The Nitrogen Cycle
Nitrogen

The most abundant element in earth’s
atmosphere = approximately 78%.

Used to carry out many life functions.
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Especially important for plant life.
Nitrogen Fixation



Almost completely unusable for living things
in its gaseous form (atmosphere).
Gaseous nitrogen must be converted, or
fixed, by bacteria before it can be used.
The process bacteria use to fix nitrogen is
called nitrogen fixation.
Nitrogen Fixing Bacteria


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Special bacteria fix the nitrogen in the air so
plants and other producers can use it.
Many of these bacteria live in the soil.
Some plants, especially legumes (beans),
have special bumps or nodes on their roots
where these bacteria can live.
Nitrogen for Other Organisms


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Once the nitrogen is fixed by bacteria, plants
use it and it becomes part of the plant.
Other organisms that feed on the plants take
in nitrogen and use it.
Nitrogen is passed through the food chain.
Decomposers


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When an organism dies, decomposers
break down the body.
Nitrogen from the body is returned to
the atmosphere.
The bacteria that decompose bodies are
a different type of bacteria than the
ones that fix nitrogen.
_________________ Cycle
Animals eat plants and
take in nitrogen.
Plants take i
fixed nitroge
Animals eat plants
and take in
nitrogen.
Plants take in fixed
nitrogen.
Plants and animals die
and are decomposed.
Nitrogen returns to
atmosphere.
Nitrogen-fixing bacteria
pull nitrogen from
atmosphere and fix it so
plants can use it.
The Cycle Goes On and On …
Animals eat plants
and take in nitrogen.
Plants take in
fixed nitrogen.
Animals eat
plants and take
in nitrogen.
Plants take in
fixed nitrogen.
Plants and animals die
and are decomposed.
Nitrogen returns to
atmosphere.
Nitrogen-fixing bacteria
pull nitrogen from
atmosphere and fix it so
plants can use it.
Human Impact — Fertilizers

We overuse nitrogen-rich fertilizers, which dissolve in
surface runoff and ground water.

Fertilizers end up in waterways as they wash into
streams and ponds.

Increased nitrate levels cause plants to grow rapidly,
filling the water until the plants use up the nitrate
supply and die.

The number of herbivores increase when the plant
supply increases. When the plants die, the herbivores
are left without a food source. In this way, changes in
nutrient supply affect the entire food chain.
Other Human Impact

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Burning fossil fuels and forests releases
various forms of nitrogen, contributing to
global warming and acid rain.
The waste associated with livestock farming
releases a large amount of nitrogen into soil
and water.
In the same way, sewage waste adds
nitrogen to soils and water.
What We Need to Do

Be careful with nitrogen-rich fertilizers.

Burn less fossil fuel, forest land, etc.

Treat sewage properly.
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Find ways to deal with livestock wastes.