The aim of this study was to evaluate the effect of two Cr species (Cr3+ and Cr6+) on N and C mineralization and dehydrogenase activity in semi-arid soils. The Cr species (250 mgkg(-1) soil) were either added alone or mixed with tannery sludge (0.0125 g g(-1)) to three soils: cultivated soils, and outside and under the canopy of mesquite trees were then incubated for 180 days at 25 degrees C. Sole Cr6+ addition had a higher inhibition of CO2 production rate in cultivated soil (58-73%) than in soils under the canopy and outside the canopy. Soil outside the canopy amended with Cr6+ showed the highest inhibition of dehydrogenase activity (40-100%) followed by cultivated and under the canopy soils. However, Cr6+ added alone increased the inhibition of nitrification in soil outside the canopy (68-84%, from 30 to 120 days), followed by under the canopy and cultivated soils. The addition of tannery sludge to Cr6+ significantly reduce the CO2 production rate and dehydrogenase activity in all three soils, and increased the inhibition of nitrification in the following order: outside the canopy, cultivated and under the canopy soils. The addition of Cr3+ or Cr3+, plus tannery sludge either stimulated or inhibited CO2 production rate, dehydrogenate activity and ammonification in the three soils in no clearly defined order. Measurement of dehydrogenase activity was the best tool for assessing the harmful effect of Cr6+ on soil microbial activity in semi-arid soils exposed for an extended period. (C) 2006 Elsevier B.V. All rights reserved.