Tuesday, 11 July 2006 - 4:20 PM

Coverage of Soil Mineral Surfaces by Organic Matter as Detected by Gas Sorption.

Rota Wagai, Kyoto University, 509-3, 2-chome, Hirano, Otsu,, Shiga, 520-2113, Japan and Lawrence M. Mayer, University of Maine, 193 Clarks Cove Rd., Walpole, ME 04573.

We review the application of gas sorption measurements to determination of organic matter coverage of mineral surfaces in soils, noting opportunities and limitations of this analytical approach as well as its corroboration by other techniques. We present new models and data to extend previous results using two gas sorption approaches. First, association with organic matter with mineral surfaces often reduces N2 sorption extent on untreated soils due to occlusion of mesoporous minerals with organic matter that adsorbs relatively little N2. Second, the average enthalpy of N2 sorption on native surfaces indicates the proportions of untreated surfaces that are organic vs. mineral. Adsorption experiments with model systems show examples of adsorption with and without occlusion of pores, showing varying impacts on measured surface area values upon saturation of adsorption sites. A variety of soils varying in their organic loading were also analyzed via these two approaches. Spodic horizons showed a tendency to coat mineral surfaces with thin coatings, consistent with highly sorptive metal oxide surfaces. Most other soils, on the other hand, showed occlusion of the majority of surface area with low-surface area organic matter, while leaving sufficient mineral surfaces uncovered to dominate the overall surface area. Full occlusion of mineral surface area typically occurs when organic matter volumes become equivalent to ca. 20-30% of mineral volumes. This occlusion mode is consistent with selective coverage of a fraction of mineral grains via aggregation or secretion processes, rather than a non-selective adsorption over all mineral surfaces. These patterns may relate to the different densities of highly reactive sorption sites on metal oxyhydroxide vs. clay particles. Surface area of clays is dominated by relatively non-reactive siloxane faces, which are likely present as internal surfaces in domain structures. For soils whose surface area is dominated by phyllosilicates, surface area values measured by N2 gas serve more as a marker for clay domain abundance than as an indicator of highly sorptive surfaces.

Back to 2.5B Interactions between Clays and Organic Matter and Their Impact on Sorption and Availability of Organic Compounds in Soil Environments - Oral
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Back to The 18th World Congress of Soil Science (July 9-15, 2006)