The Nitrogen Cycle: How Does It Work?

We explain the nitrogen cycle on land and sea, showing how processes like fixation, nitrification, denitrification and ammonification convert nitrogen into a usable form. We also look at how the over-use of nitrogen-based fertilizers has altered the global nitrogen pathway, causing a cascade of adverse effects including eutrophication, an increase in nitrous oxide emissions, loss of soil nutrients, increased acidification of soils and streams, damage to estuarine ecosystems, and a marked increase in air pollution.
Tractor Spraying Liquid Nitrogen
Tractor spraying liquid nitrogen on crops. Runoff can contaminate drinking water and cause serious health concerns.

The nitrogen cycle is one of the biogeochemical cycles that transports vital chemicals through Earth’s various spheres. Other similar pathways include: the carbon cycle, the sulfur cycle, the phosphorus cycle and the water cycle.

In the nitrogen cycle, the chemical element nitrogen is converted into several different forms – including ammonium, nitrites and nitrates – as it circulates. This is because although nitrogen is the most abundant chemical in the atmosphere, it can’t be used as it is. It must be converted into a usable form first.

The processes that convert nitrogen into a usable form all take place in the soil – one reason why soil is so important to the planet.

Why is Nitrogen Important?

Nitrogen is vital for plants because it is needed to make chlorophyll, the compound which plants use in photosynthesis to produce food energy from sunlight. In addition, it’s a major component of amino acids, the building blocks of proteins needed for growth, and for those proteins that act as enzymes, enabling many of the biochemical reactions upon which life is based. Nitrogen is also a component of energy-storage compounds, such as ATP (adenosine triphosphate), which allows cells to store and use energy produced during metabolism. On top of all this, nitrogen is an important component of nucleic acids such as DNA (Deoxyribonucleic acid), the genetic material that allows plants to grow and reproduce. Bottom line: nitrogen is essential to plant growth, and therefore essential to life. Because without plants, no human could survive. 1

How the Nitrogen Cycle Works

The nitrogen cycle describes the repetitive series of processes by which the element nitrogen, which makes up more than three-quarters of the Earth’s atmosphere, circulates between the atmosphere and the biosphere (the living Earth). Plants, bacteria, animals, and man-made as well as natural phenomena all play a role.

In order to transit the biotic (living) and abiotic (non-living) parts of the cycle, nitrogen must change forms. In the atmosphere, for example, it exists only as dinitrogen (N2), but in the soil it occurs as nitrogen oxide (NO), and nitrogen dioxide (NO2), and in fertilizers, as ammonia (NH3), or ammonium nitrate (NH4NO3).

Nitrogen Cycle: Simple Diagram
The Nitrogen Cycle. © Hattiel (CC BY-SA 3.0)

The nitrogen cycle consists of four basic stages: fixation, mineralization, nitrification, and denitrification. All four take place in the pedosphere, the layer of soil and surface rock which sits on top of the lithosphere.

Nitrogen Cycle, Step 1: Fixation

In this step, nitrogen moves from the atmosphere into the soil, where microbes such as bacteria change it into ammonium. Fixation is the first step in the process of making nitrogen available to plants. Fixation is essential in order to allow plants to absorb the nitrogen through their root systems.

The fixation of nitrogen, in which the gaseous dinitrogen is converted into forms usable by living organisms, occurs partly as a result of atmospheric processes such as lightning. The lightning provides the energy needed for dinitrogen to react with oxygen, producing nitrogen oxide (NO), and nitrogen dioxide (NO2). These nitrogen compounds then enter soils through precipitation of rain or snow.

Nitrogen can also be fixed synthetically through any of the Haber-Bosch type processes that create fertilizer. In this general process, fixing happens under high heat and pressure, when dinitrogen and hydrogen react to form ammonia (NH3), which may then be processed into ammonium nitrate (NH4NO3), a form of nitrogen that can be added to soil and used by plants and crops.

However, most nitrogen fixation is done by bacteria in the soil. Some bacteria develop a symbiotic relationship (mutually beneficial) with certain legume plants such as peas, beans, lentils, peanuts, alfalfa and clover. Attaching themselves to the plant’s roots, where they live in swellings called nodules, the bacteria obtain energy from the plant through photosynthesis and, in return, they fix (change) nitrogen in the soil into a form the plant can use.

