Heavy metal leaching and plant uptake in mudflat soils amended with sewage sludge

The leaching and uptake of Cd, Cu, Pb, and Zn by maize (Zea mays L.) in mudflat saline-alkali soils amended by sewage sludge was examined using a greenhouse leaching column experiment. Application of sewage sludge caused decreased pH, increased DOC, and increased Cd, Cu, Pb, and Zn concentrations in leachates. The similar temporal dynamics of DOC and the metal concentrations in leachates suggested complexation of the metals with DOC. There was downward movement of metals as evidenced by the metal enrichment in the bottom layer (20-40-cm depth) of leaching columns (p < 0.05). The enrichment of metals was contributed by the acid-soluble/exchangeable fraction (EX), reducible fraction (RG), oxidizable fraction (OXI), and residual fraction (RES), indicating redistribution of leached metals from the top 20-cm layer. The sewage sludge application also enhanced plant uptake of metals. However, even under the greatest sludge application rate (150 g kg^-1), very small proportions, averagely 0.65% and 0.35%, of the applied metals were leached and taken up by maize, respectively, over the experimental period. Long-term field-scale research is warranted for further investigation of the effects of sewage sludge amendment on heavy metal fractionation and distribution in mudflat soil-plant-water system.