Ranking of mechanisms governing the phytoavailability of cadmium in agricultural soils using a mechanistic model

Abstract

The objective of this study was to rank the mechanisms influencing the phytoavailability of cadmium (Cd) in agricultural soils. We developed a model that simulates the transport by diffusion and convection, the kinetics of sorption and complexation in solution and the root uptake of Cd. The ranking of mechanisms was performed by simulating the Cd2+ uptake by 1 cm2 root for 30 days for French agricultural soil characteristics. The initial Cd2+ concentration was the most influential parameter followed by the soil buffer power for Cd2+ and by the soil water content and impedance factor. The Cd2+ was generally strongly depleted at the root surface and the convection was almost negligible. In general, the Cd complex dissociation contributed little to the uptake due to a strong kinetics limitation. Conversely, the kinetics of sorption was little influential. The initial concentration and diffusion of Cd2+ were the dominant processes governing the phytoavailability in non-polluted soils. A model considering only the transport and sorption of Cd2+ without kinetics would be adequate to predict the phytoavailability. The particular situations where these simplifications do not hold (relative error >10 %) corresponded to a high supply of labile Cd complex toward plant roots.