The effect of long-term sewage sludge application on Zn, Cu, Ni and Pb levels in a clay loam soil under pasture grass in Zimbabwe

Abstract

Although sewage sludge provides nutrients for plant growth, its continual use over extended periods can result in the accumulation of heavy metals to levels detrimental to the environment. A study was conducted to determine the concentration of EDTA-extractable Zn, Cu, Ni and Pb in a Gleyic Lixisol that had been amended with sewage sludge for more than 19 years in Zimbabwe. Soil profiles were dug and samples taken for metal analysis using inductively-coupled plasma atomic emission spectrophotometry (ICP-AES). Sewage sludge addition resulted in a significant ( P < 0.01) increase in the concentration of the four metals in the top soil (0–15 cm), and there was a significant ( P < 0.01) treatment × soil depth interaction. Zinc decreased from 196 mg kg −1 in the top soil (0–5 cm) to 5 mg kg −1 in the 15–20 cm horizon while Cu, Ni and Pb decreased from 43 to 11, 23 to 9 and 53 to 19 mg kg −1, respectively. It was concluded that the high EDTA-extractable metal concentration in the sludge treatment was due to the spreading of sewage sludge slurry on the soil, and that the accumulation of the metals in the top soil may be due to the interaction of the metals with organic matter.