The role of nickel in soils of tropical areas under the application of sewage sludge is still not very well known. This study was carried out under greenhouse conditions in Jaboticabal, São Paulo State, Brazil, with the objective of evaluating the impact of the application of sewage sludge previously contaminated with rates of nickel (329, 502, 746 and 1119 mg kg-1, dry basis) on a soil cropped with sorghum, in relation to soil enzyme activity and soil microbial biomass. Soil samples were collected at the beginning and the end of the experiment. The experimental design was completely randomized, with five treatments (control and four rates of Ni in the sewage sludge) and four replications. C and N of the soil microbial biomass and enzyme activities (acid and alkaline phosphatases) were sensitive indicators for impact evaluation caused by sewage sludge contaminated with nickel. There were positive correlations between "total" and extractable nickel (Mehlich 1) with C-microbial biomass and negative with the microbial C/N relationship. N-microbial biomass correlated positively with "total" and extractable Ni at the last sampling. At the end of the experiment, the acid phosphatase activity correlated negatively with "total" Ni while the alkaline phosphatase correlated with both forms of the metal.
heavy metal; biosolid; acid phosphatase; alkaline phosphatase; sorghum