Intra‐Plant Nitrogen Isotope Fractionation in Maize

ABSTRACT Background and aim The natural abundance of nitrogen isotopes (δ 15 N) provides a powerful integrative tracer of nitrogen uptake and assimilation in plants, yet the internal partitioning of δ 15 N among plant tissues remains insufficiently understood. Methods Here, we quantified isotopic discrimination during ammonium (NH 4 + ) and nitrate (NO 3 – ) uptake in maize ( Zea mays L.) grown under axenic hydroponic conditions at 0.2 and 2 mM N, representing low and high affinity transport systems, respectively. Results Distinct and systematic isotopic offsets between roots and shoots revealed contrasting assimilation patterns for the two nitrogen forms. Under NH 4 + nutrition, shoot and root δ 15 N were nearly identical, consistent with predominant assimilation in roots and limited translocation of reduced nitrogen. In contrast, NO 3 – nutrition produced strong 15 N enrichment in shoots relative to roots, reflecting spatially partitioned NO 3 – reduction and assimilation between organs. The δ 15 N of tissue NO 3 – was inversely related with its concentration, supporting isotopic fractionation during nitrate reduction and dilution by vacuolar storage. These results demonstrate that intra‐plant δ 15 N partitioning primarily reflects the spatial localization of nitrogen assimilation. Conclusion By establishing mechanistic baselines for intrinsic nitrogen isotope fractionation under controlled conditions, this study refines the physiological interpretation of plants δ 15 N and enhances its application to ecological and biogeochemical investigations.