The fixed nitrogen is then transported to other parts of the plant and is used to help the plant to grow. Other bacteria live independently in soils or water and fix nitrogen without this symbiotic relationship. 2

Nitrogen Cycle, Step 2: Ammonification/Mineralization

All plants under cultivation obtain the nitrogen they require through the soil, except for legumes who get it through fixation, as above. Ammonification (also called mineralization) is the process during which microbial organisms decompose organic nitrogen from animal manure, dead plants and other organic matter such as crop residues to ammonium. Rates of ammonification vary with moisture, soil temperature, and how much oxygen is in the soil.

The initial type of nitrogen produced by the process of mineralization is ammonia (NH3). This then reacts with water to produce ammonium (NH4). Ammonium can be used by numerous species of plants, especially those in acidic soils (where nitrification is almost non-existent, see below). However, most plants are not able to use NH4. Instead, they require nitrate as their essential source of nitrogen nutrition.

Nitrogen Cycle, Step 3: Nitrification

During nitrification, nitrate is synthesized from ammonium by several types of bacteria. The first stage in nitrification is the oxidation of ammonium to nitrite (NO2-), a process carried out by bacteria known as Nitrosomonas (or Nitrosospira, Nitrosococcus, and Nitrosolobus).

In stage two, the nitrites are converted into nitrates (NO3-) by bacteria known as Nitrobacter (or Nitrospina, and Nitrococcus), and duly taken up in the plant’s roots. Nitrates can be utilized by plants as well as the animals who consume them. Nitrification occurs most rapidly when soil is warm (19-30°C/67-86°F), moist and well-aerated, but ceases almost entirely below 5°C/41°F and above 50°C/122°F, or in acidic soils or in water.

Nitrogen Cycle, Step 4: Denitrification

In the final stage of the nitrogen cycle, nitrates are converted back to dinitrogen (N2), or other nitrogen gases, like nitric oxide (NO) and nitrous oxide (N2O). This is carried out by bacteria through a process known as denitrification. This process leads to a net loss of nitrogen from the soil, as the gaseous form of nitrogen recycles back into the atmosphere.
Denitrification commonly occurs when the soil is saturated or poorly drained, when bacteria use nitrate as a source of oxygen.

Losses of Nitrogen from the Soil

Nitrogen can be lost from the soil in various ways, thus adding another mini-cycle to the nitrogen cycle.

One such mechanism which leads to a loss of nitrogen from the soil, is known as volatilization. This occurs through the conversion of ammonium to ammonia gas, which is then released into the atmosphere. Volatilization increases at higher soil pH (lower acidity) and during hot and windy conditions that favor evaporation.

Another way nitrogen can be lost is through leaching – that is, the loss of water-soluble plant nutrients from the soil, as a result of rain and irrigation.

Finally, nitrogen can also be lost due to surface runoff. This usually happens during periods of heavy rainwater or stormwater, when the soil is already saturated and thus unable to absorb the rainfall. Soil nutrients become dissolved in the water and get washed away as it runs off.

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Nitrogen that leaches out of the soil or is lost in run-off will invariably enter aquatic systems, such as streams, rivers and lakes. If enough nitrogen accumulates and enriches the water, it can lead to excessive growth of aquatic plant life, such as phytoplankton algae.

Known as eutrophication, this phenomenon can cause a lake to turn bright green with the dense mass of microalgae. When the algae die, they are broken down in the water by decomposers, like bacteria. This decomposition depletes oxygen levels in the water, with fatal consequences for aquatic or marine life.

What is the Oxygen Cycle?

These dead zones can appear in freshwater systems and also in estuarine or delta environments, where rivers full of nitrogen fertilizer runoff flow into oceans. 3

Eutrophication is the leading ecological problem in freshwater ecosystems. Earlier surveys have shown that eutrophication is a damaging factor in 40 percent of all lakes in North America, 54 percent of Asian lakes, 53 percent of lakes in Europe, 41 percent of lakes in South America and 28 percent of lakes in Africa.

