Technical Abstract: An understanding of how plants regulate and take up N is important if models are to simulate N dynamics realistically. Many current models consider critical percentages of N in plants in order to calculate demand and account for the effects of N availability on plant growth. Our objective was to quantify the relative amounts of active and passive N (as nitrate) uptake, and carbon assimilation rates in potato under a range of six soil N contents (2, 4, 6, 8, 11, and 14 mM N) and two CO2 levels (370 and 700 umol M-1). A second objective was to evaluate an approach to simulate active and passive N uptake using the soil model 2DSOIL and a simple crop model to determine how well the overall approach describes measured phenomenon. In the model, nitrogen uptake is regulated by carbon assimilation rate, concentration of nitrogen in the soil, and two parameters of the Michaelis-Menton equation, maximum N flux rate and the concentration of N in the soil where the flux rate is one half the maximum. The experiments were carried out using pots in daylit growth chambers with temperature and CO2 control. The crop was potato (Solanum tuberosum L.). Carbon assimilation and transpiration rates, light interception, and end of season biomass and N contents were measured. Total N uptake was calculated from carbon assimilation rates and N contents, N uptake by mass flow was calculated from transpiration and soil N content and diffusive fluxes were calculated by difference. Mass flow rates of N uptake were less than diffusive rates and the difference increased with decreasing N application rate. Simulated trends were similar to measured. The results indicated that simulation of N uptake using carbon assimilation, C/N ratio, and soil N content to regulate N uptake is a useful approach.