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

## TL;DR

Different soil extraction methods affect the composition of water-soluble organic matter, with implications for understanding soil and aquatic organic matter dynamics.

## Contribution

The study reveals how extraction methods influence the chemical composition of soil water-soluble organic matter, using absorbance, fluorescence, and PARAFAC analysis.

## Key findings

- K2SO4 extraction yields higher DOC concentrations dominated by humic-like fluorescence.
- Water extracts show stronger protein-like signals and clearer depth-related trends.
- Extraction chemistry affects the interpretation of soil organic matter dynamics and reservoirs.

## Abstract

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.