Crop growth simulation models provide a means to quantify the effects of climate, soil, and management on crop growth and biogeochemical processes in soil. The Denitrification and Decomposition (DNDC) model was evaluated for its ability to simulate N dynamics and balance in the rice-wheat cropping systems in the Indo-Gangetic Plains (IGP) with various N and water management practices using a newly compiled soil/climate/land-use database. The observed crop yield, N uptake, and losses of N were in good agreement with the values predicted by the model. In rice-wheat systems in the IGP, current inputs through fertilizer, manure, biological fixation, atmospheric deposition, and irrigation were 98, 37, 17, 8, and 7 kg N ha^-1, respectively, while outputs through uptake, volatilization, leaching, and denitrification were 175, 14, 12, and 4 kg N ha^-1, respectively. Northwestern transects of the IGP (Punjab and Haryana) showed larger yield and N uptake because of a higher amount of N use than those in eastern transects (Uttar Pradesh, Bihar, and West Bengal). Volatilization was the dominant N loss mechanism in Punjab and West Bengal while NO₃ leaching was dominant in Bihar, Uttar Pradesh, and West Bengal. Simulated N balance was negative in all the states. The study suggested that better N management is required to arrest soil N depletion and DNDC model could be useful to accelerate the application of available knowledge at field, farm, and regional levels for quantifying losses and optimizing N management.