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

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 Cdtotal 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 Cdtotal > 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. Capsule: Cd availability to S. plumbizincicola was estimated by a piecewise function in soils with wide ranges of total Cd concentration and pH. (C) 2018 Published by Elsevier B.V.