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The Nitrogen Cycle. Image © NH Animations

The Nitrogen Cycle. Image © NH Animations

STEM in Context

Understanding the Nitrogen Cycle

Kelly Resmer (with files from Lauren Thompson)


What is the nitrogen cycle? How do human activities such as using fertilizer affect the nitrogen cycle? How can this cause greenhouse gases?

Nitrogen gas (N2) makes up 78 % of the  Earth’s atmosphere. That’s a lot of nitrogen in the air! But what does it do? Understanding the nitrogen cycle can help answer this question.

The nitrogen cycle describes how nitrogen moves through the environment. It has many important impacts on the planet. For example, it helps many living things get the nitrogen they need to survive.

Let’s learn about this cycle, beginning with its three steps: nitrogen fixation, nitrification and denitrification. Then we’ll look at how human activities can affect the nitrogen cycle.

Step 1: Nitrogen fixation

Simplified (unbalanced) chemical reaction for conversion of nitrogen gas to ammonia

N2 + H+ → NH3 + H2

All living things need nitrogen. It is a part of all amino acids. Amino acids combine to make proteins, which are an important part of cells. Nitrogen is also an essential part of chlorophyll, which plants need for photosynthesis.

However, your body can’t use the nitrogen gas in the atmosphere. Other animals and plants can’t, either. That’s why the first step of the cycle is so important. It converts nitrogen gas into a form living things can use. Plants can use a nitrogen-based molecule called ammonia (NH3).  Ammonia has a nitrogen atom bonded to three hydrogen atoms.

Various ways to represent the ammonia molecule including the chemical formula, 2D structure and ball-and-stick model
Various ways to represent the ammonia molecule including the chemical formula, 2D structure and ball-and-stick model (Sources: Radio89 [CC BY-SA 3.0] via Wikimedia Commons and Ben Mills via Wikimedia Commons).

Nitrogen gas consists of two nitrogen atoms bonded together. The process of converting nitrogen gas to ammonia is called nitrogen fixation.
Nitrogen fixation can happen in different places. Bacteria in the soil can make it happen. So can bacteria on the roots of certain plants, such as many legumes. Nitrogen fixation can also occur when lightning strikes the air.There is also such a thing as marine nitrogen fixation. This is when ammonia is taken in by phytoplankton. New research shows that the highest rates of marine nitrogen fixation occur in coastal areas.

When nitrogen is in the form of ammonia, plants can metabolize and use it. Animals, including humans, can then eat these plants to get the nitrogen they need.

Did you know?

Legumes, like peas and beans, can perform nitrogen fixation.

Step 2: Nitrification

Simplified chemical reaction for conversion of ammonia to nitrite

NH3 + O2 → NO2− + H+

Chemical reaction for conversion of nitrite to nitrate

2 NO2− + O2 → 2 NO3

Nitrification is the second step of the nitrogen cycle. Not all of the ammonia produced during nitrogen fixation is used by plants. Bacteria in the soil can use some of it to create nitrite (NO2-). Nitrite can then be converted into nitrate (NO3-), which also helps plants grow.

Nitrification is an example of an aerobic process. That means it needs oxygen gas (O2) to happen. To create both nitrate and nitrite, a nitrogen atom has to connect to oxygen atoms.

Illustration of the nitrogen cycle, including fixation, nitrification and denitrification
Illustration of the nitrogen cycle, including fixation, nitrification and denitrification (Source: Eme Chicano via Wikimedia Commons).

Step 3: Denitrification

Denitrification is the final step in the nitrogen cycle. It happens when certain types of bacteria take in nitrate and convert it back to nitrogen gas. Then, the nitrogen gas is released into the air. Unlike nitrification, this process is anaerobic. This means that it doesn’t require oxygen.

How do humans impact the nitrogen cycle?

Simplified chemical reaction for conversion of nitrate to nitrogen gas

NO3− → NO2− → NO + N2O → N2

Humans can affect the nitrogen cycle in potentially harmful ways. Some of our activities can introduce more nitrogen in certain parts of the cycle.

An image of an algal bloom on Lake Erie. This lake, which borders parts of southern Ontario, has suffered the effects of eutrophication at various times since the 1970s
An image of an algal bloom on Lake Erie. This lake, which borders parts of southern Ontario, has suffered the effects of eutrophication at various times since the 1970s (NOAA via Wikimedia Commons).

For example, plant fertilizers contain nitrogen in the form of ammonia and nitrate. When too much fertilizer is used, the excess nitrogen can run off into nearby bodies of water. This can lead to eutrophication. This is a process in which excess nutrients in the water cause plants and algae to grow rapidly, forming algal blooms. When these algal blooms die, the microbes in them decompose. This severely depletes dissolved oxygen in the area. Eventually, the area might have so little oxygen that few things can live there. Fish and other areas in the region will die. These areas are sometimes called “dead zones.”