The Nitrogen Cycle in the Ocean

The ocean also absorbs nitrogen gas, which is taken up by marine microbes and transformed into various chemical compounds in ways similar in outline to the terrestrial nitrogen cycle. 4 Dinitrogen (N2) cannot be used by phytoplankton, for example, so it must undergo nitrogen fixation which is mostly carried out by cyanobacteria.

As it happens, the marine food web contains a wide variety of microbes, each adapted to a perform specific steps in the cycle. This enormous diversity of microorganisms allows the full use of every form of nitrogen in the ocean, either as a nutrient or source of energy. However, ‘simple’ nitrogen compounds, like ammonium are easier for most microorganisms to use than ‘oxidized’ compounds like nitrate, because ammonium take-up requires less energy.

Nitrogen Cycle In The Ocean: Simple Diagram
Key steps in the marine nitrogen cycle. (Image: © Center for Microbial Oceanography: Research and Education.)

How Humans Have Altered the Nitrogen Cycle

Until the beginning of the 20th century, agricultural crop yields were dependent on the limited quantity of nitrogen which occurred naturally in soils and ecosystems. We might call this the “Pre-1908 Nitrogen Cycle.” As a result, scientists generally believed that the lack of nitrogen would act as a barrier to any further major population growth.

Then in 1908, the German chemist Fritz Haber (1868-1934) invented a process which converted the hitherto unusable atmospheric nitrogen into ammonia – a form of nitrogen which plants were able to use. Carl Bosch (1874-1940), another German chemist and engineer, took Haber’s laboratory-scale process and developed it on an industrial scale. Both were later awarded a Nobel Prize. The combined “Haber-Bosch process” is still the primary method for the production of synthetic nitrogen fertilizer. 5

Scientists and statisticians estimate that nitrogen fertilizer now supports around 3.8 billion people, or roughly half the global population (total 7.8 billion: May 2020). In other words, the Haber-Bosch process has enabled the world to grow enough food to feed an additional 3-4 billion people. 6 7

This alteration of the nitrogen cycle has provided enormous benefits to the global food supply. Unfortunately, we have become so addicted to artificial fertilizers that we have largely ignored the problems it causes. These include: pollution, and the emission of the heat-trapping gas nitrous oxide – one of the worst greenhouse gases in existence and a significant contributor to global warming across the planet. The fact is, the “Post-1908 Nitrogen Cycle” needs a correction – most probably in the form of more efficient fertilizer management – in order to provide good crop yields without damaging the ecosystems upon which we depend.

Nitrogen Balance

Ecological balance is extremely important to Planet Earth and to its biodiversity and nitrogen balance is no exception. The nitrogen cycle plays a critical role in the health of biomes and ecosystems throughout the biosphere because, as we have seen, nitrogen regulates several key ecological processes, including photosynthesis and decomposition. Too little nitrogen impedes plant growth; too much nitrogen leads to pollution, loss of biodiversity and more global warming.

A notable scientific study estimated that in 1997 the use of nitrogen fertilizers had roughly doubled the rate of nitrogen input into the terrestrial nitrogen cycle, and the rate was increasing. 8

Nitrogen Fertilizers Statistics Chart
Chart showing how the use of nitrogen-based fertilizers has increased ten-fold during the 53-year period 1961-2014. Source: United Nations Food & Agricultural Organization. Image: © Our World in Data. 9

Figures produced by the United Nations Food & Agricultural Organization (UNFAO) in 2015, show that the use of nitrogen-based fertilizers has increased ten-fold during the period 1961-2014.

This rapid reshaping of the global nitrogen cycle continues at a record pace, due to a growing demand for nitrogen in agriculture and industry, coupled with persistent inefficiencies in its use. Enough man-made nitrogen is lost to air, water, and land to cause a cascade of environmental and human health problems. 10

Industrial activity in China, for example, has greatly altered the nitrogen cycle and produced nitrogenous gases of environmental significance, with the most serious atmospheric nitrogen pollution in the world. 11

The toxic pollution caused by excessive amounts of nitrogen in the water system is forecast to be exacerbated significantly by the effects of global warming, such as increased rainfall. For details, see below.

