology selection should consider the study objectives and specific biological and physicochemical processes investigated.