The Nitrogen Cycle Explained

The nitrogen cycle is the natural circuit by which nitrogen moves between the air, soil, plants, animals and water, and in India it has been thrown badly off balance by the heavy use of urea and other nitrogen fertilisers that now push far more reactive nitrogen into fields, rivers, groundwater and the atmosphere than nature ever did. Nitrogen makes up about four-fifths of the air we breathe, yet most living things cannot use it in that form, so a small group of microbes and, in recent times, factories have to make it usable.

This explainer (last updated 4 November 2026) explains each step of the nitrogen cycle, shows how the Haber-Bosch process and India’s fertiliser policy altered it, and traces the consequences for groundwater, climate, air and the sea. Figures are kept qualitative or rounded because nitrogen budgets vary by region, crop and year.

Key Takeaways

• The nitrogen cycle turns inert nitrogen gas into forms plants can use and eventually returns it to the air, mainly through bacteria that fix, transform and release nitrogen in the soil.
• The Haber-Bosch process, which makes ammonia from air and natural gas, allowed farming to feed billions more people but roughly doubled the amount of reactive nitrogen circulating on land.
• India relies heavily on urea, a nitrogen fertiliser sold at a low controlled price, and the cheap price has encouraged over-application, especially in rice and wheat belts, so a large share of the nitrogen never reaches the crop.
• Surplus nitrogen leaches into groundwater as nitrate, escapes as nitrous oxide gas that warms the planet, and runs into rivers and coastal seas where it feeds algal blooms and oxygen-poor water.
• Pulses, crop rotation, biofertilisers, neem-coated urea, soil testing and nano fertilisers can cut the need for chemical nitrogen without lowering yields.
• Fixing India’s nitrogen cycle is both an environmental and a financial matter because the country spends a great deal on fertiliser subsidy and imports.

What is the nitrogen cycle and why does it matter?

The nitrogen cycle is the sequence of natural processes that convert nitrogen between its gaseous form in the air and the compounds that plants and animals use, and it matters because nitrogen is a building block of proteins, DNA and chlorophyll, which makes it the nutrient that most often limits crop growth. Without it, a wheat plant cannot make grain protein and a leaf cannot stay green.

Nitrogen gas in the air is extremely stable, held together by a triple chemical bond that most organisms cannot break. Life therefore depends on “fixed” nitrogen, such as ammonium and nitrate in soil and water. Before people intervened, the supply of fixed nitrogen was set by microbes and lightning, which is why natural ecosystems are often short of it and why legumes with their nitrogen-fixing partners have such value.

The central modern fact is that humanity now makes more reactive nitrogen artificially than all natural land processes combined. That is a triumph of food production and a headache for the environment, because every extra kilogram of fixed nitrogen that is not taken up by a crop goes somewhere else.

What are the main steps of the nitrogen cycle?

The main steps of the nitrogen cycle are nitrogen fixation, ammonification, nitrification, assimilation and denitrification, which together carry nitrogen from the air into living things and back again. Each step has its own microbes and its own sensitivities to oxygen, water and temperature.

In fixation, nitrogen gas is converted to ammonia or ammonium. In ammonification, bacteria and fungi break down dead plants, animal waste and urea and release ammonium into the soil. In nitrification, two groups of bacteria convert ammonium first to nitrite and then to nitrate, the form most plants take up easily. In assimilation, roots absorb ammonium and nitrate and build them into amino acids and proteins, which then pass along the food chain. In denitrification, bacteria in waterlogged, low-oxygen conditions convert nitrate back into nitrogen gas, with nitrous oxide appearing as a side product.

A neat way to remember it is: capture, release, convert, absorb, return. A well-run farm manages these steps so that the nitrogen added is taken up by the crop in the season it is needed, rather than leaking into the environment between steps.

How does nature fix nitrogen?

Nature fixes nitrogen mainly through bacteria living in or around plant roots and in water, with a smaller contribution from lightning, which splits nitrogen molecules in the air and lets them combine with oxygen. The bacterial route accounts for most of the natural input to land.

The best-known fixers are Rhizobium bacteria, which live in swellings called nodules on the roots of legumes. The plant supplies sugars, the bacteria supply ammonium, and both gain. Free-living soil bacteria such as Azotobacter and Azospirillum fix smaller amounts, and blue-green algae (cyanobacteria) fix nitrogen in flooded rice fields. The tiny water fern Azolla hosts a cyanobacterium called Anabaena and has long been used as a green manure in rice paddies across parts of India and Asia.

This is why Indian farming traditions such as growing arhar (pigeon pea), moong, urad, chana, masoor and groundnut alongside or before cereals were agronomically wise. Pulses leave residues rich in nitrogen, meaning the next crop needs less fertiliser. Traditional systems such as the mixing of millets with pulses, or fallowing under legumes, were nitrogen management long before the term existed.

How did the Haber-Bosch process change the nitrogen cycle?

