In recently years, much attention has been focused on conservation agriculture, in which three main principles including minimum soil disturbance, continuous mulch, and diverse species are implemented to improve the soil quality, increase infiltration rate and sequester carbon into the soil. The amount of carbon sequestration in long-term in those practices are under investigated. One of the models have been used to simulate carbon sequestration into the soil is the Environmental Policy Integrated Climate (EPIC) model. The carbon component in the EPIC model has been under developed and tested world-wide with complex land management operations and crop rotations. The CENTURY model was incorporated into EPIC and was modified to better simulate long-term soil organic carbon. Although the modified coupled C:N cycle has been tested for the US and with different tillage systems, it was not tested with a diverse species and organic systems under conservation and conventional tillage. Also, further testing should be made for the relationship between the soil bulk density and soil organic carbon, once any change occur, it will lead to changes of infiltration, soil porosity, aeration and gas fluxes. Similarly, the inorganic carbon needs to be improved. EPIC only account for the top 10 soil layers, the carbon in the deeper subsurface horizon has not been studied; meanwhile, in the conservation agriculture  project done in Cambodia, the soil porosity has been changed due to deep rooted cover crops, and the carbon sequestration has been more than the amount sequestered from the forest. The accumulation of carbon into the soil should be tested to see how long the soil can no longer sequester carbon has not been done yet with the EPIC model. Because of those weaknesses, this paper will initially conceptualize an improvement in the carbon sequestration routine of the EPIC model and test it for a long-term organic and conventional management under conservation and conventional tillage systems.