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DIVISION S-1—SOIL PHYSICS

Effective Diffusion Coefficients of Soil Aggregates with Surface Skins

^a Texas A&M University, Dep. of Biological & Agricultural Engineering 201 Scoates Hall, 2117 TAMU, College Station, TX 77843-2117
^b Centre for Agricultural Landscape and Land Use Research, Dep. of Soil Landscape Research, Eberswalder Str. 84, D-15374 Müncheberg, Germany
^c 1912 Vinewood, Bryan, TX 77802

* Corresponding author (mkoehne{at}cora.tamu.edu)

Dual-permeability model simulations and sensitivity studies indicate that preferential solute leaching in structured soil is intensified by a limited solute mass transfer between preferential flow paths and matrix. It is currently unknown how much aggregate skins may affect solute mass transfer. The objective of this study was to evaluate the effect of a skin layer on the effective diffusion coefficient, D_e, of water saturated soil aggregates. In a diffusion experiment, the concentration decrease of Br^- and Cl^- in a solution being in contact with soil aggregates was measured. Aggregates with intact and removed surface skins were studied. Two different theoretical approaches were compared: (i) fitting a modified analytical solution of Crank for diffusion out of a solution of limited volume into a plane sheet to obtain D_e for intact and for skin-removed aggregates; and (ii) calibrating a numerical solution for diffusion through a two-layer system to estimate D_e of the skin layer and the interior part, separately. The D_e-values of the skins differed for Br^- and Cl^-. Using the first approach, D_e was six (Br^-) and 15 (Cl^-) times smaller for the intact than for the skin-removed aggregates. The second approach resulted in effective diffusion coefficients of the skin layer being 30 (Br^-) and 45 (Cl^-) times smaller than those of the interior parts. Aggregate skins may reduce diffusive anion transfer between interaggregate region and soil matrix, an effect which may significantly influence preferential solute transport in structured soil.

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