Map of Nitrates In Groundwater USA
Areas in the United States most at risk from contamination of groundwater by nitrates. Image © USGS Circular 1225. 12

Further Effects from Human Alteration of the Global Nitrogen Cycle

Human alterations of the nitrogen cycle are believed to be responsible for the following consequences. 8

  • A doubling of the rate of nitrogen input into the terrestrial nitrogen cycle, since 1908. (This may be a significant understatement, given the 10-fold increase in the rate of nitrogen fertilizer consumption.) This has serious implications for human health. 13
  • A significant increase in atmospheric levels of the potent greenhouse gas nitrous oxide.
  • Losses of soil nutrients, like calcium and potassium, that are vital for soil fertility.
  • Increased acidification of soils, streams, and lakes in several regions.
  • An accelerated loss of biodiversity, especially the loss of plants adapted to efficient use of nitrogen, and losses of the creatures that feed on them.
  • Changes in the composition and functional health of estuarine and coastal ecosystems, and the decline in coastal marine fisheries.
  • Agricultural and industrial use of nitrogen has a marked impact on air pollution and air quality. Agricultural sources of nitrogen, for instance, can release ammonia (NH3), nitrogen oxides (NOx) and nitrous oxide (N2O) into the air. Industrial combustion processes can also result in the release of NOx. When these reactive forms of nitrogen become airborne, they can lead to the formation of smog and particulate matter (PM), both of which cause a number of human health problems.
  • In 2015, in Europe alone, the environmental and human health costs of nitrogen pollution were estimated to be €70-320 billion per year. 14

The Nitrogen Cycle and Climate Change

Increased Precipitation Caused by Climate Change Makes Eutrophication Worse

As we have seen, eutrophication is a major problem in freshwater systems. Unfortunately, climate change is predicted to exacerbate the situation by causing higher levels of rainfall, leading to greater leaching and surface run-off. This increased precipitation is estimated to increase the amount of nitrogen ending up in U.S. rivers and other waterways by between 19 and 28 percent. 15 16

Oxygen levels in the waters off Texas, Louisiana and Mississippi
Satellite imagery shows the Gulf of Mexico hypoxic or “dead zone”. The annually recurring zone is caused in large part by synthetic fertilizer run-off from crops, throughout the Mississippi River watershed. One of the largest hypoxic zones in the world, the Gulf of Mexico dead zone occurs every summer causing enormous damage to the marine ecosystem along the coast. Image: © NOAA

The Gulf of Mexico Dead Zone

NOAA scientists are predicting that the Gulf of Mexico hypoxic (dead) zone for 2019 will cover an area of around 7,829 square miles – that’s roughly the size of Massachusetts. The ‘dead zone’ is an area of ocean that is very low in oxygen, and thus fatal to many fish and other marine life. It extends outward from the mouth of the Mississippi river in the Gulf of Mexico. 17

The hypoxic zone is caused in large measure by excess nitrogen washed down the Mississippi River from the cornfields of the upper Midwest. The Mississippi River/Gulf of Mexico Hypoxia Task Force recently extended the deadline until 2035 for reaching its target of a 1,950-square-mile dead zone. However, according to at least one survey, shrinking the dead zone down to that size, will require a 59 percent reduction in the amount of nitrogen runoff that flows down the Mississippi and Atchafalaya rivers. 18

Note: In 2017, the United States Geological Survey (USGS) estimated that 165,000 metric tons of nitrate fertilizer and 22,600 metric tons of phosphorus flowed into the Gulf of Mexico in the month of May, alone.

Greenhouse Gas Emissions from Eutrophication

Eutrophication leads to significant greenhouse gas emissions – especially of of methane (CH4). And the greener or more eutrophic these water bodies are, the more methane is released, which greatly exacerbates climate warming. 19 See also: Why Are Methane Levels Rising?

Other studies show numerous connections between the increased input of nitrogen into the nitrogen cycle by man-made activities, and climate change, and vice versa. 20 21

To understand more about the timeline of our planet, see: History of Earth in One Year (Cosmic Calendar).


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