Cycles of the Earth
&
Biogeochemical Cycles
Nitrogen Cycle
Slides 60-78
Nitrogen cycle:
Atmospheric nitrogen (N2) makes up
nearly 78%-80% of air.
Organisms can not use it in that form.
Lightning and bacteria convert nitrogen
into usable forms.
Nitrogen cycle:
Only in certain bacteria and
industrial technologies can fix
nitrogen.
Nitrogen fixation:
Convert atmospheric nitrogen (N2)
into ammonium (NH4+) which can
be used to make organic
compounds like amino acids.
N2
NH4+
Nitrogen cycle:
Nitrogen-fixing
bacteria:
Some live in a
symbiotic
relationship with
plants of the legume
family
(e.g., soybeans,
clover, peanuts).
Nitrogen cycle:
Some nitrogen-fixing bacteria live
free in he soil.
Nitrogen-fixing cyanobacteria are
essential to maintaining the
fertility of semi-aquatic
environments like rice paddies.
The Cycle Series - Nitrogen Cycle
https://www.youtube.com/watch?v=IaGNSmQQmBc
The nitrogen cycle
• Nitrogen comprises 78% of our atmosphere
– It is contained in proteins, DNA and RNA
• Nitrogen cycle = describes the routes that
nitrogen atoms take through the environment
– Nitrogen gas cannot be used by organisms
• Nitrogen fixation = lightning or nitrogen-fixing
bacteria combine (fix) nitrogen with hydrogen
– To form ammonium
– Which can be used
by plants
Nitrification and denitrification
• Nitrification = bacteria convert ammonium
ions first into nitrite ions then into nitrate ions
– Plants can take up these ions
• Animals obtain nitrogen by eating plants or
other animals
• Decomposers get it from dead and decaying
plants or other animals
– Releasing ammonium ions to nitrifying bacteria
• Denitrifying bacteria = convert nitrates in
soil or water to gaseous nitrogen
– Releasing it back into the atmosphere
The nitrogen cycle
Humans put nitrogen into the
environment
Fully half of nitrogen entering the environment is of human origin
Nitrogen Cycle
Nitrogen Cycle
N2
Ammonia
Nitrates
• Nitrogen needed to build DNA
• Problem: Nitrogen in atmosphere (78%) is unusable
• Step 1: Soil Bacteria
– Nitrogen fixation: convert atmospheric nitrogen into ammonia
– Nitrification: ammonia converted into nitrates
Nitrogen Cycle
Nitrates
• Step 2: Producers absorb nitrates through their roots
Nitrogen Cycle
Nitrates
• Step 2: Producers absorb nitrates through their roots
• Step 3: Consumers ingest nitrates through the food
chain
Nitrogen Cycle
Ammonia
Ammonia
Ammonia
•
•
•
•
Step 2: Producers absorb nitrates through their roots
Step 3: Consumers ingest nitrates through the food chain
Step 4: Decomposers return ammonia to soil
Cycle repeats