Dependence of Sorption of Dissolved Organic Carbon On Soil Order Classification.

Arguments in the literature debate the extent of long-term preservation afforded by sorption of dissolved organic carbon (DOC) to soil minerals in B horizon subsoils. We tested the sorption capacity of different temperate soil orders under the premise that contrasting mineralogies could lead to different mechanisms and extents of sorptive preservation. Soil samples were selected from various sites within the eastern United States (250 samples from 73 series in 5 orders). The maximum sorption of DOC was determined by applying different concentrations of solutions of characterized humic acid (0-100 mg/L) to the soils in isotherm experiments at a solid-to-solution ratio of 1:60 for 48 h and fitting to the Langmuir equation. These samples were analyzed for pH, total organic carbon (TOC), iron oxide, and textural clay content using standard methods. Multiple regressions were performed on Ultisol, Alfisol, and Mollisol soils using different combinations of the measured soil properties. For Ultisols, the measured properties of clay and Fe-oxide accounted for ~56% variation, indicating that DOC sorption onto Ultisols is dominated by ligand and cation exchange. For Alfisols, clay and pH explained ~29% of the variation, suggesting cation exchange was an important mechanism.  Mollisol DOC sorption capacity was explained by only TOC content and clay content (r² = 0.328), indicating that organic-organic interactions were important. Readily available soil properties of these three orders now can be used to predict with greater confidence the DOC sorption capacity of subsurface soils. This information is useful for improving the process of sorptive preservation in soil productivity models, and for determining regional variations in sorptive capacity.