TOXICITY OF HEAVY-METALS TO MICROORGANISMS AND MICROBIAL PROCESSES INAGRICULTURAL SOILS - A REVIEW

An increasing body of evidence suggests that microorganisms are far mo re sensitive to heavy metal stress than soil animals or plants growing on the same soils. Not surprisingly, most studies of heavy metal toxi city to soil microorganisms have concentrated on effects where loss of microbial Function can be observed and yet such studies may mask unde rlying effects on biodiversify within microbial populations and commun ities. The types of evidence which are available for determining criti cal metal concentrations or loadings for microbial processes and popul ations in agricultural soil are assessed, particularly in relation to the agricultural use of sewage sludge. Much of the confusion in derivi ng critical toxic concentrations of heavy metals in soils arises from comparison of experimental results based on short-term laboratory ecot oxicological studies with results from monitoring of longterm exposure s of microbial populations to heavy metals in field experiments. The l aboratory studies in effect measure responses to immediate, acute toxi city (disturbance) whereas the monitoring of field experiments measure s responses to long-term chronic toxicity (stress) which accumulates g radually. Laboratory ecotoxicological studies are the most easily cond ucted and by far the most numerous, but are difficult to extrapolate m eaningfully to toxic effects likely to occur in the field. Using evide nce primarily derived from long-term field experiments, a hypothesis i s formulated to explain how microorganisms may become affected by grad ually increasing soil metal concentrations and this is discussed in re lation to defining ''safe'' or ''critical'' soil metal loadings for so il protection. (C) 1998 Elsevier Science Ltd. All rights reserved.