Influence of different oxidation states of chromium (VI, III) on soil urease activity

Enzyme activity is often used for characterization of microbial viability in soil. This activity can be adversely affected by some contaminants. A contaminant concentration causing a decrease of urease activity to 50% of its initial value has been recently proposed as an ecological dose (ED50), and can be used as an indicator of toxicity of the contaminant. The aim of the investigations was to measure the influence of chromium (VI) and (III) on urease activity. The measurements were performed on three kinds of samples; the first one was a citrate buffer containing pure enzyme (E), the second was a suspension containing soil never irrigated with wastewater but amended with pure enzyme (E+S) and the third one was the suspension of the same soil which was subjected to irrigation with wastewater after II step purification amended with the same enzyme (Sir). Urease activity at different concentrations (0.1–1 mg kg−1) of chromium (VI) and (III) was determined with the help of Bounmanti et al. colorimetric method with sodium nitroprusside (at 630 nm after 30 min of incubation in the darkness). The activity was expressed as micromoles of N–NH4 produced by 1 kg of the solution per hour in the E treatment or per kilogram of soil (treatments E+S and Sir). The amendment of Cr forms changes significantly the urease activity. For Cr(VI), the following ED50 values were found: 0.4 mg Cr kg−1 solution for the treatment (E), 0.5 mg Cr kg−1 solution for E+S and 0.5 mg Cr kg−1 solution for E +Sir treatments. For Cr(III), the ED50 were as follows: 0.2 mg Cr kg−1 for E treatment, 0.4 mg Cr kg−1 for E+S treatment and 0.6 mg Cr+3kg−1 for E+Sir treatment. Urease activity in pure solution was more sensitive for Cr(III), which caused higher inhibition at the same concentration compared to Cr(VI).