Nitrogen Cycle *
Douglas Wilkin, Ph.D.
Jean Brainard, Ph.D.
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Printed: March 20, 2016
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.
TEKS
(11) Science concepts. The student knows that biological systems work to achieve and maintain
balance. The student is expected to:
(A) Describe the role of internal feedback mechanisms in the maintenance of homeostasis .
(12) Science concepts. The student knows that interdependence and interactions occur within an
environmental system. The student is expected to:
(E) describe the flow of matter through the carbon and nitrogen cycles and explain the consequences of disrupting
these cycles;
(F) Describe how environmental change can impact ecosystem stability.
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Lesson Objectives
1. Students will be introduced to the nitrogen cycle.
2. The video will introduce the phosphorous chemical cycle.
The Nitrogen Cycle
The following video introduce you to the Nitrogen Cycle: http://www.youtube.com/watch?v=leHy-Y_8nRs .
MEDIA
Click image to the left or use the URL below.
URL: http://www.ck12.org/flx/render/embeddedobject/83920
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.
FIGURE 1.1
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
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Chapter 1. Nitrogen Cycle *
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 for a summary
of this cycle. The cycle is also summarized at http://www.youtube.com/watch?v=w03iO_Yu9Xw .
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.
Practice
Use these resources to answer the questions that follow.
• http://www.hippocampus.org/Earth Science → Environmental Science → Search: Nitrogen Cycle
1. Describe the role of each of the following in the nitrogen cycle:
a.
b.
c.
d.
e.
f.
g.
nitrogen fixation
nitrification
uptake
decomposition and excretion
ammonification
denitrification
weathering
• (Resource under construction) http://www.hippocampus.org/Biology → Biology for AP* → Search: The
Nitrogen Cycle
1.
2.
3.
4.
5.
In which compounds is nitrogen found?
In plants, which are the unusable and usable forms of nitrogen?
What is nitrogen fixation? Describe this process.
What happens during ammonification?
Compare nitrification and denitrification.
Review
1.
2.
3.
4.
Why can’t plants use nitrogen gas directly?
What is nitrogen fixation?
Explain why bacteria are essential parts of the nitrogen cycle.
What is ammonification?
References
1. Mariana Ruiz Villarreal (LadyofHats) for CK-12 Foundation. An illustration of the nitrogen cycle in a
terrestrial ecosystem. CC BY-NC 3.0
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