Influence of leaching solution and catchment location on the fluorescence of water-soluble organic matter

Organic matter (OM) plays a significant role in biogeochemical processes in soil and water systems. Water-soluble organic matter (WSOM) leached from soil samples is often analyzed as representative of potentially mobile OM. However, there are many WSOM extraction methods in the literature with no clear guidelines for method selection. In this study, four common leaching solutions (0.5 M K2SO4, 0.01 M CaCl2, 2 M KCl, and H2O) were used to extract WSOM from various locations within a forested catchment. Fluorescence spectroscopy was used to analyze the impact of extraction method on WSOM chemistry. While all four methods consistently identified chemical differences between WSOM from a north-facing slope, south-facing slope, and riparian zone, there were clear differences in fluorescence signals between the leaching methods. All three salt solutions contained WSOM with a higher fluorescence index and humification index than WSOM leached with H2O, suggesting the presence of salts releases different fractions of the soil organic matter. A parallel factor analysis (PARAFAC) model developed from the leachates identified a distinctive soil humic fluorophore observed in all samples and fluorescent artifacts present in H2O-leached samples.