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A new version of DRAINMOD-N was developed to simulate nitrogen (N) dynamics and turnover in the soil-water-plant system under different management practices and soil conditions. The old model simulated the dynamics of only one N pool, nitrate-nitrogen (NO3-N), using a simplified N cycle which restricted its applicability. The new model considers a more complete N cycle and operates on different levels of complexity according to the conditions of the system being simulated. Processes considered are atmospheric deposition, application of mineral N fertilizers including urea and anhydrous ammonia (NH3), soil amendment with organic N sources, plant uptake, mineralization, immobilization, nitrification, denitrification, NH3 volatilization and N losses due to leaching and surface runoff. Nitrogen pools considered are NO3-N, ammoniacal-nitrogen (NHx-N) and organic nitrogen (ON). The new model simulates the dynamics of organic carbon (OC) using a simplified C cycle to better describe the interaction between mineral and organic N pools. It divides OC into different pools characterized by its carbon-to-nitrogen ratios and its turnover rates. Each OC pool has a corresponding ON pool. A simplified approach is used to determine temporal changes in soil pH induced by N fertilizer application, nitrification and N plant uptake; consequently the model determines the composition of NHx-N. The model simulates N transport using finite difference solution to a multi-phase form of the one-dimensional advection-dispersion-reaction equation. Model output includes daily concentrations of NO3-N and NHx-N in soil solution and drainage outflow, and cumulative rates of simulated N processes.