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Nitrogen Cycle
Biology 20
Nitrogen Cycle
• Nitrogen cycle: cycling of nitrogen between
organisms and environment.
• Nitrogen fixation: process of converting
nitrogen gas into nitrates and ammonium
ions.
– Nitrogen-fixing bacteria: bacteria grow in
root nodules of legumes.
– Examples are: clover, alfalfa
– This can then be used by plants.
Nitrogen Fixing Bacteria
• What kind of lawns would you find an
abundance of clover? A lawn high in
nitrates or low in nitrates? Explain.
Nitrogen Cycle
• Denitrifying bacteria: break down
nitrates/nitrites into nitrogen gas.
– This nitrogen gas re-enters the atmosphere.
– Denitrifying bacteria work best in an anaerobic
environment (no air).
• THOUGHT Question: Why do people
aerate their lawns in the spring? How
does this help their lawns? Explain.
Decaying
• When plants and animals decay they are
turned in to ammonia, then into nitrites and
finally into nitrates.
• These nitrates re-enter the cycle.
• Question: What organisms are important to
the decaying process?
Lightning
• Lightning: Creates nitric acid by reacting
with nitrogen gas in the air.