ABSTRACT

The implications of additions of metal-contaminated sewage sludge to soil on soil microbial activity are not as well known as the effects on higher plants. This is because there are no straightforward techniques to measure microbial activity. The methods of measuring either total amounts of microbial biomass or the contributions of different taxa are often time-consuming and subject to large estimation errors. Hence, the relative sensitivities of various microbial processes to metals in soils under realistic conditions have not been studied. However, newer, more precise techniques to measure the entire biomass have been developed recently (1, 2) and if used with suitable controls they can give useful information on the effects of various soil treatments (3). The problems of interpreting the results of such measurements centre on the fact that the size of the biomass reflects management and soil factors such as the substrate inputs as well as the physicochemical environment (3, 4). Therefore field experiments with treated plots in a reasonable state of equilibrium, are ideal. This avoids short-term effects such as the presence of excess soluble salts applied with fresh sludge, which could obscure metal effects or the lack of equilibrium between recently applied inorganic metal salts and other soil constituents.