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cyanobacteria
phytoplankton
N
2
in Atmosphere
plants
decomposers
decomposers
dead organisms
and animal waste
NO
2
-
N
2
fixation
NH
4
+
NH
4
+
NO
3
-
NO
3
-
nitrifying
bacteria
denitrifying
bacteria
denitrifying
bacteria
runo
nitrogen-fixing bacteria
in nodules and soil
Biotic
Community
Biotic
Community
human
activities
denitrification
denitrification
nitrification
N
2
fixation
Figure 21
Nitrogen is used and reused as it is cycled
continuously through the biosphere.
The nitrogen cycle
Most of the Earth’s nitrogen is found in the atmosphere. Before it can be used by plants
and animals, this nitrogen must undergo
nitrogen fixation
. In fixation, nitrogen gas,
N
2
, is converted to ammonium, NH
4
+
. Specialized bacteria and other microorganisms
perform nitrogen fixation. The nitrogen cycle is shown in
Figure 21
. After fixation, the
next step in the cycle is nitrification. Nitrification is the chemical process that turns
ammonium, NH
4
+
, into nitrogen-oxygen compounds, NO
2
−
and NO
3
−
. Plants use these
compounds to make proteins. Nitrogen-oxygen compounds may also be created by the
energy from lightning.
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Module 2 • Principles of Ecology