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

Nitrogen (N) assimilation is associated with 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 in nitrate is limited by the relatively small number of compound-specific data. We extensively measured δ15 N 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 interorgan δ15 N differences for contrasting 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 compartmentalisation whereby despite a numerically small flux value, the backflow of nitrate to roots via the phloem can lead to a c. 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 N fluxes between organs and highlights the potential importance of δ15 N of circulating phloem nitrate.