اثرات تجمعی و باقی‌مانده لجن فاضلاب شهری بر غلظت عناصر سرب و کادمیم در خاک و گیاه گندم

Sewage sludge application on farmland as fertilizer is commonly practiced in many countries. Sewage sludge is rich in macro- and micro- nutrients. However, high concentration of heavy metals in sludge may cause pollution of soil, groundwater and human food chain because of uptake of toxic metals by crops. The objective of this study was to determine residual and cumulative effects of sewage sludge on concentration of Pb and Cd in soil and wheat. Different levels of 0, 25, 50 and 100 Mg ha-1 of sewage sludge were applied to the soil for four years. To study the cumulative and residual effects of the sewage sludge, applications were repeated on three fourth of each plot in the second year, on one half of plots in the third year and in one fourth of plots in the fourth year. Wheat grown in the plots, after the fourth year, soil samples from the 0-20 cm depth of the different parts of the plots were taken and analyzed. Wheat was also harvested roots, stems and grains were separately analyzed for the heavy metal concentritons. Cumulative sewage sludge application increased OM, CEC, ECe, total and DTPA-extractable concentration of Pb and Cd in soil significantly (P≤ 0.05). Residual sewage sludge in the soil also increased CEC, total and DTPA-extractable concentration of Pb and Cd significantly. Single sludge applications at different rates increased the DTPA-extractable concentrations of heavy metals. In subsequent years with no further sludge application, DTPA–extractable metal concentrations in soil decreased continuously approaching the levels in the control. However, even after four years, DTPA-extractable concentration of Pb and Cd, were still significantly higher in plots which received more than 50 Mg ha-1 sludge than control. DTPA-extractable concentrations of Pb was closely correlated with total concentrations. Sewage sludge increased concentration of Cd in roots and stems and Pb in grains significantly. Cumulative effects on concentrations of Pb in grains, and Cd in stems were more than residual effects. The results of this study show that cumulative and residual effects of sewage sludge application increased concentrations of heavy metals in soil and wheat.