δ15 N values in plants is determined by both nitrate assimilation and circulation.

Nitrogen (N) assimilation is associated with a ¹⁴ N/¹⁵ N fractionation such that plant tissues are generally ¹⁵ N-depleted compared to source nitrate. In addition to nitrate concentration, the δ¹⁵ N value in plants is also influenced by isotopic heterogeneity amongst organs and metabolites. However, our current understanding of δ¹⁵ N values is limited by the relatively small number of compound-specific data in nitrate. We did extensive δ¹⁵ N measurement in nitrate at different time points, in sunflower and oil palm grown at fixed nitrate concentration, with nitrate circulation being varied using potassium (K) conditions and waterlogging. There were strong inter-organ δ¹⁵ N differences with contrasted situations between the two species, and a high ¹⁵ N-enrichment in root nitrate. Modelling shows that this ¹⁵ N-enrichment can be explained by nitrate circulation and compartmentalization whereby despite a numerically small flux value, the backflow of nitrate to roots via the phloem can lead to a ≈30‰ difference between leaves and roots. Accordingly, waterlogging and low K conditions, which down-regulate sap circulation, cause a decrease in the leaf-to-root isotopic difference. Our study thus suggests that plant δ¹⁵ N can be used as a natural tracer of nitrogen fluxes between organs and highlights the potential importance of δ¹⁵ N of circulating phloem nitrate.