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The Nitrogen Cycle
Science 10
“Old Outcomes”
The Importance of Nitrogen
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Nitrogen is a main ingredient in
fertilizer.
Why does fertilizer produce better
crops; what about nitrogen helps
plants?
The Importance of Nitrogen
(Cont’d)
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Nitrogen is an essential part of proteins,
DNA, and other compounds.
Like carbon, nitrogen passes along food
chains and circulates between abiotic
and biotic parts of the environment.
This is called the nitrogen cycle.
The Nitrogen Cycle
The Nitrogen Cycle
Part One-Nitrogen Fixation
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Although nitrogen makes up 80% of the
atmosphere, this is unusable by plants.
Atmospheric nitrogen must first be
“fixed” by nitrogen fixing bacteria.
Nitrogen fixation is simply the forming
of nitrogen containing compounds such
as ammonium (NH4) and nitrate (NO3-).
These can be absorbed by the roots of
plants.
Nitrogen Fixation (Cont’d)
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There are two types of nitrogen fixing
bacteria:
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Free living – those that live in soil or water.
Symbiotic – those that live in the root
nodules of legume plants such as clover,
peas, beans, and alfalfa.
Once used by plants, nitrogen is passed
along food chains.
Nitrogen and Decomposers
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The process of returning nitrogen
contained in dead organisms and
wastes is more complex than that for
carbon.
Decomposers (through consumption)
convert the nitrogen compounds to
ammonia (NH3).
The Nitrogen Cycle
The Nitrogen Cycle
Part Two - Nitrification
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The ammonia is then converted back to
nitrates by nitrifying bacteria in a process
called nitrification. It can now be re-used by
plants.
Nitrates may also be “denitrified” by
denitrifying bacteria. They are anaerobic
(they don’t thrive in oxygen).
Denitrification converts nitrate back to
nitrogen gas. Why is this process harmful to
farmers?
The Nitrogen Cycle
Human Impacts on the Nitrogen
Cycle
Impacts from Farming
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At the end of the 20th century, the amount of
nitrogen being fixed each year to make
fertilizers was estimated as over half the
amount being fixed in nature.
At the same time, expanded areas of
farmland were being used to grow legume
crops as a way of adding more nitrates to the
soil.
Global Volumes of Fixed Nitrogen
Source
Estimated amount of
nitrogen fixed per year
(millions of tonnes)
Micro-organisms
Lightning
90-140
5-10
Natural
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Human
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Legume crops
Artificial fertilizers
32-53
80
Impacts from Fossil Fuels
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When fossil fuels are burned, the
nitrogen they contain is released into
the air as nitrogen compounds.
These compounds dissolve in moisture
in the air and fall back to Earth.
The result of these impacts is nitrogen
overload.
Effects of Nitrogen Overload
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Effects on soil – makes soil acidic
dissolving toxic metals such as
aluminium.
Effects on the atmosphere – acid
precipitation killing many organisms
Effects on fresh-water ecosystems –
eutrophication and algal bloom.