Indirect effects of pesticide applications on heavy metal contents of some vegetable plants

Use of pesticides to control pests of certain crops may lead other untreated neighbor vegetables as well as the cultivated soil to be indirectly contaminated. Field experiments were carried out on a clay soil (Typic Torrifluvents) in the Experimental Farm of Assiut University, Assiut, Egypt to evaluate the effect of applying some pesticides and their time of application to plants on the heavy metal contents of the untreated neighbor plants and their status in the cultivated soil. In two consecutive seasons, lettuce (Lactuca Sativa) followed by lew mallow (Chorcoruis Olitorus) plants were grown in-between other pesticide-treated plants. Two fungicides (mancozeb and copper hydroxide) and three insecticides (carbofuran, chlorpyrifos and aldicarb) were applied each season to the surrounding plants either once (after 4 or 8 weeks from planting) or three times (after 4, 6 and 8 weeks from planting). Four heavy metals (Cr, Cu, Ni and Pb) were analyzed in untreated collected soil and plant samples at the end of each season. The obtained results showed that the DTPA-extractable heavy metals in soil increased in the first season but they decreased in the second one. All early single applications (after 4 weeks) gave significantly higher extractable amounts of Pb compared with the other application times. Lew mallow plants contained higher concentrations of Pb, Ni and Cr than lettuce for all pesticide treatments. Copper hydroxide was the most effective pesticide in raising the levels of Pb (23.1 ppm in lettuce and 41.9 ppm in lew mallow) and Cr (8.2 ppm in lettuce and 13.1 ppm in lew mallow) when it was added to the treated plants after 8 weeks from planting. However, Cu content of lettuce increased up to 31.2 ppm when copper hydroxide was added after 4 weeks. The late single application of aldicarb increased Ni concentrations up to 5.0 and 8.9 ppm in lettuce and lew mallow plants, respectively.