Soil Biological and Biochemical Response to Cd Exposure

Though heavy metals can stimulate the activity of soil enzymes in smaller amounts, yet act as inhibitors, if present in high concen-trations. Natural and anthropogenic heavy metal contamination and its disturbances on soils can be evaluated by using enzymatic activities as sensors. To study the effects of Cd, soil added with known Cd concentrations (0, 10, 20, 50,100 and 200 mg/kg soil) were incubated for a period of 30 days at 28°C. At intervals of 0, 5, 10, 20 and 30 days samples were withdrawn for enzyme assays like dehydrogenase (DHA), catalase (CAT), phenol oxidase (PHE), and peroxidise (PER). In a separate experiment the effect of Cd on active microbial biomass carbon (AMBC), basal soil respiration (BSR), and metabolic quotient were studied. AMBC showed a reduction trend with increase in Cd concentration, and a maximum reduction of 47% was observed at 30th day for 200 mg/kg treatment. BSR also showed the same trend, with a maximum decrease of 42% at the 30th day. With the rate of Cd amendments and treatment period, DHA showed an inhibition trend; whereas maximum decrease was observed for 200 mg/kg treatment at 30th day. CAT, PER, and PHE were found to be increased with Cd addition and remained at higher levels than in the control soil. These changes can be attributed to the effect of Cd on microbial activities. Based on cluster analysis, AMBC appears to be the sensitive indicators for the soil exposed to Cd contamination