Nitrogen is an essential nutrient for plant survival, often considered the most limiting element for growth in natural systems. After only water and carbon dioxide, nitrogen is the element plants require in the greatest quantity to build their structure and fuel their metabolism. Plants cannot directly utilize the abundant dinitrogen gas found in the atmosphere, forcing them to rely on nitrogen-containing compounds dissolved in the soil solution. This dependence means nitrogen availability hinges on its conversion into specific, usable forms that plant roots can absorb from the soil.
The Preferred Forms of Nitrogen Uptake
Plants primarily absorb nitrogen in two inorganic ionic forms: nitrate (NO3-) and ammonium (NH4+). In well-aerated, healthy soils, nitrate is typically the most prevalent form available for uptake because it is highly mobile and does not bind to negatively charged soil particles. Plants possess specialized membrane proteins called nitrate transporters to actively move the negatively charged nitrate ion into the root cells, a process that requires metabolic energy.
Ammonium, which carries a positive charge, is also readily absorbed by the roots using ammonium transporters. While ammonium is metabolically less costly for the plant to use, its positive charge causes it to bind tightly to soil clay and organic matter, making it less mobile in the soil solution. Plants generally do not absorb nitrites (NO2-). Nitrite is known to be highly toxic to plant cells if absorbed in significant concentrations, so its presence in the soil is usually fleeting.
The Essential Role of Nitrogen in Plant Life
Nitrogen plays a foundational role in all biological macromolecules. Nitrogen is a core component of chlorophyll, the green pigment that captures light energy to drive photosynthesis. Without sufficient nitrogen, chlorophyll production declines, leading to the yellowing of leaves and a reduction in the plant’s ability to generate energy and sugar.
Nitrogen is also the defining element in all amino acids, which are the fundamental building blocks of proteins. These proteins function as structural components within cells and, more dynamically, as enzymes that catalyze nearly all metabolic reactions. Furthermore, nitrogen is an integral part of nucleic acids, specifically DNA and RNA, which contain the genetic instructions for growth, reproduction, and all cellular activities. Finally, nitrogen is found in energy-transfer compounds like adenosine triphosphate (ATP).
Assimilating Nitrogen: From Uptake to Usable Molecules
Once nitrate is absorbed by the roots, the plant must metabolically convert it into a non-toxic, usable form through a process called assimilation. This conversion takes place in two sequential reduction steps, beginning with the enzyme Nitrate Reductase (NR) in the cell’s cytoplasm. Nitrate Reductase catalyzes the first step, transforming the absorbed nitrate (NO3-) into nitrite (NO2-).
The newly formed nitrite is a highly reactive and toxic molecule, which is why it is immediately transported from the cytoplasm into the plastids or chloroplasts. Inside the plastids, the enzyme Nitrite Reductase (NiR) quickly reduces the nitrite into ammonium (NH4+). If Nitrite Reductase cannot keep pace with the influx of nitrate, the buildup of nitrite can interfere with cellular respiration and growth.
The resulting ammonium, whether absorbed directly from the soil or generated internally through reduction, is the final inorganic form before incorporation into organic molecules. This ammonium is rapidly fixed into an organic carbon skeleton using the Glutamine Synthetase-Glutamate Synthase (GS-GOGAT) pathway. Glutamine Synthetase combines the ammonium with the amino acid glutamate to form glutamine, which then serves as the primary gateway for nitrogen to be distributed throughout the plant for the synthesis of all other amino acids and proteins.
How Soil Microbes Create Plant-Available Nitrogen
The availability of nitrate and ammonium to plants is driven by the microbially-mediated transformations within the soil known as the nitrogen cycle. The process of ammonification begins when soil microbes decompose dead organic matter, such as plant residues and animal waste. During this decomposition, the organic nitrogen compounds are converted into inorganic ammonium (NH4+), which is then made available for plant uptake.
A two-stage process called nitrification then acts on this ammonium to produce nitrate. First, a specialized group of bacteria, such as Nitrosomonas, oxidize ammonium into the unstable intermediate, nitrite (NO2-). Immediately following this, another distinct group of bacteria, including Nitrobacter, rapidly converts the nitrite into the stable nitrate (NO3-).
This rapid, two-step microbial conversion is the reason why nitrite rarely accumulates to toxic levels in well-drained, aerobic soils. The soil microbial community dictates that the available nitrogen pool consists almost entirely of ammonium and nitrate, the two forms plants are equipped to handle efficiently.