The Nitrogen Cycle
NITROGEN
• Plants and animals need
nitrogen to make protein.
• The air is about 78%
nitrogen, but plants and
animals cannot use nitrogen
directly from the air.
How Do Plants Get Nitrogen?
• Special bacteria, in the soil
and water, must change or
“fix” nitrogen gas (N2) into a
form that plants can use.
• These bacteria are called
nitrogen-fixers.
N=N → 2NH3
Nitrogen Fixers
• Mutualistic Relationship
– The plants provide food and cover for the
bacteria, and the bacteria convert nitrogen
gas into fertilizer for the plant.
How Do Animals Get Nitrogen?
• Animals get nitrogen from plants or from
other plant-eating animals, in the form of
protein.
Animals must
eat protein to
get their
nitrogen
requirements!
We can’t
breathe in
nitrogen!
How is Nitrogen Recycled?
Nitrogen is recycled by special bacteria that break
down the nitrogen compounds (proteins) in dead
plants and animals, and in animal wastes.
If plants do not use the nitrogen compounds as
fertilizer, special forms of bacteria may recycle it.
These bacteria convert the unused fertilizer into
nitrogen gas and release it into the atmosphere.
All natural ecosystems
depend upon
bacteria to keep the
nitrogen cycle going!
Natural and human activities can disrupt biogeochemical
cycles and ecosystems.
• Natural disruptions
–volcanic eruptions
–forest fires
–landslides and earthquakes
–living organisms
• Disruptions caused by humans
–poor farming practices
–burning fossil fuels
1. What kinds of organisms
are essential for the
nitrogen cycle?