SEQUENTIAL FRACTIONATION AND PLANT AVAILABILITY OF HEAVY-METALS AS AFFECTED BY SEWAGE-SLUDGE APPLICATIONS TO SOIL

The effect of sewage sludge applications on extractability and uptake by chard and lettuce of soil cadmium (Cd), chromium (Cr), copper (Cu), nickel (Ni), leaf(Pb), and zinc (Zn) was investigated. Ten different treatments (0, 150, 300, and 500 kg N ha(-1)) as mineral fertilizer, a nd 400, 800, and 1,200 kg N ha(-1) of aerobically and anaerobically-di gested sewage sludges were applied annually to a sandy-loam soil since 1984. Seven years after the start of the treatments, higher levels of heavy metals were detected in the soil, depending on the type of meta l, depth of sampling, type of sludge used, and, especially, rate of ap plication. Following a sequential extraction procedure incorporating 0 .1 M CaCl2, 0.5M NaOH, and 0.05M Na(2)EDTA, most of the heavy metals i n soil were detected in the Na(2)EDTA solution and the residual fracti ons. Large amounts of Cd appeared to be extracted by CaCl2, whereas su bstantial amounts of Cu and Ni were isolated by NaOH. The effect of tr eatments on the percentages of the metals found in each fraction depen ded on the type of metal, sampling depth, sludge used, and application rate. No significant increases were found in the heavy metal contents of chard and lettuce leaves, but some of the treatments resulted in a significant decrease of Cd and Cr levels in lettuce leaves.