The Haber-Bosch process changed the nitrogen cycle by letting industry turn nitrogen gas and hydrogen, usually derived from natural gas or coal, into ammonia at industrial scale, which is the starting point for urea and most other nitrogen fertilisers. Developed in Germany in the early twentieth century, it broke the old limit on how much food the land could produce.

Historians of agriculture often credit synthetic nitrogen with feeding a large fraction of the world’s present population. India’s Green Revolution of the 1960s and 1970s depended on it. Short, responsive wheat and rice varieties were bred to take up nitrogen and turn it into grain rather than tall, floppy stems, and fertiliser factories and imports expanded rapidly so that farmers could apply it.

The downside is that making ammonia consumes large amounts of energy, mostly from fossil fuels, and releases carbon dioxide. More importantly, once the fertiliser is on the land, the natural cycle has no way of absorbing a flood of nitrogen. Globally, human activities now add at least as much reactive nitrogen to the environment each year as all natural terrestrial processes, a shift that scientists place among the most serious departures from natural limits in the Earth system.

How much urea does India use and why is it overused?

India is one of the world’s biggest users of nitrogen fertiliser, with urea by far the most used product, and it is overused largely because the government sells it at a fixed low price that is far below the cost of production and import. The gap is paid for through a fertiliser subsidy that runs into lakhs of crores of rupees a year in some years.

Because a 45-kilogram bag of urea costs a farmer only a few hundred rupees regardless of market conditions, there is little reason to economise. In the intensive rice and wheat belts of Punjab, Haryana and western Uttar Pradesh, farmers often apply more nitrogen than their crops can use, and in many places the nutrient balance is lopsided because urea is far cheaper than phosphate, potash or organic manure. Studies estimate that nitrogen use efficiency in India, meaning the share of applied nitrogen actually taken up by crops, is well under half, so most of what is spread leaves the field by other routes.

Policy has tried to blunt this. Since 2015 all domestically sold urea has been coated with neem oil, which slows the conversion of urea to nitrate and makes it harder to divert to industry. The Soil Health Card scheme tells farmers what their soils need. Nano urea, a liquid formulation promoted in recent years, is meant to supply nitrogen at much lower volumes, although researchers are still debating how well it matches conventional urea in all crops and conditions. The PM-PRANAM scheme offers states a financial incentive for reducing chemical fertiliser use and promoting alternatives.

What happens to the nitrogen that crops do not use?

Nitrogen that crops do not use leaves the field by four routes: it leaches as nitrate into groundwater, runs off into rivers and lakes, escapes to the air as ammonia, or is released as nitrous oxide and nitrogen oxides. Each route harms a different part of the environment.

Leaching occurs when irrigation or monsoon rain carries soluble nitrate below the root zone and into aquifers. Surface runoff carries both dissolved nitrogen and nitrogen attached to soil particles into streams and canals. Ammonia volatilises into the air from urea on warm, wet soil, especially when it is broadcast on the surface rather than placed in the soil. Nitrous oxide is created by microbes when soils are wet, as in flooded paddy or after heavy irrigation and rain.

The timing and method of application make a large difference. Applying the right dose in split portions, close to when the crop needs it, placing fertiliser into the soil rather than throwing it on the surface, and avoiding irrigation immediately after top-dressing can all cut losses sharply. These are the practical heart of what agronomists call the “4Rs”: right source, right rate, right time, right place.

How does nitrogen pollution affect water and health in India?

Nitrogen pollution affects water and health in India mainly through nitrate contamination of groundwater, which can exceed safe drinking limits in parts of the north-west, Rajasthan, Gujarat and the Deccan. The Bureau of Indian Standards sets an acceptable limit for nitrate in drinking water, and many wells in intensively farmed or poorly sewered areas exceed it.

High nitrate in drinking water is linked to methaemoglobinaemia, or “blue baby syndrome”, in which an infant’s blood carries less oxygen, and it is a concern for pregnant women and very young children. Researchers have also studied possible links with thyroid problems and some cancers, although those associations are still debated and need more evidence. Nitrate in groundwater is hard to remove once present, because ordinary boiling does not help and may concentrate it, which is why prevention at the source matters.

Nitrogen also affects the air. Ammonia from fertiliser and livestock reacts with sulphur and nitrogen oxides from power plants and vehicles to form fine particles, a part of the PM2.5 that fouls winter air over the Indo-Gangetic plain. Reducing ammonia is therefore one of several paths to cleaner air, although it is rarely mentioned in smog debates.

Why is nitrous oxide such a problem for the climate?

Nitrous oxide is such a problem because it is a greenhouse gas roughly three hundred times stronger than carbon dioxide over a century, it lingers in the atmosphere for more than a hundred years, and it also damages the ozone layer. Agricultural soils are the leading source worldwide, and in India most of the country’s nitrous oxide comes from nitrogen fertiliser applied to farmland.

