Nitrogen is an indispensable element for all life on Earth, forming the structural backbone of proteins, enzymes, and the genetic material DNA and RNA. It is also a core component of chlorophyll, the pigment that captures light energy for photosynthesis. Although atmospheric air is nearly 78% nitrogen gas (\(\text{N}_2\)), plants cannot absorb this inert form directly. To meet their nutritional requirements, plants must obtain nitrogen from the soil, but only after it has been converted into specific, usable compounds.
The Essential Forms for Plant Uptake
Non-legume plants primarily absorb nitrogen in two inorganic forms: Nitrate (\(\text{NO}_3^-\)) and Ammonium (\(\text{NH}_4^+\)). Both of these ions are dissolved in the soil water and are taken up through specialized transporter proteins in the root cells. The uptake of nitrate is generally preferred in well-aerated soils, which is why it is often the most common form absorbed by plants for growth.
Nitrate is highly mobile and easily moves with the flow of water through the soil profile, making it readily available to roots. However, this mobility also means nitrate can be easily lost from the soil through leaching into groundwater.
Ammonium, conversely, is positively charged and tends to bind to the negatively charged clay and organic matter particles in the soil, making it less mobile. Plants can absorb and utilize both forms simultaneously, but their preference is often influenced by external factors like soil pH and oxygen levels.
While ammonium is more metabolically efficient to use, absorbing high concentrations can be toxic to plant cells. Nitrate must be chemically converted inside the plant before it can be used, a process that requires more energy.
Processing Nitrogen Inside the Plant
Once nitrogen has been absorbed by the roots, it must be assimilated, or incorporated, into organic compounds like amino acids to prevent toxicity and support growth. If the plant takes up the ammonium ion, it is rapidly incorporated into amino acids, primarily glutamine and glutamate, through the GS-GOGAT pathway. This rapid assimilation is necessary because ammonium is toxic if allowed to accumulate in the plant cells.
When the plant absorbs nitrate, it must first be reduced in a two-step, energy-intensive process. The enzyme nitrate reductase converts nitrate (\(\text{NO}_3^-\)) into nitrite (\(\text{NO}_2^-\)) in the plant’s cytosol. Nitrite is an intermediate form that is highly toxic to the cell.
The nitrite is then quickly transported into the chloroplasts of the leaves or the plastids of the roots, where the enzyme nitrite reductase converts it into the less harmful ammonium ion. After this reduction, the resulting ammonium is immediately channeled into the GS-GOGAT pathway to form amino acids. The entire process of nitrate reduction requires reducing factors like NADH or NADPH.
How Usable Nitrogen is Created in the Soil
The usable forms of nitrogen, nitrate and ammonium, are made available to non-legume plants through the actions of soil microorganisms in the nitrogen cycle. The process begins with ammonification, where fungi and bacteria decompose dead plant and animal matter. This decomposition converts the organic nitrogen locked in proteins and nucleic acids into inorganic ammonium (\(\text{NH}_4^+\)).
The newly formed ammonium can be taken up directly by plant roots, but it also serves as the starting point for nitrification. Nitrification is a two-step aerobic process carried out by specialized bacteria, which is most rapid in warm, moist, and well-aerated soils. In the first step, bacteria such as Nitrosomonas oxidize ammonium into nitrite (\(\text{NO}_2^-\)).
In the second step of nitrification, a different group of bacteria, like Nitrobacter, quickly converts the nitrite into nitrate (\(\text{NO}_3^-\)). This microbial conversion is important because it changes the less mobile ammonium into the highly mobile nitrate form. Non-legume plants are entirely dependent on these microbial processes to supply the necessary inorganic nitrogen ions from the soil.
Clarifying the Non-Legume Distinction
The specific focus on non-legume plants highlights a major difference in how plants acquire nitrogen. Unlike most other plants, legumes, such as peas, beans, and clover, have a unique partnership with Rhizobium bacteria. This symbiotic relationship allows legumes to perform biological nitrogen fixation.
The Rhizobium bacteria live within specialized structures called root nodules, where they convert atmospheric nitrogen gas (\(\text{N}_2\)) directly into ammonia, which is then made available to the host plant. This process bypasses the need for the plant to rely solely on the nitrate and ammonium produced by the general soil nitrogen cycle. Non-legume plants, lacking this specialized symbiotic relationship, must instead absorb their nitrogen from the soil solution as nitrate and ammonium.