This chapter focuses on the importance of soil moisture on plant nitrogen (N) uptake and the consequent effect on dissolved inorganic N export as evaluated from the viewpoint of water quality and watershed management. In watershed nutrient management for water quality assessment, it is important to understand the critical pathways of nutrient cycles and nutrient transport processes from lands to the receiving water bodies. The chapter provides examples of improvements in N-uptake/export simulation by incorporating the effect of soil moisture into the existing models, such as the target-based N uptake in Hydrological Simulation Program—FORTRAN (HSPF), the demand-based N uptake in Soil and Water Assessment Tool, and the Michaelis–Menton N uptake in HSPF. The chapter utilizes forestland simulation as an example to study the effect of moisture on plant N uptake when using HSPF’s Michaelis–Menten module. It addresses the basic N transformation processes are mineralization, immobilization, nitrification, denitrification, volatilization, plant uptake.