Analysis of micro-nutrient behaviour in the rhizosphere using a DGT parameterised dynamic plant uptake model

Mathematical models of micro-nutrient uptake by plants have been based on models designed for uptake of major nutrients. This poses a number of problems, as the behaviour of micro-nutrients in the rhizosphere is different, particularly with respect to interactions between solid and solution compartments. There is no standard procedure for the measurement of soil parameters that affect the supply of micro-nutrient from the soil. The Dynamic Plant Uptake Model (DPUM) presented here can use soil parameters for micro-nutrients that are measured under similar conditions to those that apply during plant uptake, as, for example, by the technique of DGT (Diffusive Gradients in Thin-films). The construction and practical applications of the model are discussed in the context of mathematical simulations of realistic situations. The simulations have been used to investigate conditions under which uptake of zinc by a zinc hyperaccumulator (Thlaspi caerulescens), and a non-accumulator plant (Thlaspi arvense), is limited by either processes occurring in the plant or in the soil. The size of the solid phase reservoir of zinc, and the rate at which the solid phase reservoir responds to zinc depletion in the solution phase, are important in determining the soil solution concentration at which the plant uptake kinetics begin to limit the uptake of the metal. The relative importance of advective transport of zinc from the surrounding soil to the root-soil interface depends on both the size of the solid phase reservoir and the speed at which the soil responds to zinc depletion in the pore water. These results have been used to assess conditions where DGT is likely to behave as a reliable surrogate for the transport and metal release processes that affect plant uptake.