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

Abstract

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, 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

Mechanisms of Nutrient Uptake and Assimilation Processes in Some Plants: A Review

“…The major limitation of these techniques is that they tend to overlook huge quantities of ion carried across roots into the xylem and lastly to shoots. This is to say that, in reality, only little percentage of nutrients absorbed are reserved in roots, and greater percentage are transferred to shoots (Asher and Ozanne, 1967;Loneragan andSnowball, 1969, Chalk andSmith, 2020). Consequently, determination of ions in both roots and shoots should be carried out in ion absorption experiments (Fageria et al, 2006;Abdolzadeh et al, 2008;Griffiths and York, 2020).…”

Rapid foliar uptake of inorganic and amino acid nitrogen in three dryland plant species

“…Shallow-rooted dryland grasses can have greater radial root spread to capture soil water from small rain events (Sala and Lauenroth 1982;Thomey et al 2014), whereas woody plants can extend lateral and vertical roots to access water and/or nutrients in deeper soil layers (Lee and Lauenroth 1994;Wan et al 1995). While these adaptations could also allow coexisting plants to partition resources based on soil depth, the similarities in N uptake observed among B. eriopoda, A. hymenoides and G. sarothrae indicate that rather than specializing in a certain chemical N form to avoid competing for the same limiting resource, these species may employ similarly flexible strategies for acquiring whichever soil N source is most available when growth conditions are favorable (Ashton et al 2008;Chalk and Smith 2021;Stahl et al 2011).…”

“…Several soil-based dryland experiments found low relative uptake of amino acid-N tracers into plant tissues compared to inorganic N, which was attributed to strong microbial competition for organic N rather than niche specialization among species (Chen et al 2015;Huygens et al 2016;Jin and Evans 2010;Ouyang et al 2016). An alternate understanding of resource use by N-limited plants is that cooccurring species exhibit niche plasticityrather than specializationand can take up multiple forms of N from shared soil pools wherever and whenever they are available (Ashton et al 2010;Chalk and Smith 2021;Chesson et al 2004). Differences in dryland plant uptake of NH4 + , NO3 -, and amino acid-N are more likely to be driven by individual growth strategies and competition for spatiotemporally variable local N sources than by species-specific "preferences" for N (Hong et al 2017;James and Richards 2007;James et al 2009;Patrick et al 2009;Stahl et al 2011), and it is possible that dryland plant species may exhibit resource use plasticity, but more comparative studies of N uptake dynamics among species with different growth strategies are needed, especially if soil water and nutrient uptake are asynchronous following a moisture pulse to these systems (BassiriRad and Caldwell 1992;Gebauer and Ehleringer 2000).…”

Partitioning of Environmental and Taxonomic Controls on Brazilian Foliar Content of Carbon and Nitrogen and Stable Isotopes

“…Moreover, a recent study developed an isoscape of soil δ 15 N for South America, suggesting a high variation of values across the Brazilian territory (Sena-Souza et al, 2020), which also helps to explain the dominance of the environmental filter on predicting foliar δ 15 N. It is important to notice that still ∼16% of the variance of foliar δ 15 N was explained by taxonomy (Figure 2). Plant species can be important in driving N isotopic ratios in several ways (Högberg, 1997), such as "preferential" uptake of a particular mineral; atmospheric or organic N form (Chalk and Smith, 2020); and metabolic-induced changes of N isotopic ratios (Robinson, 2001;Yoneyama et al, 2001). For instance, Roupala montana (Proteaceae), a common species found in the Cerrado, is more effective in acquiring NH 4 + related to NO 3 − .…”