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

Abstract

AbstractThe 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, 15N‐enriched mineral N forms and variations in the 15N 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 15N‐enrichment overcomes this obstacle, it is nevertheless difficult to separate uptake as 15NH4+, uptake as 15NO3− and uptake as 15NO3− derived from the nitrification of 15NH4+. With 15N 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 15N‐enrichment and 15N natural abundance are reviewed The direct uptake of 15NO3− is confounded with the uptake of 15NO3− derived from nitrified 15NH4+ Plants exhibit plasticity rather than preference in the acquisition of ammonium and nitrate