The Nitrogen Cycle

The flow of nitrogen through the Earth system is critical to the health of ecosystems around the planet. Living organisms need nitrogen to produce molecules like amino acids, proteins and nucleic acids.

There is a large quantity of nitrogen on Earth, and it can be found in many different forms. Most of the nitrogen is located in the atmosphere in the form of a gas; N2. Unfortunately, N2 is very unreactive and cannot be used by most living organisms to produce proteins. Animals (including humans) rely on eating plants and other animals to take up nitrogen. Plants can absorb nitrogen only from the soil when it is in a solid form and soluble in water, like ammonium (NH4+) or ion nitrate (NO3-). Plants and animals depend on other processes to convert the nitrogen in the air to nitrates in the soil. The various processes that move nitrogen from the air to soil to living organisms and back to the air again form the nitrogen cycle.

The nitrogen cycle. Credit: USGS.


  1. The atmosphere is the largest store of nitrogen and holds about a million times more nitrogen than all living organisms combined. 78% of the air is nitrogen
  • Nitrogen fixation (N2 to NH3 / NH4+): Nitrogen as a gas (N2) is extremely stable and it takes a lot of energy to turn into nitrate, like volcanic action or lightning discharges. However, some bacteria are able to break the bonds of the two nitrogen atoms and combine it with hydrogen to form ammonia (NH3 or NH4+). The bacteria use the enzyme nitrogenase, which only functions if there is no oxygen around, usually under layers of soil. Rice grown in wetlands have a mucus around their roots on which microorganisms grow. Legumes form nodules on their roots where rhizobia bacteria live that fix atmospheric nitrogen. These plants, and the plants around them, benefit from the ammonia created.
  1. The soil is store of nitrogen in many forms; nitrates, nitrites, ammonium. The conversion from one form to another takes place in the soil.
  • Nitrification (NH3 / NH4+ to NO2- and then to NO3-): different types of bacteria work on this process in two steps. First, the ammonia will be converted into nitrites that then can be converted into nitrates by other bacteria. This is an important step in the nitrogen cycle because nitrates and nitrites are much more mobile in soils and are more readily absorbed by plants than ammonia.
  • Assimilation: Plants take up the ammonia and nitrates through their roots and use them to make amino acids. These are then used to make plant proteins that help them grow.
  1. Plants are a source of nitrogen for animals who consume them (herbivores). Carnivores then eat herbivores and obtain nitrogen this way. Nitrogen is essential for all life; amino acids, proteins and DNA all require nitrogen.
  • Ammonification: Organic matter gets broken down either through the digestive canal of animals in the form of excrement or in the soil after death. Microorganisms (decomposers) use dead organic material for energy and turn organic nutrients into amino acids, DNA or chlorophyll, then back into ammonia. This nitrogen can then be taken up by plants again into the same cycle, or go through denitrification.
  • Denitrification: Denitrifying bacteria turn ammonia back into N2, nitrogen gas, which gets released back into the air. Denitrification only occurs in soil where there is little oxygen, such as waterlogged soil and wetlands.

All the various processes discussed above form one large cycle, which keeps approximately the same amount of nitrogen in every reservoir. The soil releases as much nitrogen to the atmosphere as it receives. Plants take as much from the soil as they return to the soil after decay. However, humans are interfering with the cycle and have altered it, mainly by burning fossil fuels and using large quantities of nitrogen for agricultural purposes.

Human impact on the nitrogen cycle

Burning fossil fuels releases nitric oxide into the atmosphere, causing smog and acid rain, and adding to the greenhouse effect warming the planet. The high-temperature combustion also fixes a small amount atmospheric nitrogen abiotically. Every year humans cause up to four times as much nitrogen fixation by burning fossil fuels than lightning does; about 20 billion kg of fixed nitrogen per year.

Humans apply millions of tons of nitrogen fertilizer to crops each year.

The cultivation of soybeans, peas, and other crops that host symbiotic nitrogen-fixing bacteria have added to the amount of fixed nitrogen caused by human activities. Large areas of natural and diverse vegetation have been turned into monocultures of rice and soybeans adding twice as much of new biologically generated nitrogen to the soil as combustion does; averaging 40 billion kg of nitrogen per year.

Humans use nitrogen fertilizers to help grow crops. This represents the largest human contribution to new nitrogen in the nitrogen cycle, with 80 billion kg of nitrogen per year. The fertilizers are often leached from the soil into the groundwater system, as well as streams and rivers. Livestock produce huge quantities ammonia as a waste product, which enter the soil and leach into bodies of water.

These human activities have serious and long-term consequences for the environment. Nutrients like calcium and potassium in the soil are lost causing fertility problems. Plants adapted to low-nitrogen soils are lost, as are the animals depending on these plants. Water bodies with excessive ammonia will have reduced oxygen levels, causing loss of biodiversity and changes in the food web. Most commonly, algae will grow in abundance, killing many forms of life. Excessive amounts of nitrates in the water make it unsafe for consumption. Today, nearly 80% of the nitrogen found in human tissues originated from synthetic fertilizers.


David Tilman, 1997, Human Alteration of the Global Nitrogen Cycle: Causes and Consequences
Anne Bernhard, 2010, The Nitrogen  Cycle : Processes, players, and human impact.
John A. Lamb, Fabian G. Fernandez, and Daniel E. Kaiser, 2014, Understanding nitrogen in soils
Plant and Soil Sciences Part 5: Nitrogen as a Nutrient
Fondriest, 2010, The Nitrogen Cycle

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