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

Abstract

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 (Cdtotal, Cdsoln, CdCaCl2 and CdDGT) were used to estimate Cd bioavailability and predict shoot Cd concentration (Cdshoot) using a piecewise function. Cdtotal and pH were the two major contributors to Cd uptake. Cdshoot 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 Cdshoot but when pH > 7.5 all the four measures were well correlated with Cdshoot. Piecewise equations in different ranges of Cdtotal or pH significantly improved the prediction of Cdshoot compared with the global equations derived from all the data. Compared with the piecewise equations, when pH > 7.5 Cdshoot was greatly overestimated with the global equation of Cdtotal. Our study provides useful information on the soils in which phytoextraction with S. plumbizincicola is feasible in the field. CapsuleCd availability to S. plumbizincicola was estimated by a piecewise function in soils with wide ranges of total Cd concentration and pH.