Plant Science; Biochemistry; Ecology

Plant Science; Biochemistry; Ecology

Understanding the mechanisms controlling whole-plant and foliar nitrogen isotope composition will advance our knowledge of plant nitrogen acquisition and allocation. Nitrogen is the element that most often limits plant growth in many terrestrial ecosystems 1. Anthropogenic activity has altered the amount and relative abundance of the forms of nitrogen (NH4+, NO3− and amino acids) that are available for plant absorption [1] and [2]. The forms of nitrogen absorbed by plants can have different isotope compositions 3, and many studies now routinely measure foliar δ15N (Box 1) in an attempt to understand differences in patterns of nitrogen use among co-occurring species. Many studies assume that 15N at natural abundance levels acts as a tracer (i.e. the isotope ratio of source nitrogen is preserved during nitrogen absorption, assimilation and translocation, and that the δ15N of leaf tissues reflects that of the nitrogen source in the soil). This assumption is important because although the reported variation in plant δ15N can be between −10‰ and +10‰, the difference among co-occurring species is often less (0–10‰), and biologically significant differences can be ∼1‰ (Ref. 4). However, it is clear that this assumption could be invalid because physiological factors, such as different nitrogen uptake mechanisms, different pathways of assimilation, and recycling of nitrogen in the plant, can discriminate against 15N. This review addresses how physiological transformations of nitrogen can influence whole-plant and leaf δ15N. Ecological aspects of δ15N measurements and δ15N of plant nitrogen sources are addressed in [3] and [5].

Nitrogen exists as two naturally occurring stable isotopes, 15N and 14N. Variation in the absolute abundance of 15N is small , therefore nitrogen isotope composition is expressed using δ notation in parts per thousand.

(I)Where δ15N is the isotope ratio relative to the atmospheric air standard, and Rsample and Rstandard are the molar ratios of the heavier to the lighter isotope. The value for Rstandard is 0.0036765. Differences in δ15N between a substrate and product will occur when 15N and 14N react at different rates. The ratio of the rate constants (k14/k15) is the isotope effect (α), and is equivalent to (Rsubstrate/Rproduct)a. Discrimination (Δ) is the deviation of α from unity (Δ=α–1)(Ref. a). Discrimination can be stated in relation to the δ15N of the substrate (δ15Ns) and product (δ15Np)

(II)The term (1+δ15Np/1000) does not differ significantly from 1, therefore an approximation is (Eq. (III)):

(III)Discrimination is positive in most biological systems, therefore, the product should have a lower δ15N value than the substrate.

 Abstract :

Whole-plant and leaf nitrogen isotope composition are determined by the isotope ratio of the external nitrogen source and physiological mechanisms within the plant. Whole-plant isotope composition can reflect that of the nitrogen source when plant demand exceeds nitrogen supply. Uptake by mycorrhizae can cause the isotope ratio of the plant to deviate from the source. Intra-plant variation in isotope composition can be caused by multiple assimilation events, organ-specific loss of nitrogen, and resorption and reallocation of nitrogen. Future work must address acquisition of organic nitrogen from the soil solution, the role of mycorrhizae, and internal transformations within the plant.