On inorganic N uptake by vascular plants: Can 15 N tracer techniques resolve the NH 4 + versus NO 3 − “preference” conundrum?

The relative uptake by plants of the two ionic nitrogen (N) forms, ammonium (NH 4 + ) and nitrate (NO 3 − ), 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 NH 4 + and NO 3 − by plants, including those based on unlabelled sources, 15 N‐enriched mineral N forms and variations in the 15 N 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 15 N‐enrichment overcomes this obstacle, it is nevertheless difficult to separate uptake as 15 NH 4 + , uptake as 15 NO 3 − and uptake as 15 NO 3 − derived from the nitrification of 15 NH 4 + . With 15 N 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 NH 4 + or NO 3 − forms of mineral N is examined Experiments using 15 N‐enrichment and 15 N natural abundance are reviewed The direct uptake of 15 NO 3 − is confounded with the uptake of 15 NO 3 − derived from nitrified 15 NH 4 + Plants exhibit plasticity rather than preference in the acquisition of ammonium and nitrate