Measuring and modelling the transport and root uptake of chemicals in the unsaturated zone

To determine the mechanisms prescribing the movement and uptake of chemicals in the soil of the rootzone, controlled experiments were carried out in four lysimeters growing tomatoes. Each lysimeter had a depth-wise array of 9 Time Domain Reflectometry (TDR) probes to monitor the soil's water content. Chloride was used as an inert tracer, and was applied with the nutrient solution used for irrigation. Sulphate was used as a reactive tracer, and was applied as a pulse resident in the upper 100 mm of the soil. The measured water contents and the concentrations of the chemicals in the soil profile at the end of the experiment were compared to a deterministic model based on Richards' equation and the convection–dispersion equation linked with various macroscopic sink terms for root water and chemical uptake. The uptake function based on matric pressure head seems to describe the uptake of water and chemicals of our tomato plants best. At high soil solution concentration chloride and sulphate exclusion occurred. Our simple model could be used to describe the major features of coupled water and chemical uptake. However, our approach of inverse modelling to infer the parameters for solute transport and root uptake could not be used to distinguish between soil-based mechanisms and plant uptake mechanisms. The choice of the root water uptake model had only a small effect on the final water content profiles, but led to differences in the final solute profiles of sulphur and chloride. This indicates that tracers might provide improved determination of the uptake mechanisms.