In arid and semiarid zones, soil inorganic carbon pool is about 2-5 times more than soil organic carbon pool. For these regions, size and turnover of soil inorganic carbon pool play important role in soil carbon balance. Pedogenic carbonate is product of soil genesis process. It is closely related with the solution/deposition balance of soil lithogenic carbonate and the CO₂ re-turnover of the decomposition of soil organic carbon. Stable carbon isotope composition of pedogenic carbonate is controlled by the isotopic values of soil CO₂, which can be stimulated by unsaturated zone gas steady-state diffusion-production model. Under the carbonate system (soil CO₂(g), carbonate, and soil solution) isotopic equilibrium condition, simulation result, from molecular diffusion of biologically produced CO₂ and carbonate chemical equilibria reaction model, shows that pedogenic carbonate is enriched by approximately 14‰~16‰ stable C isotope compared to the value of soil organic matter. The diffusion-production model and/or fractionation model can been used to quantify disseminated pedogenic carbonate accumulations, to partition the proportion of pedogenic carbonate in laminations, and to assess quantificationally the effect of land use practices on carbonate solution/deposition balance. That application makes the model very useful in understanding the global carbon cycle.