THE UPTAKE OF NATIVE AND APPLIED LEAD BY ALFALFA AND BROMEGRASS FROM SOIL

The uptake of native and applied fractions of lead (20 and 100 μg Pb/g soil) by alfalfa and bromegrass was studied in a growth chamber experiment using three soils. The applied soil lead treatments did not affect dry matter (DM) yield but the concentration of lead in the plant tops was found to increase twofold when the amount of lead added to soil was increased fivefold (mean values for both crops 3.8 and 8.7 ppm in the 20 and 100 ppm Pb treatments, respectively). Added sulphur gave significant DM yield response in both crops on all soils but did not significantly alter the lead concentration in the aboveground DM. Lowering the temperatures and reducing the photoperiod to simulate autumn conditions resulted in a significant increase of lead concentration in plants grown on the 100 ppm Pb treatment, which was attributed to the corresponding reduction in the DM yields of the plants. High concentrations of lead (mean 132 ppm) were found in the fine roots of both crops in comparison with lead levels in main roots (mean 33 ppm). Lead extracted by various extractants from soil samples at the end of the growth chamber experiment indicated that applied lead was rapidly immobilized with the higher lead concentrations in the soil solution being found in the soil with the lowest organic matter and clay content.