Comparing chemical extraction and a piecewise function with diffusive gradients in thin films for accurate estimation of soil zinc bioavailability to Sedum plumbizincicola

Accurate evaluation of metal bioavailability to hyperaccumulators is necessary for prediction of metal uptake and estimation of phytoextraction potential. Sedum plumbizincicola X.H. Guo et S.B. Zhou ex L.H. Wu (Crassulaceae), a hyperaccumulator of cadmium (Cd) and zinc (Zn), was grown in a pot experiment in 108 agricultural soils representing a wide range of typical Chinese soil types. The contribution of soil properties to Zn uptake was assessed quantitatively. Soil total Zn, soluble Zn, CaCl2-extractable Zn and (DGT)-extractable Zn concentrations (Zn-total, Zn-soln, Zn-CaCl2 and Zn-DGT, respectively) were compared to predict shoot Zn concentration (Zn-shoot) using a piecewise equation. This showed a logarithmic increase with increasing Zn-total from 100-1000 mg kg(-1) but plateaued outside this range. The optimum soil pH for Zn removal was similar to 5.5 and further acidification was unnecessary because Zn-shoot increased significantly with a decrease in pH from 7.5 to 5.5 and rarely changed at pH < 5.5. Sedum plumbizincicola grown in loamy or clay loam soils had larger Zn-shoot than in sandy or clay soils. The Zn-CaCl2 was superior to other methods in estimating Zn bioavailability across all the data. At pH < 5.5, Zn-CaCl2 showed much stronger correlation with Zn-shoot than Zn-total, Zn-soln or Zn-DGT. Piecewise equations (single linear regression equations at different intervals of soil properties) versus Zn-total and Zn-CaCl2 at different pH ranges considerably increased the accuracy of prediction of Zn-shoot compared with the global equations derived from all data. Estimation of the potential of S. plumbizincicola for Zn phytoextraction in the field indicated that it is feasible to decontaminate acid-polluted soils with Zn-total <= 500 mg kg(-1) with S. plumbizincicola. Highlights Zn bioavailability to hyperaccumulators needs to be estimated over a wide range of soil types. Piecewise equations considerably improved the prediction of shoot Zn concentration. CaCl2 extraction was more suitable than DGT for the estimation of Zn bioavailability. Decontamination with S. plumbizincicola is feasible for acid soils with Zn-total <= 500 mg kg(-1).