Heavy metal pollution is a serious potential danger for the human civilization. Soil enzyme activities are thought to be sensitive and responding attributes to heavy metal contamination in soils. The enzymes are being inhibited by the heavy metals depending on the type and concentration of the heavy metals. Phosphatases are widely spread in the environment and soils and are responsible for P mineralization from soil organic matter. Acid phosphomonoesterase or briefly acid phosphatase is a member of phosphomonoesterases that is responsible for P mineralization in acid soils. The objectives of the research were to identify the response of acid phosphatase to CaCO 3 in a wide time span and if the Cd inhibition on acid phosphatase is affected by CaCO 3 application. Two acid soils (0-15 cm) were collected from Langroud region in Guilan province, the soils were air-dried and passed through a 2 mm sieve. Soil texture, organic C, EC, pH and calcium carbonate were determined. In a preliminary experiment, the soils were treated with various quantities of CaCO 3 to obtain pH levels of 6.0, 6.5 and 7. In the first experiment, the CaCO 3 -treated soils were incubated for 5 months and pH and acid phosphatase activity were measured at 0, 1, 2, 3, 4 and 5 months. In the second experiment, the inhibition percentage of the enzyme was determined in the soils treated with cadmium. In the third experiment, the kinetic parameters of acid phosphatase were determined in CaCO 3 treated and untreated soils. Results indicated that CaCO 3 application increased pH values from 5.5 to 7.5 in the sandy loam soil and from 4.5 to 5.3 in the silty clay loam soil. Following the incubation the pH values were decreased gradually. The acid phosphatase activity was decreased consistently due to CaCO 3 application. It was observed that acid phosphatase activity decreased from 424.9 to 238.6 mg kg -1 soil h -1 in the loamy sand soil while it was decreased from 1101.0 to 978.9 in the silty clay loam soil. It was also monitored that the acid phosphatase activity increased gradually during the 5 months of incubation. Acid phosphatase was consistently inhibited by Cd application however, the effects of CaCO 3 application on the degree of inhibition remained obscure since we could not obtain consistent results. CaCO 3 application could not significantly influence the kinetic parameters of acid phosphatase. Keywords: Acid phosphatase, Time, Calcium carbonate, Cadmium, IP.
