DRAINMOD−N II (Youssef, 2003) considers a detailed N cycle that includes three N pools: NO3−N, NHx−N, and ON (fig. 2). The NHx−N pool is set to be optional, so it may be ignored if soil and environmental conditions do not favor its accumulation in the system. The form of NHx−N in soil is pH dependent. NH4−N dominates in acid to neutral soils. Appreciable amounts of NH3−N appear when soil pH exceeds 7.5 (Tisdale et al., 1993). Processes considered in the model are atmospheric deposition, application of mineral N fertilizers including urea and anhydrous NH3, application of ON sources, plant uptake, mineralization, immobilization, nitrification, denitrification, NH3 volatilization, and NO3−N and NHx−N losses via subsurface drainage and surface runoff (fig. 2).

 

Figure 2. The nitrogen cycle considered in DRAINMOD−N II.