Fractionation of nitrogen isotopes between a plant and its environment occurs during uptake and assimilation of inorganic nitrogen. Fractionation can also occur between roots and the shoot. Under controlled nitrogen conditions, whole-plant and organ-level nitrogen isotope discrimination (Δ15N) is suggested to primarily be a function of three factors: nitrogen efflux back to the substrate relative to gross influx at the root (efflux/influx), the proportion of net influx assimilated in the roots and the export of remaining inorganic nitrogen for assimilation in the leaves. Here, an isotope discrimination model combining measurements of δ15N and nitrogen content is proposed to explain whole-plant and organ-level variation in δ15N under steady-state conditions and prior to any significant retranslocation. We show evidence that nitrogen isotope discrimination varies in accordance with changes to nitrogen supply or demand. Increased whole-plant discrimination (greater Δ15N or more negative δ15N relative to the source nitrogen δ15N) indicates increased turnover of the cytosolic inorganic nitrogen pool and a greater efflux/influx ratio. A greater difference between shoot and root δ15N indicates a greater proportion of inorganic nitrogen being assimilated in the leaves. In addition to calculations of integrated nitrogen-use traits, knowledge of biomass partitioning and nitrogen concentrations in different plant organs provides a spatially and temporally integrated, whole-plant phenotyping approach for measuring nitrogen-use in plants. This approach can be used to complement instantaneous cell- and tissue-specific measures of nitrogen use currently used in nitrogen uptake and assimilation studies.