Effect of heavy metal contamination on the rate of decomposition of sewage sludge and microbial activity

Fresh treated sewage sludge from an urban source was applied to a field of granitic sandy loam soil under grass at a rate equivalent to 100 tonnes dry matter ha−1. The field was divided into 32 plots (4 m × 2 m). Treatments with metals (Cd, Cr, Cu, Mo, Ni, Pb, Zn as salts) were applied to blocks of 16 plots. Soil microbial biomass C and basal respiration were measured 3 a after the addition of sludge. After 5 a, plots were sampled, sieved to pass a 10 mm mesh and dried at 40°C. Carbon and N analyses were performed on finely ground soil samples. Total metals (after aqua regia digestion) and 0.1 M CaCl2 extractable metals were measured by ICP. Samples from Cu-treated plots were sequentially extracted with 0.01 N HCl, 0.1 M Na4P2O7 (pH 7) and 0.1 M NaOH and the fractions obtained were examined by Electron Paramagnetic Resonance spectroscopy which showed that some of the Cu had become organically complexed. There was no significant difference in the total soil C contents or microbial parameters as a result of the sludge addition and the presence of the different metals.