Discriminating inhibitory from enhancing effects in respirometry assays from metal polluted-sewage sludge amended soils

The toxicity level differed in the two soils essayed, mainly due to their basal microbial activity, and to their different properties, which determine the released elements bioavailability. Water-soluble elements determination at the end of the incubation revealed the presence of soluble Zn, Cr and Ba in different concentrations in the two soils. For instance, Zn concentration was, at the highest application dose, of 131 and 23 mg kg−1, for SN and SO, respectively. So, the sludge toxicity was best observable at low rates and at incubation time periods long enough to allow the mineralisation of the waste oxidable organic matter.