Estimating cadmium availability to the hyperaccumulator Sedum plumbizincicola in a wide range of soil types using a piecewise function

Highlights: • Estimating Cd availability to Sedum plumbizincicola is necessary for field trial. • Nonlinear relationships were found among shoot Cd concentration and soil properties. • Piecewise equations significantly improved the prediction of shoot Cd concentration. • Phytoextraction is feasible for the acid soils with Cd < 5.00 mg kg⁻¹. Estimating the bioavailability and predicting the uptake of metals to hyperaccumulators is very important in developing the field application of phytoextraction. A pot experiment was conducted using 108 agricultural soils covering a wide range of soil properties by the cadmium (Cd) hyperaccumulator Sedum plumbizincicola. The contributions of a range of soil properties to Cd uptake were quantified. Soil total, soluble, CaCl₂-extractable and diffusive gradients in thin films (DGT)-extractable Cd concentrations (Cd_total, Cd_soln, Cd_CaCl2 and Cd_DGT) were used to estimate Cd bioavailability and predict shoot Cd concentration (Cd_shoot) using a piecewise function. Cd_total and pH were the two major contributors to Cd uptake. Cd_shoot showed a logarithmic increase with Cd_total from 0.30 to 10.0 mg kg⁻¹ but no further increase when Cd levels exceeded 10 mg kg⁻¹. Soil pH had a discernible negative effect on Cd bioavailability from pH 5.5 to 7.5 but a weak influence at pH < 5.5 or pH > 7.5. This indicates that the optimum pH for phytoextraction with S. plumbizincicola was ~5.5 and lower pH produced little increase in shoot Cd uptake. DGT gave the best estimation of Cd bioavailability across all the data. When Cd_total > 10 mg kg⁻¹, none of the four measures was accurate enough to predict Cd_shoot but when pH > 7.5 all the four measures were well correlated with Cd_shoot. Piecewise equations in different ranges of Cd_total or pH significantly improved the prediction of Cd_shoot compared with the global equations derived from all the data. Compared with the piecewise equations, when pH > 7.5 Cd_shoot was greatly overestimated with the global equation of Cd_total. Our study provides useful information on the soils in which phytoextraction with S. plumbizincicola is feasible in the field.