Effects of soil drying and wetting-drying cycles on the availability of heavy metals and their relationship to dissolved organic matter

Variation in soil moisture content can change metal availability. However, the effects of soil drying during wetting-drying cycles on metal availability have been little investigated. Metal availability to an imposed sink can be assessed by the diffusive gradients in thin film (DGT) technique. The purpose of this study was therefore to investigate the effects of the soil drying processes and previous soil moisture contents on metal availability using DGT. Two metal-polluted agricultural soils with contrasting pH (one acid and one calcareous) were collected from agricultural fields and were repeatedly phytoextracted with a Zn/Cd hyperaccumulator. Both soils, with and without phytoextraction, were subjected to two soil drying processes, i.e. drying from flooded conditions to 50 % maximum water holding capacity (WHC) and drying from 100 to 30 % WHC. Changes in metal availability induced by changing soil water moisture were examined using DGT. In acid soils, the availability of cadmium (Cd), zinc (Zn) and nickel (Ni) (but not copper) increased with drying from saturation to 50 % WHC. Drying soil from 100 to 30 % WHC had weak or no significant effects on available metals. Availabilities of Cd, Ni and Zn correlated well with available Al for both drying conditions. In calcareous soil, a decreasing trend of metal availability with soil drying from saturation to 50 % WHC or from 100 to 30 % WHC was found. Soils were also subjected to different wetting-drying cycles prior to drying. In acid soil, available copper (Cu) was correlated with dissolved organic carbon (DOC). Available Cd, Ni and Zn were correlated with available Al and increased with soil wetting-drying cycles in phytoextracted soil, but not in non-remediated soil. In calcareous soil, available Cu and Ni were correlated well with DOC and were higher with long dry periods. However, available Cd and Zn were lower in soils dried from the cycle of saturation to 50 % WHC. The results indicate that soil water regimes can be manipulated to alleviate soil metal availability, but they must be tailored to individual metals and different soil types, and soil moisture content before drying should be considered when dried soils are used for evaluation of metal availability.