Influence of sewage sludge application on soil properties and on the distribution and availability of heavy metal fractions

Abstract

The influence of sewage sludge application on some soil properties and on the growth of wheat and corn plants were studied with pot experiments. The distribution of heavy metals among the various soil fractions and their availability to plants were also investigated in relation to soil pH. The results showed that sewage sludge application significantly ifluenced pH, organic matter content, electrical conductivity and available phosphorus. Soil pH increased and tended to hold steady near neutrality while organic matter content, electrical conductivity and available phosphorus (P) increased. For the heavy metals investigated, only total copper (Cu) and zinc (Zn) increased but were below the limits set by the EC. A significant increase was observed in the concentrations of cadmium (Cd), nickel (Ni), Cu, and Zn extracted by DTPA while iron (Fe) and manganese (Mn) were reduced showing a strong relationship to soil pH. For the metals sequentially extracted it was observed that chromium (Cr), Ni, Cu, and lead (Pb) extracted by NaOH, EDTA, and HNO₃ increased significantly with the increase of sludge application, while the respective forms of Zn and Pb were not affected. NaOH‐Cr, NaOH‐Cu, and NaOH‐Pb were significantly related to organic matter content but KNO₃‐Cr and KNO₃‐Pb were significantly related to soil pH. Wheat and corn clearly responded to sludge application. All metals except Fe showed increased concentrations in dry matter with increasing sludge rate, but were below toxicity levels. In contrast, K concentration in corn plants was reduced with increasing sludge rate. All metals, except Fe, extracted by DTPA were significantly correlated with the metal concentration in wheat dry matter while for the com the results were variable.