A proper understanding of the soil carbon balance and measures to build up or maintain the amount of carbon in an agricultural production system is important for its sustainable production. Long-term experiments are especially useful to understand the dynamics of carbon since these processes are very slow in nature. We used a simple analytical model (Yang, 1996) to calculate carbon soil organic carbon in the rice-based cropping systems of the Indo-Gangetic Plains (IGP) and made a carbon balance estimation based on carbon input and output. The Yang's model is very simple in approach and it needs only two parameters: the initial relative mineralization rate (R9) and the speed of ageing of a substrate (S). We used 8 data sets under rice-based cropping systems belonging to different transects of the IGP with different levels of mineral fertilizers, green manures, farm yard manure (FYM), and residue treatments applied to rice, wheat or both. The model describes the dynamics of both added and indigenous soil carbon and the final soil carbon concentration is compared with the observed results. The calculated values of soil organic carbon were comparable to the observed values (R2 = 0.81). A periodical carbon balance was made based on the input to and output of carbon from the system at different periods of cropping. In most of the cases, the model results followed the observed trends, except for Pantnagar where addition of organic carbon by soil erosion caused abrupt changes in the SOC concentration. In general, when the remaining amount of carbon added through root biomass and organic amendments were not enough to balance the loss of SOC through decomposition, SOC concentration in soil was found to decrease. The study emphasizes the importance of adequate and repeated addition of carbon to maintain the initial levels of soil organic carbon and for a long-term sustainable cropping system.