Solute diffusion in unsaturated soils refers to the transport of dissolved constituents in liquid phase from a higher to a lower concentration point. Several empirical and conceptual models were proposed to predict the solute diffusion coefficients in unsaturated soils, but they were not systematically tested and evaluated under the same conditions using soils of different textures. Our experimental data showed that there is no perfect model that can depict the behavior of solute diffusion coefficient in soil at ranging soil-water contents across soil textures. Therefore, we proposed a new model using matric potential as the variable. The new model combines the two-region linear relationship of the soil-water content versus logarithm tortuosity with the Campbell soil-water retention model. It has compatible predictive capability with previous empirical models while reduces the uncertainty of constants estimation. Also, the use of matric potential instead of soil-water content as the independent variable directly incorporates the soil pore structural properties, and consequently the diffusive pathway, in the solute diffusion coefficient prediction. Moreover, the new model is applicable to soils of different textures and degrees of aggregation.

Although solute diffusion in soil has been widely examined under the influence of both soil-water content and matric potential, little information is available about the impact of soil-water content/matric potential hysteresis on solute diffusion. Our research shows that the unequal soil-water content at a given matric potential when a soil is subjected to drying or wetting process has substantial impact on solute diffusion. The measured solute diffusion coefficients of drying were greater than those of wetting in certain soil-water content range. When fitted the solute diffusion coefficients of drying and wetting limbs by proposed predictive models, the drying limb is better described by a power function of soil-water content with empirical constants estimated by particle-size distribution and bulk density; while the wetting limb is better described by a conceptual model which assumes the solute diffusive pathway is comprised of the serial arrangement of pore water and film water. This research concluded that the behavior of solute diffusion in unsaturated soils is essentially determined by the liquid phase distribution.