The usable forms of nitrogen are nitrate, ammonium, and amino acids. Although nitrogen gas makes up about 78% of Earth’s atmosphere, the triple bond holding each molecule together is so strong that most living things cannot use it directly. Nitrogen must first be converted into reactive, or “fixed,” forms before plants, animals, and microorganisms can incorporate it into proteins, DNA, and other essential molecules.
Why Atmospheric Nitrogen Is Unusable
Nitrogen gas exists as two nitrogen atoms bonded together with an extremely stable triple bond. This bond requires a massive amount of energy to break, which is why more than 95% of nitrogen in the environment, including in seawater, remains in this inert form. Living cells simply lack the chemistry to crack it open on their own, with a few important exceptions.
The Three Forms Plants Actually Absorb
Plants pull nitrogen from the soil in three forms: nitrate, ammonium ions, and amino acids from decomposed organic matter. Under normal soil conditions, nitrate is the dominant form available. In flooded or acidic soils, ammonium takes over as the primary source.
Once inside the plant, both nitrate and ammonium are funneled into a cycle that builds glutamine and glutamate, two amino acids that serve as the starting materials for all the other nitrogen-containing compounds the plant needs. From there, nitrogen gets incorporated into dozens of other amino acids, nucleic acids, and chlorophyll. This internal conversion process is why nitrogen is considered the most important macronutrient for plant growth.
Amino acids absorbed directly from compost-amended or manure-rich soils give plants a shortcut, since the nitrogen is already in organic form and requires less internal processing.
How Nitrogen Gets Fixed in Nature
Biological nitrogen fixation is the main natural pathway for converting atmospheric nitrogen into something usable. Certain bacteria and cyanobacteria produce an enzyme called nitrogenase, which breaks the triple bond of nitrogen gas and combines it with hydrogen to produce ammonia. The process requires significant energy: eight electrons and at least 16 units of the cell’s energy currency per molecule of nitrogen fixed, along with the release of one molecule of hydrogen gas as a byproduct.
The best-known example is the partnership between legumes (beans, peas, clover) and rhizobia bacteria that live in nodules on their roots. In the ocean, cyanobacteria of the genus Trichodesmium were long considered the primary nitrogen fixers in tropical and subtropical waters, though researchers have since discovered smaller unicellular species and symbiotic bacteria living inside diatoms that also contribute substantially.
Lightning also fixes a small amount of nitrogen by providing enough energy to split nitrogen molecules in the atmosphere, but biological fixation dwarfs this contribution.
Nitrification: Ammonia to Nitrate
Once ammonia enters the soil, specialized bacteria convert it into nitrate through a two-step process called nitrification. In the first step, ammonia-oxidizing bacteria (primarily Nitrosomonas) convert ammonia into nitrite. In the second step, nitrite-oxidizing bacteria (primarily Nitrobacter) convert nitrite into nitrate. Both steps require oxygen, which is why waterlogged soils tend to accumulate ammonium instead of nitrate.
This bacterial relay matters because nitrate is highly mobile in soil. It dissolves easily in water and moves wherever water flows, making it readily available to plant roots but also vulnerable to leaching into groundwater.
Industrial Nitrogen Fixation
The Haber-Bosch process, developed in the early 1900s, mimics what nitrogenase does biologically but at industrial scale: it combines atmospheric nitrogen with hydrogen gas under high temperature and pressure to produce ammonia. About 70% of global ammonia production goes toward fertilizers, with the rest used in explosives, plastics, rubber, and synthetic fibers.
The process consumes roughly 26 gigajoules of energy per tonne of ammonia produced and accounts for about 2% of the world’s total energy consumption. It also generates around 1.3% of global carbon dioxide emissions. Without it, modern agriculture could not feed the current global population, since natural nitrogen fixation alone cannot keep pace with crop demand.
How Humans Get Usable Nitrogen
Humans obtain nitrogen entirely through dietary protein. When you eat protein, your digestive system breaks it down into amino acids, which your body then reassembles into its own proteins, enzymes, and DNA components. The average adult needs about 104 mg of nitrogen per kilogram of body weight per day to maintain nitrogen balance, meaning the amount taken in equals the amount lost through urine, feces, and skin. For a 70 kg (154 lb) person, that works out to roughly 7.3 grams of nitrogen daily, equivalent to about 0.75 grams of protein per kilogram of body weight.
When Usable Nitrogen Becomes a Problem
The same reactive nitrogen that sustains life becomes a pollutant in excess. When more fertilizer is applied than crops can absorb, nitrate leaches through soil into groundwater. The EPA and World Health Organization set the safe limit for nitrate in drinking water at 10 mg/L. Research on agricultural runoff shows that with repeated fertilizer application, nitrate concentrations in groundwater can climb well above this threshold and continue rising even after fertilization stops, because nitrogen stored in soil keeps releasing nitrate over time.
In surface waters, excess nitrogen fuels explosive algae growth, a process called eutrophication. When the algae die and decompose, the process consumes dissolved oxygen, creating dead zones where fish and other aquatic life cannot survive. Nitrogen is the nutrient that most commonly limits growth in marine environments on short timescales, so even modest increases in nitrogen runoff reaching coastal waters can trigger these blooms. This is now one of the most widespread water quality problems globally, driven largely by agricultural fertilizer use and, to a lesser extent, wastewater discharge.