Bioavailability and toxicity of cadmium to microorganisms and their activities in soil: a review

Significant quantities of cadmium (Cd) have been added to the soils globally due to various anthropogenic activities, raising concerns for environmental health. Microorganisms play a unique role in the soil ecosystem, because of their contributions to soil fertility. Contrasting trends, reported on the toxic effects of heavy metals including Cd on soil microorganisms and their activities, are attributable to short-term studies often limited to a single soil type and conducted under controlled laboratory conditions. There is a paucity of reliable field data on Cd alone, since most field studies on Cd-microorganism interactions in soils are based on sewage sludge containing multimetals and organic substances. No single parameter can be used to generalize Cd toxicity and different parameters can provide contrasting results. A battery of relevant tests, rather than just one single assay, involving important microbial activities should therefore be included in ecotoxicity studies. The bioavailability of Cd and associated toxicity to soil biota vary with time, soil type, speciation, ageing, Cd-source, organisms and the environmental factors. The available fraction or soil solution Cd, and not the total concentration of Cd, seems to correlate well with the toxicity parameters