On inorganic N uptake by vascular plants: Can 15N tracer techniques resolve the NH4+ versus NO3- preference conundrum?

The relative uptake by plants of the two ionic nitrogen (N) forms, ammonium (NH4+) and nitrate (NO3-), has been the subject of much interest during the past 50 years, resulting in a considerable scientific literature. The general idea is that plants have choice, resulting in preference for either one mineral N form or the other. Unfortunately, there is no specific definition of preference or agreement on how it should be measured. In this review, we critically examine the alternative techniques that have been used to measure the relative uptake of NH4+ and NO3- by plants, including those based on unlabelled sources, N-15-enriched mineral N forms and variations in the N-15 natural abundance of mineral N sources. The main difficulty with using unlabelled N is the antecedent N in plant tissue prior to the imposition of treatments. Although N-15-enrichment overcomes this obstacle, it is nevertheless difficult to separate uptake as (NH4+)-N-15, uptake as (NO3-)-N-15 and uptake as (NO3-)-N-15 derived from the nitrification of (NH4+)-N-15. With N-15 natural abundance, isotopic fractionation during plant uptake complicates the interpretation of data. There is increasing evidence that plants exhibit flexibility or plasticity with respect to the use of mineral N forms rather than preference. Highlights The concept of plant preference for NH4+ or NO3- forms of mineral N is examined Experiments using N-15-enrichment and N-15 natural abundance are reviewed The direct uptake of (NO3-)-N-15 is confounded with the uptake of (NO3-)-N-15 derived from nitrified (NH4+)-N-15 Plants exhibit plasticity rather than preference in the acquisition of ammonium and nitrate

Automated summary

The relative uptake of ammonium (NH4+) and nitrate (NO3-) by plants has been widely studied, but there is still debate on how to measure plant preference for these nitrogen forms. Techniques such as unlabelled sources, N-15-enrichment, and N-15 natural abundance have been used, but each has its limitations and challenges in accurately determining plant uptake preferences. It is suggested that plants exhibit flexibility or plasticity in nitrogen form acquisition rather than strict preference.