What Are the Main Nitrogen Sources for Plants?
Learn how nitrogen is converted from its environmental origins into the specific forms plants absorb to support fundamental growth and development.
Nitrogen is a chemical element required for plant growth and survival. It is a primary component of many organic molecules, including the amino acids that form proteins, which act as structural components and enzymes. Nitrogen is also a core part of the chlorophyll molecule, enabling photosynthesis, and a part of nucleic acids like DNA, which hold genetic instructions. Healthy plant tissues often contain between 3 and 4 percent nitrogen by mass.
Natural Nitrogen Reservoirs
The largest single reservoir of nitrogen is the atmosphere, where it exists as dinitrogen gas (N₂). This gas makes up approximately 78% of the air, but its two nitrogen atoms are held together by a strong triple bond, rendering it inaccessible to plants directly. For plants to use it, this atmospheric nitrogen must first be converted, which is a limiting factor in many ecosystems.
A more accessible reservoir of nitrogen is found within the soil’s organic matter. This includes a complex mix of decomposing plant and animal materials, microbial biomass, and stable humus. Within this organic material, nitrogen is locked in large, complex molecules, representing a significant long-term supply for terrestrial ecosystems.
The release of nitrogen from soil organic matter occurs through a process called mineralization. During decomposition, soil microbes, such as bacteria and fungi, break down the complex organic compounds. This process transforms the organic nitrogen into inorganic forms, primarily ammonium (NH₄⁺).
Biological Nitrogen Fixation
Biological nitrogen fixation is a process where specific microorganisms convert atmospheric nitrogen gas into ammonia (NH₃), a form plants can use. This natural process is a significant source of new nitrogen for ecosystems. The conversion is accomplished with an enzyme complex called nitrogenase, which breaks the strong triple bond of the N₂ molecule.
Symbiotic nitrogen fixation is a well-known example, involving a cooperative relationship between certain plants and bacteria. Legumes, such as soybeans, peas, and clover, form a partnership with Rhizobium bacteria. These bacteria infect the plant’s roots, forming specialized structures called nodules where they fix atmospheric nitrogen for the plant.
Nitrogen fixation also occurs through free-living microorganisms in the soil and water. Bacteria such as Azotobacter and Clostridium live independently in the soil and contribute to the nitrogen pool. In aquatic environments, cyanobacteria are also significant contributors to the nitrogen cycle through this process.
Synthetic Nitrogen Fertilizers
Modern agriculture relies on synthetic nitrogen fertilizers to provide the large quantities of nitrogen needed for high crop yields. These fertilizers are manufactured through industrial processes to deliver nitrogen in a readily available form, and their use has significantly increased global food production.
Common types of synthetic nitrogen fertilizers include urea, ammonium nitrate, and anhydrous ammonia. Urea is a solid, granular fertilizer, while ammonium nitrate is another solid that is highly soluble in water. Anhydrous ammonia is a gas injected directly into the soil, where it converts to the ammonium form.
The production of these fertilizers begins with the Haber-Bosch process. This industrial procedure combines atmospheric nitrogen with hydrogen gas under high temperatures and pressures to produce ammonia. This ammonia then serves as the foundational ingredient for manufacturing other nitrogen fertilizers.
Plant Nitrogen Uptake and Use
Regardless of its origin, nitrogen must be in specific inorganic forms to be absorbed by plants. Plant roots primarily take up nitrogen from the soil solution as nitrate (NO₃⁻) and ammonium (NH₄⁺), though preference can vary by species and soil conditions.
Once inside the plant, this inorganic nitrogen is converted into organic molecules through assimilation. Nitrate is first reduced to ammonium, which is then incorporated into carbon skeletons to form amino acids. These newly synthesized amino acids are then transported to areas of active growth, such as new leaves, shoots, and developing seeds.