The nitrogen cycle is a continuous loop where nitrogen moves between the atmosphere, soil, water, and living organisms through five main processes: fixation, nitrification, assimilation, ammonification, and denitrification. Picture it as a circle with the atmosphere as the starting and ending point. Nitrogen gas makes up 78% of Earth’s atmosphere, but plants and animals can’t use it in that form. The cycle is all about converting nitrogen into usable forms and eventually returning it back to the air.
Why Nitrogen Needs to Be “Fixed” First
Atmospheric nitrogen exists as two nitrogen atoms locked together by an extremely strong triple bond. This makes the molecule inert, essentially unreactive. Living things need nitrogen to build proteins and DNA, but they can’t break that bond on their own. So the cycle begins with nitrogen fixation: specialized bacteria crack open the nitrogen molecule and combine it with hydrogen to produce ammonia.
The key enzyme that does this work is called nitrogenase, and only certain prokaryotes carry it. The most important are bacteria in the genus Rhizobium and Bradyrhizobium, which live in nodules on the roots of legumes like soybeans, clover, and peanuts. The plant provides the bacteria with sugars, and the bacteria provide the plant with usable nitrogen. Free-living soil bacteria like Azotobacter and Clostridium also fix nitrogen without a plant partner, and cyanobacteria do it in aquatic environments.
Lightning contributes a small amount of fixation too, about 5 teragrams of nitrogen per year globally, by supplying enough energy to split nitrogen molecules in the atmosphere. But biological fixation dwarfs it. Before industrialization, natural processes on land fixed roughly 58 teragrams of nitrogen per year, with marine organisms adding another 140 teragrams.
Nitrification: Ammonia Becomes Nitrate
Once ammonia is in the soil, a two-step process called nitrification converts it into forms that plants absorb more readily. First, ammonia-oxidizing bacteria (including species like Nitrosomonas) convert ammonia into nitrite. Then a separate group, nitrite-oxidizing bacteria, convert that nitrite into nitrate. Both steps require oxygen, which is why nitrification happens best in well-aerated soils.
Nitrate is the form of nitrogen that moves most easily through soil and water. It dissolves readily and flows wherever water goes, which makes it both highly available to plant roots and highly vulnerable to being washed away by rain.
Assimilation: Plants Take It In
Plants pull nitrate and ammonium from the soil through their roots using specialized transport proteins. Once inside the plant, nitrate gets converted back to ammonium through a series of internal reactions, then incorporated into amino acids, the building blocks of proteins. Animals get their nitrogen by eating plants (or eating other animals that ate plants), passing it up the food chain.
This step is where nitrogen enters the living world. Every protein in your body, every strand of DNA, contains nitrogen atoms that were once floating in the atmosphere, fixed by bacteria, converted in the soil, and taken up by a plant.
Ammonification: Decomposers Recycle It
When plants and animals die, or when animals produce waste, decomposers go to work. Fungi and bacteria break down the organic nitrogen in dead tissue and excrement, releasing it back into the soil as ammonium. This process, called ammonification (or mineralization), is the cycle’s recycling step. It feeds nitrogen back into the soil where nitrification can begin again, or where plants can absorb it directly.
In marine sediments, ammonification plays a particularly important role. The degradation of organic material sinking to the seafloor releases ammonium, which then fuels nitrification and denitrification in the ocean’s depths.
Denitrification: Nitrogen Returns to the Air
The cycle closes with denitrification. Certain bacteria, like Paracoccus denitrificans and Pseudomonas stutzeri, reverse the process by converting nitrate back into nitrogen gas, which escapes into the atmosphere. This happens through a chain of reductions: nitrate becomes nitrite, then nitric oxide, then nitrous oxide, and finally nitrogen gas.
Denitrification typically occurs in low-oxygen or oxygen-free environments, such as waterlogged soils, deep sediments, and wetlands. The bacteria involved are using nitrogen compounds instead of oxygen for respiration. Some research shows that certain denitrifiers can also operate in environments where some oxygen is present, reducing nitrate and using oxygen simultaneously.
The Ocean’s Shortcut: Anammox
In marine environments, there’s an additional pathway that wasn’t discovered until relatively recently. Anaerobic ammonium oxidation, or anammox, pairs one nitrogen atom from ammonium with one from nitrite to produce nitrogen gas directly. This skips several steps of the conventional cycle and acts as a shortcut for removing fixed nitrogen from ocean sediments. Research on marine sediments found that anammox contributes substantially to nitrogen gas production in the seafloor, adding a significant shunt to the traditional ammonification-nitrification-denitrification sequence.
How Humans Have Doubled the Cycle
For most of Earth’s history, the nitrogen cycle was balanced. Natural fixation and denitrification roughly cancelled each other out. That changed in the early 20th century with the invention of the Haber-Bosch process, which uses high heat and pressure to industrially fix atmospheric nitrogen into ammonia for fertilizer.
The numbers are staggering. In 2010, the Haber-Bosch process alone fixed 120 teragrams of nitrogen per year, double the 63 teragrams from all natural terrestrial sources combined. Add in nitrogen-fixing crops grown in agriculture (another 60 teragrams), and total human-caused fixation reaches about 210 teragrams annually. Natural sources account for 203 teragrams. Humans have effectively doubled the global cycling of nitrogen in just a century.
Much of that extra nitrogen doesn’t stay where it’s applied. Nitrate dissolves easily in water, and excess fertilizer runs off fields into streams, rivers, and eventually coastal waters. There, it acts as a fertilizer for algae, triggering massive blooms that consume oxygen as they decompose. The result is hypoxia: water so low in dissolved oxygen that fish and other marine life suffocate. The EPA reports 345 eutrophic or hypoxic “dead zones” in the United States, with adverse nitrogen and phosphorus pollution affecting 65% of the nation’s major estuaries.
The Big Picture
If you were to draw the nitrogen cycle, it would look like a loop with four major reservoirs: the atmosphere (by far the largest, at 78% of the air), soil, water, and living organisms. Arrows connect them. Fixation moves nitrogen from air to soil. Nitrification transforms it within the soil. Assimilation moves it from soil into living things. Ammonification returns it from dead organisms to soil. Denitrification sends it from soil and water back to the atmosphere. Each arrow represents a flux driven by specific groups of microorganisms, and the whole system depends on bacteria at nearly every step.
What makes the nitrogen cycle distinct from, say, the water cycle is that almost every transformation requires a living organism. Water evaporates on its own. Rocks weather passively. But nitrogen stays locked in its inert atmospheric form until a bacterium, a lightning bolt, or an industrial furnace breaks it free. The cycle is, at its core, a story about microbes doing the chemical work that makes all other life possible.