The processes of nitrification and denitrification produce a potent greenhouse gas called nitrous oxide (N2O). But about 40% of the N20 in the atmosphere is due to human activities. For example, agriculture can result in more nitrogen in the soil. More nitrogen in the soil means more nitrification and denitrification. This means more N20 in the atmosphere. 

One way to help manage eutrophication and nitrous oxide emissions is for gardeners and farmers to apply only as much fertilizer as their plants need. But that’s easier said than done. It can be very difficult to determine this exact amount.

Plants, animals and bacteria need nitrogen to help them grow. Thanks to the nitrogen cycle, nitrogen gas in the atmosphere gets converted to a form they can use. The nitrogen cycle affects life on the planet in many ways. One of those ways is making sure your body gets its daily dose of nitrogen! 

Starting Points

Connecting and Relating
  • Has eutrophication ever affected your choices for leisure activities such as swimming or boating? Explain.
Relating Science and Technology to Society and the Environment
  • How might human actions that impact the nitrogen cycle affect your daily life or leisure activities? Explain.
  • Describe two environmental, social, and economic consequences of human activities that impact the nitrogen cycle.
  • Explain how eutrophication of lakes, ponds or rivers due to farming is an example of how science, technology, society and the environment intersect.
  • Nitrogen is an important and expensive input for the production of agricultural crops. Why do farmers fertilize their crops? How do farmers determine the amount of fertilizer to put on a crop? What factors affect the cost of nitrogen fertilizer? (Note: This question may require additional research)
Exploring Concepts
  • The nitrogen cycle has several steps. Using chemical formulas found in the article, explain how nitrogen flows through an ecosystem.
  • Explain the role of, and interaction between, plants, animals and bacteria in the nitrogen cycle.
  • What role does nitrogen play in the accumulation of greenhouse gases in the atmosphere? What forms of nitrogen in the atmosphere are a particular concern and why? 
  • What would happen if the amount of nitrogen in an ecosystem drastically decreased? Explain.
  • What form of nitrogen contributes to the problem of acid precipitation? What is the source of this form of nitrogen? 
Nature of Science/Nature of Technology
  • Our scientific understanding of the nitrogen cycle has allowed us to create technology such as fertilizers based on this understanding. Is science to blame for the negative effects of the technology that is created (i.e., overuse of chemical fertilizers)? Why/why not?
  • You suspect a lake may be undergoing eutrophication due to human activities. If you were going to set up a scientific study to test your suspicion, what things would you measure (i.e., the dependent/responding variables, etc.)? What variables would you want to control? Which variables (independent/manipulated) would you vary?
Teaching Suggestions
  • This article can be used in Biology, Chemistry and Earth & the Environment to support teaching and learning related to the nitrogen cycle. Concepts introduced include nitrogen cycle, nitrogen fixation, nitrification, denitrification and algal blooms. 
  • After reading the article teachers could have students conduct a Write-Around Discussion learning strategy to help students consolidate newly learned material. Ready-to-use reproducibles of the Write-Around Discussion for this article are available to download in [Google doc] and [.pdf] formats.
  • To consolidate learning from the article, teachers could also assign students a focus question to answer, such as “Explain the processes of nitrogen fixation, nitrification and denitrification” or “explain how acid rain or eutrophication is connected to the nitrogen cycle.”
  • Students could also consolidate their understanding of the nitrogen cycle by creating their own diagram, like the one in the article, that shows all of the different ways that nitrogen moves through the environment.
  • One of the negative effects humans have on the nitrogen cycle is the production of nitrous oxide, a greenhouse gas. Have students list some ways they’ve learned about greenhouse gases outside of the classroom (commercials, documentaries, radio commercials etc.). Could the students create another way to educate the public about greenhouse gases?
  • Eutrophication and nitrous oxide emissions can be controlled if farmers used the right amount of fertilizers. Have students devise a way to communicate the importance of not over-fertilizing to farmers.
Additional Resources

Learn more

The Nitrogen Cycle: Processes, Players, and Human Impact (2010) The Nature Education Knowledge Project -- An extensive and detailed explanation for more advanced learners.

The Nitrogen Cycle Partnership for Environmental Education and Rural Health -- Summary of the nitrogen cycle

Nitrogen Indicator -- A more advanced resource from the Government of Canada (Agriculture and Agri-Food Canada) showing how the government assesses nitrogen contamination risk in soil and water 


Coastal waters are unexpected hotspots for nitrogen fixation. (2019). Retrieved from