Comparison of diffusive gradients in thin film technique with traditional methods for evaluation of zinc bioavailability in soils

The technique of diffusive gradients in thin film (DGT) has been shown to be a promising tool to assess zinc (Zn) bioavailability in soils, but there exists considerable debate on its suitability. In this study, Zn bioavailability was systematically investigated in wheat- and maize-grown soils using this technique and seven traditional methods, including soil solution concentration and six widely used single-step extraction methods (HAc, EDTA, NaAc, NH4Ac, CaCl2, and MgCl2). The concentrations of Zn in the shoots and roots of these two plant species increased continuously with increasing additions of Zn to the soils, accompanied by significant decreases in shoot biomass and root biomass at Zn concentrations greater than 400 mg kg−1 for maize and 800 mg kg−1 for wheat. Zinc uptake and accumulation was higher in maize roots than in wheat roots. Both the concentrations of bioavailable Zn measured by DGT (CDGT) and soil solutions (Csol) increased linearly with increasing additions of Zn to the soils, while no strong linear relationships were observed for the extraction methods. Higher concentrations of extractable Zn, lower values of Csol, and larger values of R (i.e., the ratio of CDGT to Csol) were observed in maize-grown soils compared with those of wheat-grown soils, while the values of CDGT between the two plants were similar. These findings demonstrate that there likely exists a stronger resupply of Zn from the soil solid phases in maize-grown soils to satisfy a higher Zn uptake and accumulation in this plant. Linear correlation analyses showed that CDGT had the highest correlation coefficients with plant Zn concentrations compared with other traditional methods, implying that the DGT technique is more sensitive and robust in reflecting Zn bioavailability in soils to plants.