Applying a mechanistic model to cadmium uptake by Zea mays and Thlaspi caerulescens:: Consequences for the assessment of the soil quantity and capacity factors

The Barber-Cushman model was evaluated for its ability to predict Cd uptake by maize (Zea mays L.) and by the Viviez ecotype of the Cd-Zn hyperaccumulator Thlaspi caerulescens (J & C Presl), both cultivated on a loamy soil enriched with cadmium sulphate (7 concentration levels). Shoots and roots were harvested after 12 and 24 days for maize and after three months for T caerulescens. The quantity factor was estimated from different chemical extractions and from the isotopic labile pool E(t) measured using isotopic exchange kinetics (IEK). The capacity factor was calculated as the ratio of the quantity factor to the Cd concentration in the soil solution. It was also directly deduced from the IEK data. When using Cd extracted with a CaCl2 0.01 mol L-1 solution as the quantity factor, the model correctly predicted the Cd uptake by maize grown in non phytotoxic contamination levels (r(2) approximate to 0.8-0.9). The ratio between E(t) and the Cd concentration in the liquid phase of the suspension used to determine E(t) also led to a correct prediction of the maize uptake. The latter was overestimated when using the amount of Cd extracted with a DTPA solution or the isotopic labile pool E(t) as the quantity factor. Where the soil Cd content exceeded 3 mg kg(-1), phytotoxicity resulted in a 25% decrease in the biomass yield, and in a reduction of the Cd uptake which could not be predicted by the model. Phytotoxicity may have modified the absorption kinetic parameters of the maize roots. When using the capacity factors which the best simulated the uptake by maize, the model underestimated the Cd uptake by T caerulescens. This was correctly simulated when using a higher capacity factor, calculated from the isotopic labile pool E(t) or from the DTPA extractable Cd. These results suggest that the soil capacity factor depends on the level of the soil's Cd depletion by the plant.