The Nitrogen Cycle Explained
Nitrogen makes up about 78% of the air you breathe, yet almost nothing on Earth can use it in that form. The nitrogen cycle is the set of processes that convert unusable atmospheric nitrogen gas into forms living things can actually absorb, use, and eventually return back to the atmosphere.
Why Nitrogen Gas Is Unusable on Its Own
Atmospheric nitrogen exists as N2, two nitrogen atoms joined by an extremely strong triple bond that requires a large amount of energy to break. Plants and animals need nitrogen to build proteins, DNA, and chlorophyll, but no plant or animal can break that triple bond directly. Only certain specialized bacteria (and, less biologically, lightning and industrial processes) have the ability to break N2 apart and convert it into a chemical form other organisms can actually use — which is why nitrogen availability, despite the atmosphere being full of it, is one of the most common limiting factors for plant growth.
Nitrogen Fixation: Breaking the Triple Bond
Nitrogen fixation converts N2 gas into ammonia (NH3) or ammonium (NH4+), forms usable by living cells. The majority of natural fixation is carried out by nitrogen-fixing bacteria, some living freely in soil and others living symbiotically in root nodules of legume plants such as peas, beans, and clover, where the plant supplies sugars in exchange for usable nitrogen. Lightning also fixes a smaller amount of nitrogen by providing enough energy to force N2 to react directly with oxygen in the atmosphere. Industrially, the Haber-Bosch process fixes atmospheric nitrogen at scale to manufacture synthetic fertilizer, a development credited with supporting a substantial share of current global food production.
Nitrification: Ammonium to Nitrate
Once ammonium is available in soil, a second group of specialized bacteria convert it further through nitrification, a two-step oxidation process: ammonium is first converted to nitrite (NO2−), then nitrite is converted to nitrate (NO3−). Nitrate is the form of nitrogen most readily absorbed by plant roots, which is why nitrate levels in soil are such a strong predictor of plant growth, and why nitrate is the active ingredient in most synthetic fertilizers.
Assimilation and the Food Web
Plants absorb nitrate through their roots and use it to build amino acids, proteins, and nucleic acids — a process called assimilation. Animals then acquire nitrogen by eating plants (or by eating other animals that ate plants), passing organic nitrogen compounds up through the food web exactly the way carbon and energy move through the food chains described elsewhere on this site. When organisms die or excrete waste, decomposer bacteria and fungi break the organic nitrogen back down into ammonium, returning it to the soil to re-enter the cycle through nitrification once again.
Denitrification: Returning Nitrogen to the Air
Denitrification closes the loop, converting nitrate back into N2 gas, which is released back into the atmosphere. This process is carried out by denitrifying bacteria that thrive in low-oxygen environments, such as waterlogged soils and wetland sediments, where they use nitrate instead of oxygen for respiration. Without denitrification returning nitrogen to the atmosphere, all the nitrogen fixed by other processes would eventually accumulate entirely in soil and water, so denitrification is just as essential to the balance of the cycle as fixation is.
Human Impact: Excess Fertilizer and Runoff
Heavy use of synthetic nitrogen fertilizer, combined with the natural nitrogen cycle, has roughly doubled the total amount of fixed nitrogen entering ecosystems compared to pre-industrial levels. Excess nitrate that isn't taken up by crops washes into rivers and eventually the ocean, fueling algal blooms that deplete dissolved oxygen as they decompose, creating low-oxygen "dead zones" where fish and other aquatic life cannot survive — a direct consequence of disrupting the natural balance between fixation and denitrification at a global scale.
Summary
The nitrogen cycle converts inert atmospheric nitrogen gas into usable ammonium and nitrate through fixation and nitrification, moves that nitrogen through the food web via assimilation, and eventually returns it to the atmosphere through denitrification. Human fertilizer use has significantly accelerated the fixation side of this cycle without a matching increase in denitrification, driving nutrient pollution in waterways downstream. This cycle parallels the process described in the carbon cycle and depends on the same food web relationships covered in ecosystems and food chains.