Dissolved organic matter sorption by mineral constituents of subsoil clay fractions

The retention of dissolved organic matter in soils is mainly attributed to interactions with the clay fraction. Yet, it is unclear to which extent cer tain clay-sized soil constituents contribute to the sorption of dissolved o rganic matter. In order to identify the mineral constituents controlling th e sorption of dissolved organic matter, we carried out experiments on bulk samples and differently pretreated clay-size separates (untreated, organic matter oxidation with H2O2, and organic matter oxidation with H2O2 + extrac tion of At and Fe oxides) from subsoil horizons of four Inceptisols and one Alfisol. The untreated clay separates of the subsoils sorbed 85 to 95% of the dissolved organic matter the whole soil sorbed. The sorption of the cla y fraction increased when indigenous organic matter was oxidized by H2O2. S ubsequent extraction of Al and Fe oxides/hydroxides caused a sharp decrease of the sorption of dissolved organic matter. This indicated that these oxi des/ hydroxides in the clay fraction were the main sorbents of dissolved or ganic matter of the investigated soils. Moreover, the coverage of these sor bents with organic matter reduced the amount of binding sites available for further sorption. The non-expandable layer silicates, which dominated the investigated clay fractions, exhibited a weak sorption of dissolved organic matter. Whole soils and untreated clay fractions favored the sorption of " hydrophobic" dissolved organic matter. The removal of oxides/hydroxides red uced the sorption of the lignin-derived "hydrophobic" dissolved organic mat ter onto the remaining layer silicates stronger than that of "hydrophilic" dissolved organic matter.