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Nitrogen Cycle

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Nitrogen Cycle

Alfalfa, clover, peas, beans, lentils, lupins, mesquite, carob, soy, and peanuts. What are these?

Legumes. Legume plants have the ability to fix atmospheric nitrogen, due to a mutualistic symbiotic relationship with bacteria found in root nodules of these plants.

The Nitrogen Cycle

Nitrogen makes up 78 percent of Earth’s atmosphere. It’s also an important part of living things. Nitrogen is found in proteins, nucleic acids, and chlorophyll. The nitrogen cycle moves nitrogen through the abiotic and biotic parts of ecosystems. Figure below shows how nitrogen cycles through a terrestrial ecosystem. Nitrogen passes through a similar cycle in aquatic ecosystems.

An illustration of the nitrogen cycle in a terrestrial ecosystem

Nitrogen Cycle in a Terrestrial Ecosystem. Nitrogen cycles between the atmosphere and living things.

Even though nitrogen gas makes up most of Earth's atmosphere, plants cannot use this nitrogen gas to make organic compounds for themselves and other organisms. The two nitrogen atoms in a molecule of nitrogen gas are held together by a very stable triple bond. This bond must be broken for the nitrogen to be used. The nitrogen gas must be changed to a form called nitrates, which plants can absorb through their roots. The process of changing nitrogen gas to nitrates is called nitrogen fixation . It is carried out by nitrogen-fixing bacteria. The bacteria live in soil and roots of legumes, such as peas.

When plants and other organisms die, decomposers break down their remains. In the process, they release nitrogen in the form of ammonium ions. This process is called ammonification. Nitrifying bacteria change the ammonium ions into nitrites and nitrates. Some of the nitrates are used by plants. The process of converting ammonium ions to nitrites or nitrates is called nitrification. Still other bacteria, called denitrifying bacteria, convert some of the nitrates in soil back into nitrogen gas in a process called denitrification. The process is the opposite of nitrogen fixation. Denitrification returns nitrogen gas back to the atmosphere, where it can continue the nitrogen cycle.

See The Nitrogen Cycle: An Aquatic Perspective at http://www.youtube.com/watch?v=pdY4I-EaqJA (5:08) for a summary of this cycle. The cycle is also summarized at http://www.youtube.com/watch?v=w03iO_Yu9Xw (2:04).


  • The nitrogen cycle moves nitrogen back and forth between the atmosphere and organisms.
  • Bacteria change nitrogen gas from the atmosphere to nitrogen compounds that plants can absorb.
  • Other bacteria change nitrogen compounds back to nitrogen gas, which re-enters the atmosphere.


Use these resources to answer the questions that follow.

  1. Describe the role of each of the following in the nitrogen cycle:
    1. nitrogen fixation
    2. nitrification
    3. uptake
    4. decomposition and excretion
    5. ammonification
    6. denitrification
    7. weathering
  1. In which compounds is nitrogen found?
  2. In plants, which are the unusable and usable forms of nitrogen?
  3. What is nitrogen fixation? Describe this process.
  4. What happens during ammonification?
  5. Compare nitrification and denitrification.


  1. Why can't plants use nitrogen gas directly?
  2. What is nitrogen fixation?
  3. Explain why bacteria are essential parts of the nitrogen cycle.
  4. What is ammonification?

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