DGT-measured fluxes explain the chloride-enhanced cadmium uptake by plants at low but not at high Cd supply

The technique of diffusive gradients in thin films (DGT) has been shown to be a promising tool to assess metal uptake by plants in a wide range of soils. With the DGT technique, diffusion fluxes of trace metals through a diffusion layer towards a resin layer are measured. The DGT technique therefore mimics the metal uptake by plants if uptake is limited by diffusion of the free ion to the plant roots, which may not be the case at high metal supply. This study addresses the capability of DGT to predict cadmium (Cd) uptake by plants at varying Cd supply. To test the performance of DGT in such conditions, we used the chloride (Cl(-)) enhancement effect, i.e. the increase in Cd solution concentrations-due to chloride complexation of Cd-and Cd uptake with increasing Cl(-) concentrations, as previously characterized in pot, field and solution culture experiments. The uptake of Cd by spinach was assessed in soil amended with Cd (0.4-10.5 mg Cd kg(-1)) and NaCl (up to 120 mM) in a factorial design. Treatments with NaNO(3) were included as a reference to correct for ionic strengths effects. The effect of Cl(-) on the shoot Cd concentrations was significant at background Cd but diminished with increasing soil Cd. Increasing Cl(-) concentrations increased the root area based Cd uptake fluxes by more than a factor of 5 at low soil Cd, but had no significant effect at high soil Cd. Short-term uptake of Cd in spinach from nutrient solutions confirmed these trends. In contrast, increasing Cl(-) concentrations increased the DGT measured fluxes by a factor of 5 at all Cd levels. As a result, DGT fluxes were able to explain soil Cl(-) effects on plant Cd concentrations at low but not at high Cd supply. This example illustrates under which conditions DGT mimics trace metal bioavailability. If biouptake is controlled by diffusive limitations, DGT should be a successful tool for predicting ion uptake across different conditions.