Although compost is added to soils to improve their nutritive status and properties; yet upon its decomposition, considerable amounts of CO2 might be emitted to the atmosphere causing hazardous environmental risks. Investigating the mineralization of compost applied to a sandy soil and a silty clay one of low organic-C content in the green house for 90 days under soil moisture contents of 75 or 100% of FC was considered. Efflux rate of CO2 decreased considerably for one day in the sandy soil after application of the compost; beyond which showed an almost constant rate. In the silty clay soil, the efflux rate decreased obviously to achieve non detectable value 3 days later. Even in soils amended with mineral N-fertilizer, CO2 emissions were valued while decreased with time. By the end of the incubation time, the rate of C-mineralization was significantly higher in the sandy soil compared with the silty clay one. Increasing soil moisture content from 70 to 100% of the field capacity led to further increases in rates of CO2 efflux as well as org-C mineralization. The amount of N mineralized in the sandy soil was 1.7 times higher than its content in the silt clay one at the FC and 1.1 times in case of 70% of FC. In conclusion, a reversible equilibrated process between decomposition of soil organic matter and buildup of more stable organic components might exist at the same time.