The Nitrogen Cycle is the set of natural processes that move nitrogen between the atmosphere, living organisms, soils, and water. Although Earth’s air is about 78% nitrogen gas (N2), most plants and animals cannot use N2 directly, so nitrogen must be converted into usable chemical forms.
As a core Biogeochemical cycle, it regulates the supply of ammonia (NH3/NH4+), nitrite (NO2−), and nitrate (NO3−) that underpin proteins, DNA, and chlorophyll. Human activity now adds so much reactive nitrogen that global creation of reactive N (especially via industry and fertilizers) is commonly estimated at roughly comparable magnitude to natural terrestrial fixation—reshaping ecosystems worldwide.
The cycle begins with conversion of inert atmospheric N2 into reactive forms through Nitrogen fixation. This is performed by specialized microbes (including those living in legume root nodules) and also by lightning, which can oxidize N2 and contribute small but important inputs in some regions.
Once ammonium is present in soils or water, nitrifying microbes convert it to nitrite and then nitrate (nitrification). Plants typically take up nitrate and ammonium (assimilation) and build them into amino acids and other biomolecules; animals obtain nitrogen by eating plants or other animals.
When organisms excrete waste or die, decomposers convert organic nitrogen back to ammonium (ammonification), replenishing the pool of reactive nitrogen in soils and sediments. Finally, under low-oxygen conditions—common in waterlogged soils, wetlands, and oxygen-poor aquatic zones—microbes convert nitrate back to N2 or nitrous oxide (N2O) via Denitrification, returning nitrogen to the atmosphere and “closing” the loop.
Some nitrogen can also be lost from soils as ammonia gas (volatilization), especially after surface application of urea or manure, while nitrate can be transported by leaching into groundwater and streams. These pathways connect the nitrogen cycle tightly to water quality and climate, because N2O is a potent greenhouse gas with a warming effect far greater than CO2 per molecule over a century timescale.
In agriculture, legume crops such as soybeans, alfalfa, and clover host nitrogen-fixing bacteria that can supply a substantial share of the nitrogen needed for growth, reducing fertilizer demand. In contrast, high-yield cereal systems (like corn and wheat) often rely heavily on industrial fertilizer, and global nitrogen fertilizer use is on the order of 100+ million metric tons of N per year.
In forests and grasslands, nitrogen availability frequently limits plant growth, so small shifts in deposition from the atmosphere (from vehicles, power generation, and agriculture) can change species composition. Extra nitrogen can favor fast-growing plants and alter soil microbial communities, which in turn changes rates of carbon storage and soil acidity.
In coastal waters, excess nitrate washed from watersheds can fuel algal blooms and oxygen depletion, a process closely tied to Eutrophication. One widely cited example is the seasonal “dead zone” in the Gulf of Mexico, which in some years has measured around 15,000–20,000 km², driven largely by nutrient runoff from the Mississippi River Basin.
Cities also reshape nitrogen flows through wastewater and stormwater systems. Modern sewage treatment can remove a large fraction of nitrogen through engineered nitrification and denitrification, but incomplete removal can still deliver reactive nitrogen to rivers and estuaries, especially during heavy rain events that overwhelm infrastructure.
The nitrogen cycle is central to food security because nitrogen is often the nutrient most limiting plant productivity in managed fields. Synthetic fertilizers, produced largely through the Haber–Bosch process, help sustain modern crop yields; many analyses conclude that a large fraction of the global population is fed thanks to industrially fixed nitrogen.
At the same time, “too much” reactive nitrogen has broad environmental costs. Nitrate pollution can contaminate drinking water sources; many health agencies use 10 mg/L as nitrate-nitrogen (NO3-N) as a benchmark limit in drinking water due to risks such as infant methemoglobinemia (“blue baby syndrome”).
Nitrogen enrichment also affects biodiversity by pushing ecosystems toward nitrogen-tolerant species and away from those adapted to low-nutrient conditions. In aquatic systems, blooms and subsequent decay can strip oxygen from water, causing fish kills and habitat loss, while on land, increased nitrogen can accelerate soil acidification and nutrient imbalances (for example, shifting the availability of calcium and magnesium).
The cycle is tightly coupled with other elemental cycles: adding nitrogen can increase plant growth and influence carbon uptake, linking it to the Carbon cycle. It also interacts with phosphorus availability—often the limiting nutrient in freshwater—so management commonly considers nitrogen alongside the Phosphorus cycle when addressing algal blooms and ecosystem productivity.
Scientists recognized nitrogen as a distinct element in the 18th century, and by the 19th century, microbiology revealed that specific microbes drive key transformations such as nitrification and denitrification. This shifted nitrogen from being seen as a static “ingredient” of air and soil to a dynamic, biologically mediated cycle.
A major turning point came in the early 20th century with industrial nitrogen fixation. The commercialization of the Haber–Bosch method (developed around 1909–1913) enabled ammonia production at massive scale, supporting fertilizers and explosives and profoundly altering global nitrogen flows.
Since the mid-20th century, fertilizer use and fossil-fuel combustion have increased reactive nitrogen emissions and deposition across continents. By the late 20th and early 21st centuries, research emphasized nitrogen’s role in acid rain, coastal dead zones, greenhouse forcing via N2O, and interactions with climate-driven changes in rainfall, wildfire, and soil processes.
Most plants cannot use N2 gas directly, so they depend on nitrate or ammonium in soils. In many ecosystems, those usable forms are scarce, making nitrogen a common growth-limiting nutrient.
Reactive nitrogen is mobile: nitrate can leach into groundwater, and nitrogen can also run off into streams or escape as gases (NH3, N2O, N2). Loss rates vary widely, but even a modest percentage loss matters when applied across millions of hectares.
Denitrification can reduce soil nitrate available to crops, but it also protects water quality by converting nitrate to inert N2 gas before it reaches rivers and coasts. The key is managing conditions to minimize nitrous oxide emissions while still limiting nitrate pollution.
Agriculture is a major driver, but not the only one. Fossil-fuel combustion forms nitrogen oxides (NOx) that deposit back to land and water, and wastewater discharges and urban runoff also add substantial nitrogen loads to aquatic systems.