Abstract
Soil aggregates as the basic units of soil structure greatly affect soil properties and fertilizer. They have an important theoretical and practical significance in studying the change in solute transport with different aggregate-size soil on the Loess Plateau. Based on a soil column experiment in the laboratory, breakthrough curves (BTCs) of chloride displaced through columns of different aggregate sizes of a clay soil were measured under water-saturated steady-flow conditions. The data were fitted with a convection and dispersion equation (CDE), a mobile-immobile model, and a two-flow-region model. In the CDE, all soil water was assumed to be mobile with a physical equilibrium existing in the system. For the mobile-immobile model, soil water was partitioned into mobile and immobile regions and convective diffusive solute transport was limited to the mobile water region; however, chloride transfer between the two regions was assumed to occur at a rate proportional to the difference in tracer concentration between them. In the two-flow-region model, soil water was divided into two regions based on flow velocities, but neither region had a nonzero flow rate. All three models could fit the solute transport curves in columns packed with all aggregate sizes at a small pore water velocity (0.68 cm/h), but the values of the fitted parameters varied greatly. The Péclet number derived from the mobile-immobile model and the two-flow-region model showed the same changing tendency with aggregate size, increasing as aggregate size increased. The Péclet values derived from the CDE were approximately two orders of magnitude smaller than those obtained by the mobile-immobile model and the two-flow-region model. The mobile water fraction obtained with the two-flow-region model decreased as aggregate sizes increased whereas the mass transfer coefficient ω decreased as pore water velocity increased because of the short resident time of chloride transport through the soil columns.
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