The air we breathe is predominantly nitrogen, making up about 78% of the Earth’s atmosphere. This atmospheric nitrogen (\(N_2\)) exists as a diatomic molecule with a strong triple covalent bond. The immense energy required to break this bond makes \(N_2\) chemically inert and unusable by most life forms, despite its abundance. For nitrogen to be incorporated into the amino acids, proteins, and DNA necessary for biological life, it must be removed from the air and converted into a chemically reactive form like ammonia or nitrate. This process, called “nitrogen fixation,” is accomplished by three primary mechanisms—biological, atmospheric, and industrial—making the element accessible to ecosystems worldwide.
The Microbial Machine: Biological Fixation
Biological fixation, performed by specialized microorganisms, is the largest natural mechanism for removing nitrogen from the air. These prokaryotes, including certain bacteria and archaea, possess the unique nitrogenase enzyme complex, which breaks the triple bond of \(N_2\). Nitrogenase, which is sensitive to oxygen and contains iron and molybdenum atoms, catalyzes the conversion of atmospheric nitrogen into ammonia (\(NH_3\)).
This chemical reduction is an energetically expensive process, requiring a significant input of energy, typically around 16 molecules of adenosine triphosphate (ATP) for every molecule of \(N_2\) converted. The process requires a steady supply of electrons, transferred through an iron-containing protein component of the enzyme complex. Ammonia is the first stable, biologically usable form of nitrogen created from the atmospheric gas.
Biological fixation occurs in two main contexts. One is a symbiotic relationship, such as the genus Rhizobium living within the root nodules of leguminous plants like peas and beans. The plant provides an oxygen-free environment and energy for the bacteria’s work. Other microbes, such as free-living cyanobacteria and bacteria like Azotobacter, perform non-symbiotic fixation in soil and aquatic environments. These microbial systems are responsible for approximately 90% of all natural nitrogen fixation.
The High-Energy Process of Lightning
Atmospheric fixation is driven by the immense energy released during lightning strikes. A bolt of lightning heats the air it passes through, providing the necessary energy to break the \(N_2\) triple bond. This extreme heat causes the freed nitrogen atoms to react with oxygen (\(O_2\)), forming various nitrogen oxides (\(NO_x\)).
These nitrogen oxides dissolve in the moisture of the clouds, leading to the formation of nitric acid (\(HNO_3\)). This acid ionizes to form nitrate (\(NO_3^-\)), a soluble form of reactive nitrogen. Precipitation carries these nitrates to the Earth’s surface, depositing them directly into the soil for plant uptake. While minor compared to biological fixation, this process contributes thousands of tons of fixed nitrogen to global ecosystems daily.
Human Intervention: Industrial Fertilizer Production
The Haber-Bosch process is the primary human method for removing atmospheric nitrogen. Developed in the early 20th century by chemists Fritz Haber and Carl Bosch, this industrial process converts atmospheric nitrogen and hydrogen gas (\(H_2\)) into ammonia (\(NH_3\)). The procedure requires extremely high energy inputs, utilizing temperatures between 400 and 500 degrees Celsius and pressures ranging from 150 to 200 atmospheres.
In the reaction vessel, a finely divided iron-based catalyst accelerates the reaction. The resulting ammonia is condensed out of the gas mixture as a liquid, and the unreacted nitrogen and hydrogen are recycled back into the process to maximize efficiency. The vast majority of the ammonia produced is used to manufacture synthetic nitrogen fertilizers, which support a significant portion of the global population by increasing crop yields. This massive-scale industrial fixation has effectively doubled the amount of reactive nitrogen cycling through the environment compared to pre-industrial times.
Locking Nitrogen into Living Systems
Once atmospheric nitrogen has been fixed into a reactive form like ammonia or nitrate, the next step is its incorporation into the biomass of living organisms, known as assimilation and immobilization. Plants absorb these inorganic nitrogen compounds from the soil through their roots. Nitrate is often reduced back into ammonium within the plant cells before use, a process that requires additional energy.
The ammonium is then rapidly incorporated into organic molecules, primarily through the synthesis of amino acids, the building blocks of proteins. This nitrogen is also used to construct nucleic acids like DNA and RNA. When animals consume plants, the nitrogen is transferred and immobilized within their organic tissues. This fixed nitrogen remains held within the biomass of the ecosystem, unavailable to return to the atmosphere, until organisms die and are broken down by decomposers.