Phosphomonoesterases are widely distributed in nature. They play an important role in P mineralization in soils via breaking the phospho-esteric bonds. Acid phosphatase and alkaline phosphatase are two predominant soil phosphomonoesterases which both are involved in the mineralization of P and therefore, contribute in providing inorganic P for plant growth and development. The enzymes are sensitive to the environmental pH and hence, thought to be sensitive to lime application. Moreover, it has been suggested that they respond to management practices as well as land use changes. However, our understanding regarding the mechanism(s) by which calcium carbonate influence the activity of acid and alkaline phosphatase is obscure. The main objective of this study was to perform a mechanistic approach to the effects of calcium carbonate application on acid and alkaline phosphatase activities, total phosphomonoestaerase activity and the ratio of acid to alkaline phosphatase activity under different management practices. For this purpose we collected our soil samples from three acidic soils of forest and deforested counterparts of three ecosystems including Lahijan (37? 11? N, 50? 01? E), Langroud (37? 10? N, 50? 07? E) and Gisum (respectively, 38? 40? N, 49? 01? E and 38? 40? N, 49? 03? E) forest standings in Gilan Province, Northern Iran. The effects of calcium carbonate application were assessed under two time span including 7 and 73 days following incubating soils under optimum moisture and temperature. Application of calcium carbonate resulted in pH increases while soil pH ranges were 0.2 to 0.5 unit lower under 73 days of incubation. Both enzymes showed to be pH dependent so that acid phosphatase activity was negatively associated with calcium carbonate-induced pH increase whereas, alkaline phosphatase increased as pH enhanced due to calcium carbonate application. Comparing acid and alkaline phosphatase in terms of the rate of response, the acid phosphatase activity was much more responding. Furthermore, the rate of response was increased when the samples incubated for 73 days, compared to 7 days. Path analysis (as an statistical approach) was performed to discriminate direct and indirect effects of calcium carbonate on the enzyme activities. The effects of Ca 2+ and CO3 2- are presumably the direct and that H + or OH - are thought to be the indirect effects of calcium carbonate application on the enzyme activities. The results indicated that both enzymes are more influenced by the indirect (pH) than direct effects of calcium carbonate application. In other word, the enzymes are rather influenced by the inhibitory effects of either H + or OH - under increasing pH conditions. This finding leads us to investigate the effects of calcium carbonate-induced pH enhancement by an ED 50 approach and interestingly figured out that the rectangular hyperbolic models generally fitted well to the data. Better results were obtained when partial inhibition model (model 2) used compared to the complete inhibition model (model 1). The model parameters were not only influenced by the incubation time but also they showed to be sensitive to management history of the soils. Overall, we conclude that lime application significantly influences the pH of the acid soils and the pH change is the main factor that performs further effects on the activity of acid and alkaline phosphatases, total soil phosphomonoestaerase activity and the ratio of acid to alkaline phosphatase activity. It appeases that both H + and OH - acts as inhibitors on the enzymes and the rate of the inhibitors can be successfully assessed, in particular with the partial inhibition model with an ED 50 approach based on the rectangular hyperbolic model. We suggest further investigation on the effects of pH changes on the inhibitory effects of trace metals on the soil enzymes when lime is applied as an amendment. Key words phosphomoesterase, history of land management, lime, ED 50