Nitrogen (N) availability in crop systems is largely determined by fertilizer application rates. In relatively undisturbed native ecosystems, however, N becomes available largely as a result of internal cycling of this element. Therefore, conceptual or quantitative models of plant N uptake must effectively incorporate processes of N cycling such as decomposition and mineralization. The chapter suggests that those mineralization models, be they simple functional or mechanistic models, could improve the reliability of the N-uptake models if they capture the high temporal and spatial variability. It offers road maps by which mechanistic models of soil N availability can be linked to physiological plant growth models to accomplish that effort. Despite the clear role of mineralization in determining soil N availability, models of plant N uptake in native systems seldom integrate it with mechanistic parameters such as root uptake kinetics and morphology.