Nitrous oxide is made in the soil by microbes during both nitrification and denitrification. Emissions rise when fertiliser is applied in excess, when soils are waterlogged, and when rain follows a heavy dose. Paddy fields can be tricky. Continuous flooding keeps soil oxygen low, which tends to suppress nitrous oxide but encourages methane, while intermittent wetting and drying cuts methane but can raise nitrous oxide if fertiliser is heavy, a trade-off that researchers are working to balance.

The climate case adds to the economic one. Every kilogram of nitrogen that does not need to be applied means less fertiliser to manufacture, less subsidy to pay, less nitrous oxide emitted and less nitrate in water. Few environmental measures offer so many linked gains.

Is nitrogen from farms creating dead zones in India’s seas?

Nitrogen from farms and cities contributes to algal blooms and oxygen-poor water in Indian lakes, estuaries and coastal seas, though how much of the low-oxygen water offshore is natural and how much human-made is still being studied. The Arabian Sea and the Bay of Bengal both have natural oxygen minimum zones at mid-depth.

Along India’s western shelf, seasonal low-oxygen waters appear in the monsoon months, when upwelling brings oxygen-poor deep water toward the surface. Scientists believe extra nutrients from rivers, sewage and farm runoff can intensify blooms, and when blooms die and decay, microbes use up oxygen, creating conditions that can trigger fish kills. Fish and shellfish either flee or die in these hypoxic patches. Noctiluca blooms, glowing green tides of a plankton species that have spread across the northern Arabian Sea in recent decades, are another sign of changing nutrient and oxygen conditions, although warming and circulation changes also play a part.

The Ganga-Brahmaputra delta, the Godavari, Krishna and Mahanadi estuaries and the lagoons of Chilika and Vembanad all receive large nutrient loads. Because these systems support fisheries and livelihoods, nitrogen management on land is directly linked to the health of the sea.

How can India put the nitrogen cycle back in balance?

India can put the nitrogen cycle back in balance by raising nitrogen use efficiency, using more biological fixation, and reforming incentives so that farmers are rewarded for applying the right amount rather than the most. No single measure is enough, but several together can cut fertiliser needs substantially.

Pulses are the most natural remedy. India is the world’s largest producer and consumer of pulses, and expanding them in rotations and as intercrops supplies nitrogen for free and offers protein-rich food. Biofertilisers containing Rhizobium, Azospirillum, phosphate-solubilising bacteria, Azolla and blue-green algae are widely promoted by agricultural universities and research institutes. Organic manures, compost and green manure crops such as dhaincha and sunhemp supply nitrogen along with organic matter. Precision tools such as leaf colour charts, soil tests, drip fertigation and drones help match dose to need. Reforming subsidy so that support is linked to nutrient content or paid directly to farmers could reduce the incentive to overuse, although such reforms have been politically sensitive.

Wastewater treatment and manure management complete the picture, since sewage and livestock waste carry large amounts of nitrogen too. Treating sewage before it enters rivers, and capturing biogas from dung, turns a pollutant into a resource.

How do the main nitrogen pathways compare?

The main nitrogen pathways differ in who drives them, what they produce and what they mean for India, as the table below summarises. It helps to see the natural steps beside the human-made ones.

Bottom Line

The nitrogen cycle once ran on bacteria and lightning, and it now runs largely on factories. India’s low urea price helped feed a growing population, but it also fuelled waste, polluted groundwater, nitrous oxide emissions and damage to rivers and seas. The solution lies in treating nitrogen as a precious nutrient: grow more pulses, apply fertiliser sparingly and precisely, protect water with sewage and manure treatment, and let farmers earn more from using less.

Frequently Asked Questions

Why can plants not use nitrogen directly from the air?

Atmospheric nitrogen is held together by a very strong triple bond that plants cannot break. Only certain bacteria, lightning and industrial processes can convert it into ammonium or nitrate, which roots can absorb.

Why are pulses good for the nitrogen cycle?

Pulses host Rhizobium bacteria in root nodules that fix nitrogen from the air for the plant. After harvest, the leftover roots and residues leave nitrogen in the soil, reducing the fertiliser the next crop needs.

What is neem-coated urea?

Neem-coated urea is ordinary urea covered with neem oil, which slows its conversion to nitrate in the soil. It helps crops use nitrogen more efficiently, cuts losses, and makes it harder to divert urea to industrial use.

Can nitrate in groundwater harm people?

Yes, at high levels. Nitrate above safe limits in drinking water can cause blue baby syndrome in infants by reducing the oxygen their blood can carry, so wells in heavily fertilised areas should be tested.

Why is nitrous oxide dangerous for the climate?

Nitrous oxide traps far more heat than carbon dioxide per molecule, persists for over a century and also depletes the ozone layer. Most of India’s nitrous oxide comes from fertilised farmland.

How can farmers reduce nitrogen losses without losing yield?

Farmers can test the soil, split fertiliser doses, place urea in the soil instead of on the surface, rotate crops with pulses and use biofertilisers or compost. These steps keep yields steady while cutting waste, cost and pollution.

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