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The Nitrogen Cycle
An essential part of proteins, DNA
and other compounds needed for
life…
Nitrogen:
• N is needed for life’s DNA, protein, amino and nucleic
acids
• Plants use nitrogen (N), phosphorus (P) and potassium
(K) for growth.
• Earth’s atmosphere is 70% nitrogen gas (N2).
• Nitrogen gas is a form that very few organisms can use
(they can’t absorb it directly).
• In order to be used by organisms, nitrogen gas must be
“fixed”
Nitrogen:
• Nitrogen gas in the
atmosphere is composed
of two nitrogen atoms
bound to each other.
• Nitrogen is a fairly nonreactive gas.
• It takes a lot of energy to
get nitrogen gas to break
up and combine with
other elements, such as
carbon or oxygen.
Usable Nitrogen :
•
There are 2 ways
atmospheric
nitrogen can be
converted into a
usable form for
plants and animals
1. Lightening
2. Nitrogen
Fixation
Lightening:
•
•
Lightning provides
enough energy to
"burn" the nitrogen
and fix it in the form
of nitrate, NO3 This process is
duplicated in
fertilizer factories to
produce nitrogen
fertilizers.
Nitrogen Fixation:
•
•
N2 gas is converted
(fixed) to NO3 or
NH4 by bacteria
living in the soil or
the roots (nodules)
of some plants
These bacteria use
special enzymes to
fix nitrogen
Nitrogen Fixation:
• This is done when nitrogen gas is bound to other elements
to make it usable by living organisms.
add to oxygen to form: NO3
nitrate
add to hydrogen to form: NH4 ammonium
• The most important of these bacteria is Rhizobium, a
bacterium that lives in nodules on the roots of plants like
legumes (peas, beans, alfalfa and clover).
• Plants can absorb the fixed nitrogen through their roots.
• Animals must get nitrogen by eating plants or other animals.
The Cycle Begins
• 1. Most plants can take up nitrates and convert it
to amino acids and then possibly proteins.
• 2. Animals acquire all of their amino acids when
they eat plants (or other animals).
• 3. When plants or animals die (or release waste)
the nitrogen is returned to the soil.
• 4. The nitrogen that is usually returned to the soil
in animal wastes or in the output of the
decomposers, is ammonia. Ammonia is rather
toxic.
• 5. Nitrifying bacteria in soil or water convert
ammonia to nitrates, which are taken up by
plants to continue the cycle.
Cycle Completed
• Soil also contains bacteria the uses NO3as a fuel source.
• These Denitrifying Bacteria release N2
back to the atmosphere.
Human Impact:
• Agricultural/industrial
fertilzer
• Fossil fuel burning
• Increase the production of
N fixing crops (soya
beans, peas)
***Human activities add
approximately 140 million
tonnes of extra N in the
environment