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

TL;DR

This study examines how maize plants fractionate nitrogen isotopes during nitrate and ammonium uptake, offering insights into ecological interpretations of nitrogen isotope data.

Contribution

The study provides new evidence on concentration-dependent isotope fractionation during nitrate and ammonium uptake in maize.

Findings

Nitrate uptake shows low and concentration-independent nitrogen isotope fractionation.

Ammonium uptake exhibits higher and concentration-dependent nitrogen isotope fractionation.

An isotope mixing model indicates that nitrate efflux contributes to isotope enrichment in solution.

Abstract

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.

Controlled hydroponic experiments reveal distinct nitrogen isotope fractionations during nitrate and ammonium uptake in maize, refining our understanding of δ¹⁵N patterns in ecological contexts.

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