Effect of Root Water and Solute Uptake on Apparent Soil Dispersivity: A Simulation Study

Plants take up water from the root zone and thus affect the three-dimensional water flow field and solute transport processes in the soil. In this study, the impacts of root architecture, plant solute uptake mechanisms (passive, active, and solute exclusion), and plant transpiration rate on the water flow field in the soil and on solute spreading were simulated. Therefore, a fully mechanistic model was used to simulate water flow along water potential gradients in the root-soil continuum by coupling a three-dimensional Richards equation in the soil with a flow equation in the root xylem vessels. Solute transport was simulated using a three-dimensional random walk particle tracking algorithm. To quantify the effect of root water and nutrient uptake on solute transport, an equivalent one-dimensional flow and transport model was fitted to horizontally averaged simulation results, and the fitted apparent parameters were compared with the parameters of the three-dimensional model. Our simulation results showed that the apparent dispersivity length is affected by the heterogeneous flow field, caused by root water uptake, and changed in a range of 50%, depending on solute redistribution in the root zone that depends on solute uptake type and soil dispersivity length. In addition, simulation results indicate that local concentration gradients within the root zone have an impact on apparent solute uptake rate parameters used in one-dimensional models to calculate uptake rates from spatially averaged concentrations. This shows the importance of small scale three-dimensional water and solute fluxes induced by root water and nutrient uptake.