Selective adsorption of dissolved organic matter to mineral soils

As soil solutions pass through forested mineral soils, the chemical and structural compositions of dissolved organic carbon (DOC) can alter substantially due to interactions with soil particle surfaces. Typically, adsorption processes dominate in mineral soils and the resulting concentration of DOC is reduced substantially. We studied changes in the molecular and structural compositions of DOC during equilibration with mineral soils collected across Canada (n = 43) and found that the overall aromatic content of DOC decreased with equilibration in almost all cases from using specific absorbance (SUVA) and the fluorescence index. The fluorescence index revealed that podzolic B horizons, with typically large adsorption capacity (Qmax), had the greatest reduction in aromaticity, which was partially explained by the much lower aromatic content of DOC desorbed from soils surfaces. In contrast, a decrease in DOC aromaticity for volcanic B horizons, also with high Qmax, was primarily due to adsorption. An unexpected finding was the release of extremely high (2.6 × 106 Da) and low (420 Da) molecular weight (MW) organic materials during equilibration using high performance size exclusion chromatography (HPLC), for luvisols and podzols, respectively. In general, the average number–average MW (Mn) of DOC decreased for all soil types, but the greatest decrease in Mn was observed for mineral soils with large Qmax, including the podzolic and volcanic B horizons. Analysis of changes in FTIR spectra revealed that the most prominent change to DOC functional groups was a reduction in carboxyl groups, which was even greater than the removal of aromatic DOC. The findings of this study emphasize that while DOC concentrations may decrease substantially during passage through mineral soils, it is valuable to consider the contribution of DOC from desorption of pre-existing soil C. Essential to the findings of this study was the inclusion of multiple analytical techniques to track changes to DOC character, and the inclusion of a wide range of mineral soils.