Water is the vehicle for transport of solutes, including trace elements, through soil. Although solubility is a prerequisite of mobility, various rate-limited or kinetic geochemical and hydrological processes in the rhizosphere affect the transport of trace elements. The transport of dissolved trace elements in soils depends primarily on their concentration gradient spatially, and the mass flow of water. The latter is a function of the soil's matric water potential, its porosity, and long-distance preferential processes of transport through macropores. Nonequilibrium chemical or physical reactions may also occur. Physical nonequilibrium can result from nonuniform water flows and preferential transport.
The long-distance transport of trace elements in soils can be described using the convection dispersion equation (CDE), which describes the movement of solutes during transient flow : In one-dimensional form, this is dM = 4(8ds dt dz LV ' dz
• M is the total trace element concentration (mg/kg)
• 8 is the volumetric water content (m3/m3)
• C is the local trace element concentration in the soil solution (mg/L)
• Ds is the solution diffusion coefficient (m2/s)
• Sm(z) is the solute uptake, or release, by plant roots as a function of depth
Here, M is the total concentration, which is partitioned according to Equation 6.1.
Dissolved trace elements will be more mobile in sandy soils because the diffusive and connective water fluxes tend to be higher than in loams or clays. Mobility is reduced by plant uptake or sorption onto plant roots that are a sink for water and solutes. Mobility will be enhanced by plant release or desorption.
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