Plants do use ammonia, but primarily in its ionized form, ammonium (NH4+), rather than its gaseous state. Nitrogen is a fundamental element that plants must acquire from their environment to grow and thrive. The ammonium ion is a direct, usable form that is rapidly taken up by roots. This essential nutrient is often the single most limiting factor for plant growth in many terrestrial ecosystems.
Why Plants Need Nitrogen
Nitrogen is an indispensable building block, comprising a significant percentage of a plant’s dry weight. It is a core component of all amino acids, which link together to form proteins. These proteins include the enzymes that catalyze nearly every biochemical reaction necessary for the plant’s survival and growth.
Nitrogen is also integral to the structure of nucleic acids, specifically deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). These molecules contain the genetic instructions and machinery required for cell division and growth, directing the plant’s development. Furthermore, nitrogen forms the central part of the chlorophyll molecule, the green pigment that captures light energy to fuel photosynthesis. Without sufficient nitrogen, a plant cannot effectively convert carbon dioxide and water into the sugars it needs for energy and structure.
How Plants Obtain Nitrogen from Soil
Plants absorb nitrogen from the soil solution predominantly in two inorganic forms: the positively charged ammonium ion (NH4+) and the negatively charged nitrate ion (NO3-). The uptake of both forms occurs through specialized transport proteins embedded in the root cell membranes. Ammonium is absorbed using high-affinity ammonium transporters (AMTs), while nitrate is moved into the cell by nitrate transporters (NRTs).
In most well-aerated soils, nitrate is the more abundant form because soil microbes rapidly convert ammonium to nitrate. While many plants exhibit a preference for nitrate, the absorption of ammonium is generally more efficient in terms of initial energy cost. Nitrate must first be converted into ammonium inside the plant, a two-step process requiring specific enzymes and a significant energy expenditure of both electrons and adenosine triphosphate (ATP). In contrast, ammonium can be incorporated directly into organic compounds, bypassing the energy-intensive reduction steps.
Converting Ammonium into Plant Tissue
Once the ammonium ion is absorbed into the root cell, it cannot be allowed to accumulate because free ammonium is toxic to plant cells. The presence of excess NH4+ can disrupt the cell’s proton balance, which is essential for cellular functions. Therefore, the plant must quickly convert the absorbed ammonium into non-toxic organic molecules, a process known as assimilation.
This rapid conversion takes place primarily through the Glutamine Synthetase-Glutamate Synthase (GS-GOGAT) pathway. The first step involves the enzyme glutamine synthetase (GS), which combines the ammonium with glutamate to form glutamine. This reaction requires energy in the form of ATP to proceed.
The second enzyme in the pathway, glutamate synthase (GOGAT), then uses the glutamine to produce two molecules of glutamate. One of these glutamate molecules is recycled to continue the assimilation cycle. The other is used as the foundational amino acid to synthesize all other amino acids and nitrogen-containing compounds. This immediate incorporation ensures that free ammonium levels remain low, protecting the plant from cellular damage.
The Microbial Engine of Nitrogen Supply
The forms of nitrogen available to the plant are largely determined by the activity of soil microorganisms. These microbes drive the nitrogen cycle, which continuously transforms nitrogen between various chemical states. When organic matter, such as dead leaves and roots, decomposes, bacteria and fungi perform a process called ammonification.
Ammonification releases ammonium (NH4+) from the organic nitrogen compounds, making it available for plant uptake. This ammonium is then quickly acted upon by another group of bacteria in a two-step process known as nitrification. The first step converts ammonium into nitrite (NO2-), and the second converts the nitrite into nitrate (NO3-).
These microbial processes ultimately dictate the ratio of nitrate to ammonium available in the soil solution for the plant roots to absorb. The health and composition of the microbial community therefore directly affect the plant’s nitrogen nutrition, acting as the primary biological engine that supplies usable nitrogen forms.