Nitrogen (N) is a fundamental building block for all life, forming proteins, enzymes, and the nucleic acids that make up DNA. Although the atmosphere is approximately 78% dinitrogen gas (\(N_2\)), this enormous reservoir is largely unusable by plants and animals. The two nitrogen atoms in the \(N_2\) molecule are held together by a powerful triple bond, making the gas inert and chemically unreactive. To become fertile, the soil must gain this atmospheric nitrogen by converting, or “fixing,” the inert gas into reactive forms like ammonia (\(NH_3\)) or nitrate (\(NO_3^-\)). This crucial conversion occurs through biological processes, intense natural energy events, and industrial activity.
Biological Nitrogen Fixation
Biological nitrogen fixation (BNF) is the largest natural pathway for atmospheric nitrogen to enter the soil. This process is carried out exclusively by certain prokaryotic microorganisms, such as bacteria and archaea. These specialized organisms possess the nitrogenase enzyme, which breaks the triple bond of the \(N_2\) molecule and reduces the inert gas into ammonia (\(NH_3\)). Ammonia is a compound readily used by plants.
The most well-known form is symbiotic fixation, a mutualistic partnership between bacteria and certain plants, primarily legumes like clover, soybeans, and peanuts. Bacteria from the genus Rhizobium colonize the plant’s roots, forming specialized structures called root nodules. Inside the nodules, the bacteria receive carbohydrates from the host plant. In return, Rhizobium fixes atmospheric nitrogen into ammonia, which is transferred to the plant for growth.
Other microorganisms contribute to nitrogen fixation while living freely in the soil or water. Free-living bacteria, such as Azotobacter, and certain cyanobacteria fix nitrogen without needing a plant host. They convert atmospheric nitrogen gas directly into ammonia for their own growth. When these organisms die and decompose, the fixed nitrogen is released into the soil, becoming available to the wider plant community.
Atmospheric Fixation by Natural Events
High-energy natural events, particularly lightning, contribute to atmospheric nitrogen fixation, though on a much smaller scale than biological methods. The intense electrical discharge of a lightning bolt provides enough energy to break the stable triple bond of the \(N_2\) molecule. Once separated, the nitrogen atoms rapidly combine with oxygen to form various nitrogen oxides (\(NO_x\)).
These highly reactive nitrogen oxides dissolve into water vapor and cloud droplets, forming nitric acid. This nitric acid is carried down to the Earth’s surface and into the soil by precipitation, known as wet deposition. The dissolved compounds are deposited as nitrates (\(NO_3^-\)), a form of nitrogen that plants can absorb directly. This process accounts for less than 10% of all natural nitrogen fixation but provides a sporadic source of reactive nitrogen to ecosystems.
Entry via Human Industrial Processes
Human innovation has created a massive, non-natural pathway for atmospheric nitrogen to enter the soil, fundamentally altering the global nitrogen cycle. The most significant method is industrial nitrogen fixation, executed primarily through the Haber-Bosch process. This process synthesizes ammonia (\(NH_3\)) by reacting atmospheric nitrogen with hydrogen gas under extremely high pressure and high temperature, using an iron catalyst.
The resulting ammonia is the raw material for nearly all synthetic nitrogen fertilizer, such as urea and ammonium nitrate. This industrial method converts an estimated 100 million metric tons of atmospheric nitrogen into reactive nitrogen annually. It is the single largest source of new reactive nitrogen on Earth, and its widespread use has dramatically increased global crop yields.
Another major human source of nitrogen is the combustion of fossil fuels in vehicles, power plants, and industrial operations. Burning coal, oil, or natural gas at high temperatures causes nitrogen and oxygen in the air to react, forming nitrogen oxides (\(NO_x\)). These gases are released into the atmosphere, contributing to smog and air pollution.
These airborne nitrogen oxides eventually return to the land and water through wet and dry deposition. The deposition of these compounds, which includes nitric acid, acts as an unintended form of fertilization. This unintentional input of reactive nitrogen can lead to the acidification of soils and disrupt the balance of local ecosystems.