Nitrogen Cycle
Douglas Wilkin, Ph.D.
Jean Brainard, Ph.D.
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Printed: January 5, 2015
AUTHORS
Douglas Wilkin, Ph.D.
Jean Brainard, Ph.D.
www.ck12.org
C HAPTER
Chapter 1. Nitrogen Cycle
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Nitrogen Cycle
• Outline the steps of the nitrogen cycle.
• Explain nitrogen fixation.
• Discuss the roles of ammonification, nitrification and denitrification in the 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 1.1 shows how nitrogen cycles through a terrestrial ecosystem. Nitrogen passes through a
similar cycle in aquatic ecosystems.
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
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FIGURE 1.1
Nitrogen Cycle in a Terrestrial Ecosystem.
Nitrogen cycles between the atmosphere
and living things.
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).
Summary
• 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.
Explore More
Use this resource to answer the questions that follow.
• The Nitrogen Cycle at http://www.physicalgeography.net/fundamentals/9s.html .
1.
2.
3.
4.
5.
Why is nitrogen important to living organisms?
How do most plants take up nitrogen?
How do animals receive their necessary nitrogen?
What is nitrification?
What is denitrification?
Review
1. Why can’t plants use nitrogen gas directly?
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Chapter 1. Nitrogen Cycle
2. What is nitrogen fixation?
3. Explain why bacteria are essential parts of the nitrogen cycle.
4. What is ammonification?
References
1. Mariana Ruiz Villarreal (LadyofHats) for CK-12 Foundation. An illustration of the nitrogen cycle in a terre
strial ecosystem . CC BY-NC 3.0
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