Phosphomonoesterase belongs to an environmentally important subclass of hydrolases which catalysis the hydrolysis of phospho-esters in soils. It is suggested that application of lime and organic amendments have significantly influenced their activities as well as the ratio of acid- (ACP) to alkaline-phosphomonoesterase (ALP). However, the underlying mechanisms have been remained obscure. We hypothesized that a part of their activity response to the application of lime and organic amendments can be attributed to their production and persistence, and therefore a study was conducted to investigate i) the effects of CaCO 3 (L) and alfalfa residue (P) on the ACP and ALP activities in a 90-days incubation, ii) response of the ACP and ALP production and persistence to application of L and P as a possible underlying mechanism for the enzyme activity responses. An acidic soil was collected from uplands of Guilan Province, northern Iran. In a factorial arrangement of a complete randomized block design, the soil was applied with two levels of CaCO 3 including 0 (L0) and 32 (L1) g kg -1 and two levels of alfalfa residue including 0 (P0) and 10 g C kg -1 . The soils were incubated at 25° C and 50% water holding capacity for 90 days. The soils were destructively sampled at 1, 2, 4, 8, 16, 30 and 90 days and ACP and ALP were measured. The production and persistence indices were calculated accordingly.Results indicated that ACP activity reduced during the early 30 days of incubation followed by a stable 60-days phase. The L and P treatments reduced and increased the ACP activity, respectively. The decreasing trend in ACP activity was markedly observed in the LO treatments while in L1 no significant differences were monitored among the incubation times. Similarly, the decreasing trends was more pronounced in P1 compared to P0 treatment. The response of ACP to P application was much greater in L0 treatment compared to L1. The activity of ALP was drastically decreased in early 30 days and remained constant to the end of the incubation. The application of both L and P significantly increased the ALP activity. The decreasing trend of ALP during the incubation was similarly observed in both L0 and L1 treatments. As suggested for ACP, the ALP was also decreased during the incubation time particularly in P1 treatment. The response of ALP to P application was much greater in L1 than that of L0 treatment. The ACP:ALP ratio was also monitored and it was observed that the ratio has been slightly increased during the incubation. Moreover, the ration was also slightly increased as affected by P application and drastically increased by L application. Correlation studies revealed that the ACP and ALP activities are less associated in the pool data while dividing the data into two compartment of L0 and L1 significantly increased the correlation coefficients. In the second study, we figured out that under L application, unexpectedly, the production of ACP was not decreased. Instead, L application resulted in a much less persistence of the ACP in the soils. Application of P, expectedly increased the production and decreased the persistence of ACP in the soils. The production of ALP was significantly increased in P- and L-applied soils, while its persistence was not affected by L application. Similar to that of ACP, the ALP persistent was also decreased by P application. Overall, it was concluded that decreasing the value of persistence is the possible underlying mechanism for ACP decrease under lime application in acid soils while the enzyme production has no considerable role in describing the response of ACP to lime application. On the other hand increasing the ALP activities in lime-applied acid soils can be described by the fact that lime application cause increase in ALP production. Key Words: Phosphomonoestrases, CaCO 3 , Production and Persistence