Toxicity of cadmium to soil microbial activity:: effect of sewage sludge addition to soil on the ecological dose

Cadmium has a toxic effect on soil microbial activity which plays an important role in nutrient cycling and, therefore, in maintaining soil fertility. In addition, the mobility of this heavy metal in soil is affected by the addition of urban wastes such as sewage sludge. This study was conducted to determine the effect of sewage sludge amendment of a semiarid soil, previously polluted with Cd, on the toxic effect of this heavy metal on soil microbial biomass and its activity. Dehydrogenase activity, ATP content, microbial soil respiration and microbial biomass carbon were used as bioindicators of the toxic effect of Cd. The inhibition of microbial activity and biomass by different Cd concentrations ranging from 0 to 8000 mg Cd kg(-1) soil was described by three mathematical models in order to calculate three ecological doses of Cd: ED50, ED10, and ED5. In general, higher ED values were calculated for the sewage sludge amended soil than for unamended soil. Thus the Cd toxicity to microbial activity of the sewage sludge amended soil can be considered lower than that of the unamended soil. Moreover, increased ED values with time after soil Cd contamination were observed. (C) 2002 Elsevier Science B.V All rights reserved.