Nitrogen Isotope Fractionation During Nitrate and Ammonium Uptake in Maize: Hydroponic Evidence and Implications for Ecological Investigations

Understanding nitrogen (N) isotopic fractionation during plant uptake is critical for interpreting δ¹⁵N variations in terrestrial ecosystems. We investigated isotopic discrimination during ammonium (NH₄ ⁺) or nitrate (NO₃ ^-) uptake in maize (Zea mays) grown hydroponically under controlled conditions with 0.2 and 2 mM to represent high and low affinity transport systems, respectively. Nitrogen (¹⁵ε) and oxygen (¹⁸ε) isotopic fractionation during NO₃ ^- uptake were determined. NO₃ ^- uptake exhibited low and concentration-independent ¹⁵ε values (0.2 mM: ¹⁵ε = -2‰; 2 mM: ¹⁵ε = -1.7‰). In contrast, ¹⁸ε was lower at high concentrations (0.2 mM: ¹⁸ε = -5.3‰; 2 mM: ¹⁸ε = -2.1‰). For NH₄ ⁺ uptake, ¹⁵ε was higher and increased with concentration (0.2 mM: ¹⁵ε = -5.7‰; 2 mM: ¹⁵ε = -8.5‰). An isotope mixing model suggests a small NO₃ ^- efflux contributes to ¹⁵N and ¹⁸O enrichment in solution due to significant isotopic fractionation during assimilation. The discrimination between source and plant δ¹⁵N is influenced by the source δ¹⁵N, the magnitude of ¹⁵ε, N supply, and uptake kinetics. While plant δ¹⁵N integrates source δ¹⁵N over time, it is unsuitable as a direct tracer. This study refines the understanding of isotopic fractionation mechanisms in plant N uptake and their implications for δ¹⁵N-based ecological investigations.