Nowadays, soils and environmental pollution by organic compounds is one of the main challenges in soil health. Organic contaminants may be toxic to human and soil organisms and may have some deleterious influences on the environment. 3,3'-diaminobenzidine ( DAB) is one of the organic contaminants which is usually added to soils by food and textile industries. We hypothesized that DAB can be a potential inhibitor for soil amidohydrolases (urease, L-asparaginase and L-glutaminase) and also it can be at least partially degraded by soil microorganisms. Therefore, the objectives of this study were to investigate i) the effects of DAB concentration on soil amidohydrolases, L-arginine ammonification and N mineralization and ii) the biodegradation kinetics of DAB in soil. Five concentrations of 0, 10, 25, 50 and 100 mg kg -1 DAB were added to two surface soils (Shervedan and Jouzdan), incubated at 25 °C and 50% water holding capacity for 30 days. To study the kinetics on N mineralization, two concentrations of 0 and 100 mg kg -1 DAB were added and the soils were incubated for 60 days under the optimum conditions. The inorganic N was measured destructively in 10 different time intervals during the incubation. Results indicated that higher concentrations of DAB reduced the soil biological properties in both soils. The sensitivity index was decreased as the DAB concentration increased from 10 to 100 mg kg -1 . L-glutaminase showed the greatest sensitivity to DAB among the three amidohydrolase studied. Kinetic behavior of the DAB degradation consistently followed first-order models in both soils. In the first 7 days of the incubation period, N mineralization was adversely influenced by DAB in both soils indicating that DAB has a toxic effect on the biological process. Following the first 7 days of the incubation, DAB was started to degrade as the inorganic N concentration was greater in DAB-treated soils than those of controls. The potentially mineralizable N in DAB-treated soils was consistently greater than those of control. Overall, we can clearly conclude that DAB can be a potential toxic organic compound for soil biological activities and that the rate of toxicity is time-dependent.
