Multiple Facets of Nitrogen: From Atmospheric Gas to Indispensable Agricultural Input

Abstract

Nitrogen (N) is a gas and the fifth most abundant element naturally found in the atmosphere. N's role in agriculture and plant metabolism has been widely investigated for decades, and extensive information regarding this subject is available. However, the advent of sequencing technology and the advances in plant biotechnology, coupled with the growing interest in functional genomics-related studies and the various environmental challenges, have paved novel paths to rediscovering the fundamentals of N and its dynamics in physiological and biological processes, as well as biochemical reactions under both normal and stress conditions. This work provides a comprehensive review on multiple facets of N and N-containing compounds in plants disseminated in the literature to better appreciate N in its multiple dimensions. Here, some of the ancient but fundamental aspects of N are revived and the advances in our understanding of N in the metabolism of plants is portrayed. It is established that N is indispensable for achieving high plant productivity and fitness. However, the use of N-rich fertilizers in relatively higher amounts negatively affects the environment. Therefore, a paradigm shift is important to shape to the future use of N-rich fertilizers in crop production and their contribution to the current global greenhouse gases (GHGs) budget would help tackle current global environmental challenges toward a sustainable agriculture.

Keywords: agriculture; environment; nitrogen; sustainable food production.

Figure 1

Pattern of global macronutrients use in agriculture from 1961 to 2020.

Figure 2

Illustration of the multiple roles or involvement of nitrogen in plant growth, development, and cellular metabolism. Nitrogen is contained in various biological components and takes part in diverse physiological processes, biochemical reactions, and molecular response toward stress in plants. This illustration was created using the gallery proposed by Biorender (https://app.biorender.com/, accessed on 23 March 2022). The circular shapes and the description were added using Microsoft Office PowerPoint 2016.

Figure 3

Illustration of N metabolism in plants. N is acquired by plants from the soil, which is the major source. Soil microorganisms are responsible for mediating the conversion of the atmospheric N gas (N₂) to a form available to the plant (NO₃ or NH₄). N is also supplied exogenously through fertilizer application. NO₃ and NH₄ are taken up by the roots and transported to the shoot via the action of several transporter-encoding genes, which translocate to the cells. NO₃ and NH₄ undergo series of conversions and are assimilated in the form of amino acids, becoming part of several macromolecules. The NO₃ stored in the vacuole is remobilized in the event of reduced fluxes from soil to the plant. This model was created from available information on the nitrogen metabolism proposed in the literature, including Wang et al. [131], Ali [145], and the Kyoto Encyclopedia of Genes and Genomes (KEGG) pathways database (https://www.genome.jp/pathway/ko00910, accessed on 10 June 2022) using the BioRender design platform (https://app.biorender.com/, accessed on 10 June 2022). Continuous lines with an arrow indicate a positive process or induction. Dotted and dashed lines denote the nitrification and denitrification processes, respectively.