Poor growth is often observed in artificial young forests due to insufficient inorganic nitrogen in karst soils. However, little is known about the assimilatory demand of the whole plant for nitrate and the partitioning of nitrate assimilation in roots and leaves in woody plants grown in karst habitats. In this study, Broussonetia papyrifera (L.) Vent (B. papyrifera) seedlings were grown under nearly hydroponic conditions. The isotope mass balance approach was employed to quantify the δsup.15N values of the N assimilates in plant organs and in whole plants for B. papyrifera seedlings grown at different nitrate concentrations. The δsup.15N values of the N assimilates in the whole B. papyrifera seedlings showed a rising trend with increasing nitrate concentration. Increasing the supply of nitrate decreased the leaf-root difference in the δsup.15N values of the N assimilates for B. papyrifera seedlings. Quantifying the δsup.15N values of N assimilates in the whole B. papyrifera seedlings grown under different nitrate concentrations contributes to estimating the assimilatory demand of the B. papyrifera seedlings for nitrate. The leaf-root difference in the δsup.15N values of the N assimilates can be used to estimate the partitioning of nitrate assimilation in the roots and leaves.