Ammonia forms through several distinct processes, from bacterial activity in soil to massive industrial reactors. The most common natural route is biological nitrogen fixation, where soil bacteria convert atmospheric nitrogen gas into ammonia. The most common artificial route is the Haber-Bosch process, which produces roughly 150 million metric tons of ammonia per year worldwide. Your own body also generates ammonia constantly as a byproduct of breaking down protein.
Biological Nitrogen Fixation
The atmosphere is about 78% nitrogen gas, but most living things can’t use nitrogen in that form. The two nitrogen atoms in each molecule are locked together by an extremely strong triple bond. Only one enzyme in all of biology can break it: nitrogenase, found in certain bacteria and archaea.
Nitrogenase works by feeding electrons and protons into the nitrogen molecule one at a time, gradually weakening the triple bond rather than snapping it all at once. This process consumes a large amount of cellular energy. The bacteria that carry it out, including species like Azotobacter and Klebsiella, live freely in soil or form partnerships with plant roots, particularly legumes like soybeans and clover. The ammonia they produce gets absorbed by plants and incorporated into proteins, DNA, and other nitrogen-containing molecules that all life depends on.
Nitrogenase exists in three versions, each built around a different metal at its core: molybdenum, vanadium, or iron. The molybdenum version is the most common and the best studied. All three appear to use the same general eight-electron mechanism to convert one molecule of nitrogen gas into two molecules of ammonia.
Decomposition and Ammonification
When organisms die or excrete waste, the nitrogen locked in their tissues (in amino acids, DNA, and other organic molecules) doesn’t stay locked up for long. Fungi and bacteria decompose this organic matter and release the nitrogen back into the environment as ammonia, a process called ammonification. That ammonia then becomes available for plants and microorganisms to take up and use for growth, completing a loop in the nitrogen cycle.
This is one reason compost and manure work as fertilizers. The microbes doing the decomposition are effectively converting complex biological molecules back into the simple ammonia that plants can absorb through their roots.
Lightning and Atmospheric Chemistry
Lightning contributes to ammonia formation, though indirectly. A lightning strike’s electrical energy is intense enough to break apart nitrogen molecules in the atmosphere, creating various nitrogen oxide compounds. These fall to Earth dissolved in rainwater. Once in the soil, bacteria convert those nitrogen oxides into ammonia. This pathway produces far less ammonia than biological fixation or industrial synthesis, but it has been cycling nitrogen for billions of years.
The Haber-Bosch Process
Industrial ammonia production dwarfs every natural source combined. The Haber-Bosch process, developed in the early 1900s, forces nitrogen gas and hydrogen gas to react under extreme conditions: temperatures around 400 to 500°C and pressures roughly 150 to 300 times atmospheric pressure. An iron-based catalyst speeds the reaction along, but even with the catalyst, the conditions have to be intense because that nitrogen triple bond is so resistant to breaking.
Global production hit an estimated 150 million metric tons in 2023. In the United States alone, about 88% of that ammonia goes toward fertilizer, making it one of the foundations of modern agriculture. The process is also enormously energy-intensive, accounting for roughly 1 to 2% of global energy consumption. The hydrogen feedstock typically comes from natural gas, which is why ammonia production carries a significant carbon footprint.
Ammonia From Protein Metabolism
Your body produces ammonia every time it breaks down amino acids, the building blocks of protein. When cells need energy or need to recycle amino acids they don’t require, enzymes strip off the nitrogen-containing portion (the amino group) in a process called deamination. This happens primarily in the liver. The stripped amino group ultimately gets released as free ammonia.
Ammonia is toxic to the brain, so the liver converts it into a much safer compound, urea, through a five-step cycle. In the first step, ammonia combines with carbon dioxide to form a starter molecule. Over the next four steps, a second nitrogen atom (donated by a different amino acid) gets incorporated, and the final product, urea, is released into the bloodstream. Your kidneys filter it out and excrete it in urine. This cycle runs continuously, and the ornithine molecule that carries the reaction through its steps is regenerated at the end, ready to go again.
Gut Bacteria as an Ammonia Source
Your intestines are another significant source of ammonia. Urea produced by the liver doesn’t all go to the kidneys. Some of it gets transported into the colon, where gut bacteria break it down using an enzyme called urease. This splits urea back into ammonia and carbon dioxide. Human cells don’t produce urease at all, so this ammonia comes entirely from bacterial activity.
In a healthy person, the liver recaptures and processes this intestinal ammonia without issue. In people with severe liver disease, however, the liver can’t keep up. Ammonia accumulates in the blood, crosses into the brain, and gets taken up by brain cells called astrocytes. Inside astrocytes, ammonia is converted into another molecule that draws in water, causing the cells to swell. This swelling can lead to confusion, disorientation, and in severe cases, dangerous brain pressure. Blood ammonia levels above 150 micromoles per liter are considered a risk factor for this kind of brain injury. For reference, normal adult levels sit below 30 micromoles per liter.
How These Pathways Connect
All of these processes are part of the same global nitrogen cycle. Bacteria in soil fix atmospheric nitrogen into ammonia. Plants absorb it and build proteins. Animals eat those plants and break the proteins back down, releasing ammonia that gets converted to urea and excreted. Decomposers break down dead organisms and waste, releasing ammonia back into the soil. And the Haber-Bosch process short-circuits the whole cycle by pulling nitrogen straight from the air and converting it to ammonia at industrial scale, feeding crops that would otherwise deplete the soil’s natural nitrogen supply far faster than bacteria could replenish it.