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Parameters of Surface Heterogeneity from Laboratory Experiments on Soil Degradation

USDA-ARS, Systems Research Lab., Bldg. 007, Rm. 008, BARC-West, Beltsville, MD 20705

T. A. Polubesova

Inst. of Soil Science and Photosynthesis, Pushchino 142292, Russia

M. Hajnos, G. Jozefaciuk and Z. Sokolowska

Inst. of Agrophysics, Lublin 20280, Poland

* Corresponding author ( ypachepsky{at}asrr.arsusda.gov).

ABSTRACT

In this study, we examined the applicability of the theory of gas adsorption on heterogeneous surfaces to assess changes in soil particle surfaces in degrading soils. Soil degradation was simulated by laboratory treatments of gray forest soil (Udic Argiboroll), chernozem soil (Typic Haploboroll), and chestnut soil (Ustolic Orthid) samples. Water vapor adsorption data were used to estimate the frequency distribution of surface adsorption energy and to assess the strength of the surface attractive field. A modified Brunauer-Emmett-Teller equation provided a good fit of the water vapor adsorption isotherms on soil minerals for relative vapor pressures in the range of 0.05 to 0.85. This equation was used as a local isotherm equation. The piecewise distribution technique was applied to estimate the surface adsorption energy distribution. After a H₂O₂ treatment, the monolayer capacity and average adsorption energy decreased in the samples of all three soils. After cyclic wetting-drying, the adsorption energy range decreased. After silica acid treatments, the monolayer capacity decreased, and the energy distribution became much narrower. All treatments resulted in a loss of the low-energy adsorption sites and an increase in the number of layers retained by the surface at saturation vapor pressure. The exchangeable Ca content changed after the treatments in the same way as the monolayer capacity did. The exchangeable Mg content increased after organic matter oxidation and slightly decreased after the silica treatments. The exchangeable K content increased after both organic matter oxidation and the wetting-drying treatments.

Received for publication December 21, 1993.