DIFFUSION-PROCESSES IN WATER-SATURATED SPHERICAL SOIL AGGREGATES

The release curves of D2O, K+, and Cl- from saturated spherical aggreg ates of four soils, presaturated with a 10 mol m-2 KCl and 10 kg m-3 D 2O mixed solution, to a 10 mol m-3 NH4NO3 solution were analyzed to ex amine the diffusion processes in soil aggregates. Aggregates were take n from the weathering crust of a mudstone (MDR), the AB horizon of a B rown Lowland soil (BLS), the B horizon of a Brown Andosol (ADO), and t he C horizon of a Volcanogenous Regosol (PUM). The apparent diffusion coefficients of the solutes for each sample were obtained by fitting t he spherical diffusion model to the release curves. Although the relea se rates of solutes from the 2-4 mm aggregates were higher than those from the 6-8 mm aggregates, almost the same diffusion coefficients (D) were obtained for both aggregate sizes. The values of the tortuosity factor (f) which was estimated as D/D0 for D2O (D0 is the diffusion co efficient in free water) ranged from 0.25 to 0.51. Higher values were found in samples with a larger porosity. The D/D0 values for K+ in all the samples were one hundredth to half of the f values due to the cat ion adsorption effect. Equilibrium coefficients for K+ adsorption calc ulated from the D/D0 of K+ and f agreed well with those obtained indep endently as a product of the bulk density of aggregates and the slope of K+ desorption isotherms.The D/D0 values for Cl- in ADO and PUM deri ved from volcanogenous materials were lower than the f values owing to the anion adsorption effect. The equilibrium coefficients for anion a dsorption were estimated at 0.65 for ADO and 0.17 for PUM. On the othe r hand, in MDR and BLS the D/D0 values were higher than the f values, indicating the presence of the anion exclusion effect. Anion exclusion al water volume was estimated at 16% for MDR and 17% for BLS. The valu es could be explained by the model which determines the anion exclusio nal water volume as the volume of pores with a radius smaller than the anion exclusional distance from the pore wall.