Temporal Changes in Cadmium, Thallium, and Vanadium Mobility in Soil and Phytoavailability under Field Conditions

Abstract

A field study was conducted over a 30 mo period to examine movement of Cd, Tl, and V through the profile of a Coastal Plain soil (Typic Kandiudult) and the availability of these trace metals to bush bean (Phaseolus vulgaris L.) plants. The metals were applied to field plots as dissolved salts and mixed into the surface 7.5 cm. The greatest concentration of all three metals was observed in the surface soils, with a steep decrease occurring down to the 7.5 to 15 cm depth. Thallium was the most mobile of the three metals; approximately 15% of the applied Tl and <3% of the applied Cd and V moved below the surface 7.5-cm region during the 30-mo experiment. Extractable concentrations of all three metals in the surface soils decreased significantly (P ≤0.05) during the initial 18 mo after treatment. No further decrease occurred between 18 and 30 mo. The presence of Al- and Fe-oxides and small amounts of clay minerals and organic matter in this highly-weathered, low cation-exchange soil were likely responsible for the retention of the trace metals. Bioavailability, as measured by concentrations and total amounts of metals in root and aboveground tissues of plants, did not change significantly between 18 and 30 mo. These data suggest that bioavailability of Cd, Tl, and V decreased over time as a result of transformation of these elements into unavailable forms and not to leaching. These changes in bioavailability occurred soon after application, becoming negligible after 18 mo.