δ¹⁵N values in plants are determined by both nitrate assimilation and circulation

Nitrogen (N) assimilation is associated with ¹⁴N/¹⁵N fractionation such that plant tissues are generally ¹⁵N-depleted compared to source nitrate. In addition to nitrate concentration, the δ¹⁵N value in plants is also influenced by isotopic heterogeneity amongst organs and metabolites. However, our current understanding of δ¹⁵N values in nitrate is limited by the relatively small number of compound-specific data. We extensively measured δ¹⁵N 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 δ¹⁵N differences for contrasting situations between the two species, and a high ¹⁵N-enrichment in root nitrate. Modelling shows that this ¹⁵N-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 δ¹⁵N can be used as a natural tracer of N fluxes between organs and highlights the potential importance of δ¹⁵N of circulating phloem nitrate.