Transport of Cd, Cu, Pb and Zn in a calcareous soil under wheat and safflower cultivation— A column study

Anthropogenic release of heavy metals (HMs) into the environment has resulted in a continuous buildup of HMs in agricultural soils. On the one hand, uptake of HMs by crop plants may lead to food chain transfer to humans, and on the other, leaching of HMs with deep seepage may cause groundwater contamination. The objectives of this study were to assess the mobility of cadmium (Cd), copper (Cu), lead (Pb) and zinc (Zn) under two common crop plants with different rooting systems, wheat (Triticum aestivum) with fibrous roots, and safflower (Carthamus tinctorious) with a taproot system in a calcareous silty clay loam soil. The study was conducted with 50 cm long undisturbed soil columns (Typic Haplocalcid) extracted from a wheat and a safflower field located in the same unit of soil type. The top 10 cm of half of the columns were artificially contaminated with Cd, Cu, Pb, and Zn at concentrations of 15, 585, 117 and 1094 mg kg− 1, respectively. Half of the contaminated and uncontaminated columns were planted with wheat and the other half with safflower according to their previous cultivation history. Discharge was collected continuously. After the crops were harvested, soil samples were collected at 10-cm intervals and analyzed for HNO3- and DTPA-extractable metals. Plants enhanced the displacement of metals into the subsoil. Safflower resulted in larger concentrations at deeper depths than wheat. Metal concentrations were significantly (P < 0.05) higher in discharge from contaminated than from uncontaminated columns. Because discharge was reduced in the presence of plants due to transpiration, total amounts of metals leached were in general larger in fallow than treatments with plants. The only exception was the leaching of Zn from wheat columns, which was larger under plants than under fallow. Plants did not only affect metal transport velocity of mobile fraction through actual root activity, but also through effects on soil properties during previous cultivation history, as there were clear differences in metal leaching between wheat and safflower soils from fallow columns. These were metal-dependent, indicating that these effects were not only related to different effects on soil structure by the root systems, but also to chemical soil properties.