DRAINMOD-N II is a field-scale, process-based model that was developed to simulate nitrogen dynamics and
turnover in the soil-water-plant system under different management practices and soil and environmental conditions. It is
an enhanced version of the nitrogen (N) simulation model, DRAINMOD-N, that simulates a more complete N cycle, adds a
carbon (C) cycle, and operates at different levels of complexity. Processes considered in the model include atmospheric
deposition, application of mineral N fertilizers including urea and anhydrous ammonia (NH3), soil amendment with organic
N (ON) sources including plant residues and animal waste, plant uptake, organic C (OC) decomposition and associated N
mineralization/immobilization, nitrification, denitrification, NH3 volatilization, and N losses via subsurface drainage and
surface runoff. Nitrogen pools considered in the model are nitrate-nitrogen (NO3-N), ammoniacal nitrogen (NHx-N) and ON.
DRAINMOD-N II includes a submodel that simulates C dynamics in the soil-plant system using a C cycle similar to that of
the CENTURY model. A simplified approach is used to simulate temporal changes in soil pH; consequently, the model
determines the composition of the NHx-N pool and, if necessary, changes its operation mode. DRAINMOD-N II simulates
N reactive transport using a finite difference solution to a multiphase form of the one-dimensional advection-
dispersion-reaction equation. Model output includes daily concentrations of NO3-N and NHx-N in soil solution and drain
flow, daily OC content of the top 20 cm soil layer, and cumulative rates of simulated N processes.