The nitrogen cycle describes how nitrogen moves between the atmosphere, soil, plants, and animals. Nitrogen makes up most of the air but cannot be used by plants and animals in its atmospheric form. Bacteria play a key role in fixing nitrogen by converting it into forms that can be used by plants through processes like nitrification and assimilation. Humans have disrupted the natural nitrogen cycle through activities like fertilizer use that introduce excess nitrogen. Maintaining the balance of the nitrogen cycle is important for ecosystems and the environment.

1. The Nitrogen
Cycle
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2. There is an abundance of nitrogen in the
atmosphere. It makes up 80% of the total volume of
air. Neither plants nor animals can use nitrogen
directly from air. It is the fourth element found in
living things.

3. The nitrogen cycle describes how
nitrogen moves through different
objects including in the
atmosphere and the ground.
Because nitrogen must be fixed in
order to be used by living things, it
rarely exists in its pure form.
Bacteria are very important part of
the nitrogen cycle because it helps
to change or fix nitrogen into a
usable form.

4.  Plants absorb nitrogen directly from the soil.
 There are many steps in the nitrogen cycle including
fixation, nitrification, assimilation, ammonification, and
denitrification.
 Plants need nitrogen to make chlorophyll which they
use during photosynthesis.
 Nitrogen is important to animals in many ways
including being a part of its DNA and proteins.
 Humans alter the nitrogen cycle by using synthetic fertilizers
on lawns which adds to much extra nitrogen to the soil.
 Nitrous oxide is a greenhouse gas in the atmosphere.
 Too much nitrous oxide in the atmosphere can
cause acid rain.
 Nitrogen is used by many farmers as a
fertilizer which helps plants grow.

5. FOUR PRINCIPAL STAGES OF
NITROGEN CYCLE
NITRIFICATION
ASSIMILATION
AMMONIFICATION
DENITRIFICATION

6. NITRIFICATION
 This is the process by which ammonium
gets changed into nitrates by bacteria.
Nitrates are what the plants can then
absorb.
ASSIMILATION
 This is how plants get nitrogen. They absorb
nitrates from the soil into their roots. Then
the nitrogen gets used in amino acids,
nucleic acids, and chlorophyll.

7. AMMONIFICATION
• This is part of the decaying process.
When a plant or animal dies,
decomposers like fungi and bacteria
turn the nitrogen back into ammonium so
it can reenter the nitrogen cycle.
DENITRIFICATION
• Extra nitrogen in the soil gets put
back out into the air. There are
special bacteria that perform this
task as well.

8. Plants and animals could not live without nitrogen.
It is an important part of many cells and processes
such as amino acids, proteins, and even our DNA. It
is also needed to make chlorophyll in plants, which
plants use in photosynthesis to make their food and
energy.
Unfortunately, human activity has altered the cycle. We
do this by adding nitrogen into the soil with fertilizer as
well as other activities that put more nitrous oxide gas
into the atmosphere. This adds in more nitrogen than is
needed by normal cycle and upsets the cycle's balance.

9.  This picture shows the flow of the
nitrogen cycle. The most important part
of the cycle is bacteria. Bacteria help the
nitrogen change between states so it can
be used. When nitrogen is absorbed by
the soil, different bacteria help it to
change states so it can be absorbed by
plants. Animals then get their nitrogen
from the plants.

10. References
• Ordanez J.A II . “Environmental Biology : Philippine Setting” (2008) . Retrieved
June 25, 2014.
• “Nitrogen” . Retrieved June 25, 2014
http://www.chemicool.com/elements/nitrogen.html
• “Ecosystem: The Nitrogen Cycle” . Retrieved June 25, 2014
http://www.ducksters.com/science/ecosystems/nitrogen_cycle.php
• “Nitrogen Cycle Facts” . Retrieved June 25, 2014
http://www.softschools.com/facts/earth_systems/nitrogen_cycle_facts/409/
• Real Trees 4 Kids. “Cycles”. Retrieved June 25, 2014.
http://www.realtrees4kids.org/sixeight/cycles.htm