Nitrogen is an indispensable building block for all life on Earth, forming the structural backbone of DNA and the amino acids that create proteins. It is also an integral component of chlorophyll, the pigment plants use to capture sunlight for energy. Although the atmosphere is composed of approximately 78% nitrogen gas (N₂), this massive reservoir is almost entirely inaccessible to plants. Plants cannot absorb atmospheric nitrogen through their leaves or roots like they do with carbon dioxide or water. They require nitrogen to be chemically “fixed,” or converted into a usable compound, before they can incorporate it into their tissues. This limitation stems from the unique and highly stable molecular structure of atmospheric nitrogen itself.
The Chemical Reason Nitrogen is Unavailable
The primary barrier preventing plants from directly utilizing atmospheric nitrogen is the strength of the chemical bond holding the molecule together. Atmospheric nitrogen exists as a diatomic molecule (N₂), meaning two nitrogen atoms are joined by a triple covalent bond. This triple bond is one of the strongest chemical bonds found in nature, making the N₂ molecule highly inert and stable.
Breaking this strong bond requires a tremendous input of energy. Plants lack the biochemical machinery and high-energy mechanisms necessary to cleave the N₂ molecule within their cells. While energy-intensive processes like lightning strikes or specialized industrial methods can overcome this barrier, plants cannot perform the task. The fundamental biological limitation is the absence of an enzyme capable of catalyzing this energetically unfavorable reaction.
Nature’s Solution Nitrogen Fixing Bacteria
The natural solution that overcomes this chemical roadblock is biological nitrogen fixation, performed exclusively by prokaryotic microorganisms known as diazotrophs. These bacteria and archaea possess the unique enzyme complex called nitrogenase, which reduces atmospheric N₂ to ammonia (NH₃). This specialized enzyme is the only known biological catalyst capable of breaking the N₂ triple bond under physiological conditions.
The nitrogenase enzyme is highly sensitive to oxygen and requires an anaerobic environment to function effectively. The enzyme uses a complex metal cluster—typically containing molybdenum and iron (FeMo-co)—as the site for N₂ binding and reduction. The process is highly energy-intensive, requiring a substantial number of ATP molecules to drive the electron transfer necessary for conversion.
Some significant nitrogen-fixing bacteria, such as Rhizobium, form a symbiotic relationship with plants, particularly legumes like peas, beans, and clover. The bacteria reside within specialized root structures called nodules, where the plant supplies them with carbohydrates for energy. In return, the bacteria fix nitrogen, converting it into ammonium (NH₄⁺), a form the host plant can readily absorb. This partnership ensures the bacteria have the necessary anaerobic conditions, while the plant receives a direct supply of fixed nitrogen.
Completing the Cycle Usable Forms and Absorption
The fixed nitrogen, initially ammonium (NH₄⁺), must undergo further transformation before most plants can use it. This next stage is nitrification, a two-step process carried out by soil bacteria. First, bacteria like Nitrosomonas oxidize ammonium to nitrites (NO₂⁻), and then a second group, such as Nitrobacter, oxidizes the nitrites into nitrates (NO₃⁻).
Nitrates are highly soluble and represent the most common and easily absorbed form of nitrogen for non-leguminous plants. Once absorbed through the roots, the plant incorporates these inorganic ions into organic molecules, a process called assimilation. The nitrogen is built into amino acids and nucleotides, which are then used to synthesize proteins, enzymes, and DNA.
The nitrogen cycle is completed by other microbial processes that ensure continuous turnover. Decomposers perform ammonification, breaking down dead organic matter and waste products to release ammonium back into the soil. Conversely, denitrification, carried out by anaerobic bacteria, converts nitrates back into gaseous N₂ under oxygen-poor conditions, returning nitrogen to the atmosphere.
The Impact of Human Activity on Nitrogen Availability
For millennia, plant growth was limited by the slow rate of biological nitrogen fixation and other natural processes. This constraint was radically altered in the early 20th century with the development of the Haber-Bosch process. This industrial method synthesizes ammonia from atmospheric nitrogen and hydrogen gas under extremely high temperatures (around 400–500°C) and pressures (150–200 atmospheres) using an iron-based catalyst.
The Haber-Bosch process effectively bypasses nature’s limitations by performing industrial nitrogen fixation on a massive scale. The resulting ammonia is the foundation for synthetic nitrogen fertilizers, which increased crop yields globally. This technological breakthrough is credited with sustaining a large portion of the world’s population by providing a predictable supply of fixed nitrogen for agriculture. The industrial process currently adds approximately 165 million tonnes of reactive nitrogen to soils annually, an amount roughly equal to or greater than that fixed naturally by all biological systems.