EFFECTS OF TIME AND TEMPERATURE ON THE BIOAVAILABILITY OF CD AND PB FROM SLUDGE-AMENDED SOILS

A pot experiment was conducted to compare the behaviour and bioavailab ility of Cd and Pb from two soils mixed with sewage sludge at three ra tes (0, 50 and 150 t ha-1) and maintained at two contrasting ambient t emperatures (15-degrees-C and 25-degrees-C) over a period of one year following the treatments. Ryegrass (Lolium perenne) accumulated Cd and Pb in the sewage sludge treated soils, although accumulation was sign ificantly lower in the soils treated at the high rate (150 t ha-1) com pared to the low rate (50 t ha-1). Ryegrass grown in the warm environm ent (25-degrees-C) accumulated significantly higher levels of Cd and P b than that grown in cooler conditions (15-degrees-C). Samples of the soils spiked with nitrate salts of Cd and Pb at equivalent rates of me tal loading resulted in the ryegrass accumulating much higher levels o f both the metals than on the sludge treated soils. Metal uptake by th e ryegrass from the sludge treatments increased over successive harves ts while that from metal salt treatments decreased. The observed trend of increasing plant metal uptake over time coincided with a trend of decreasing pH in the sludge treatments. However, the concentrations of Cd and Pb extracted by DTPA failed to predict the changes in plant me tal uptake. The importance of sewage sludge as both a source and a sin k of pollutant metals and the trend of increasing bioavailability over time shown by this experiment are discussed.