Nitrogen transformation and microbial biomass content in soil contaminated with nickel and cadmium from industrial wastewater irrigation

During a survey of the contamination of soils receiving sewer water/industrial wastewaters, we identified two sites, one with limited plant growth and containing 2290 mg Ni and 40 mg Cd kg⁻¹ (contaminated soil) and another with good plant growth and containing 26 mg Ni and 2 mg Cd kg⁻¹ soil (uncontaminated soil). Various Ni levels ranging from 26 to 2290 mg kg⁻¹ were created by mixing contaminated and uncontaminated soils in different proportions to study the influence of Ni content of soil on microbial biomass, urea hydrolysis and nitrogen transformations. Soil microbial biomass decreased with increasing levels of Ni and Cd in soil. Urea hydrolysis was faster in uncontaminated soil (65% within 12 h in soil containing 26 mg Ni $\text{kg}{}^{\mathrm{-1}}$) compared to contaminated ones (41% within 12 h in soil containing 2290 mg Ni kg⁻¹). The values of first-order rate constant for urea hydrolysis (k) were higher in uncontaminated soil as compared to Ni contaminated soil. The NH₄⁺–N content increased up to 14 days of incubation in soil containing Ni >1230 $\text{mg}\text{kg}{}^{\mathrm{-1}}$ and decreased thereafter, however, in other soils it decreased with increasing incubation period. The NO₃⁻–N content increased with increasing incubation period irrespective of the Ni content of the soil. These results indicate that soils containing excessive concentration of toxic metals (Ni, Cd) may decrease the microbial biomass and also lead to leaching losses of urea owing to delayed hydrolysis.