Human Impacts on the Nitrogen
Cycle
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In the early twentieth century, scientists
developed a method of fixing nitrogen and
used it to develop commercial fertilizers.
Farmers around the world used artificial
fertilizers to grow more crops for a rapidly
expanding human population.

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 by
microorganisms.
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Farming methods are not the only way that
people affect the nitrogen cycle. When fossil
fuels are burned, the nitrogen they contain is
released into the air as nitrogen compounds.
Now, with farming and fossil fuel combustion,
about 140 million tonnes of extra nitrogen
move through the environment each year.
Effects on Soil
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If a little fertilizer produces larger tomatoes,
will a lot of fertilizer produce tomatoes the
size of basketballs?
There are limits to how much plant growth
can be increased by adding more nitrates.
With a surplus of nitrogen, plant growth soon
becomes limited by the scarcity of other
resources, such as phosphorous, calcium,
and water.

The plants are unable to use nitrogen, and if
you keep adding fertilizers, it causes a
“nitrogen saturation”. When this occurs,
the extra nitrogen washes from the soil into
streams or ground water without being
absorbed by organisms.

Extra nitrogen in soil can cause damage to
tree roots, stunts tree growth and causes
needles on spruce trees to turn yellow and
fall off.
Effects on the Atmosphere

Nitrogen-containing gases spew from
industrial smokestacks, power plants and
vehicle exhausts. When these gases
dissolve in moisture in the air, they form nitric
acid. This is a component of acid
precipitation (rain, snow, etc.).

Acid precipitation has damaged about 150
000 lakes in eastern Canada. About 14 000
of these lakes are known to be acidified.
This means that the acid kills fish, birds,
amphibians, and other organisms.
Effects on Fresh-Water Ecosystems

In the 1970’s, many lakes and streams
became choked by the rapid growth of algae
and weeds. The increase in plant growth
was due to nitrates from fertilizers and
phosphates from detergents.

The buildup of nutrients in an aquatic
ecosystem is called eutrophication.

How might an increase in the population of
producers disrupt an aquatic system?
Eutrophication
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1. Run-off carries nitrates from farms and
cities into water systems.
2. An increased size in plant growth on the
surface of the water blocks sunlight from
reaching deeper waters.
3. Plants below the surface are unable to
carry out photosynthesis. They begin to die
and therefore stop producing oxygen.
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4. As the plants die, the population of
decomposers explodes, feeding on the extra
decaying matter. The decomposers cause a
further decline in oxygen levels through their
cellular respiration.
5. The altered conditions kill fish and other
animals that require high oxygen levels.
Effects on Marine Ecosystems

Scientists have found high levels of nitrogen
in seawater off the coast of Europe. How do
you think this increased nitrogen has affected
coastal marine fisheries? What impact would
this have on local communities?
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The added nitrates initially create a
population explosion of algae in the warm
surface waters. This is called algal bloom.
As the algae die, they sink down to cooler
waters. There they are broken down by
bacteria. The process of decomposition
uses the oxygen in the deeper, cold
seawater.

Since there is little mixing between warm and
cooler layers of seawater, the area near the
sea floor may contain little or no dissolved
oxygen. The lack of oxygen causes death of
many organisms that require oxygen. The
reduced life in deeper waters has an effect
throughout neighboring food chains. Algal
blooms are becoming more common and
causing a loss in fish and shellfish.
Effects on Biodiversity

Biodiversity is a measure of a variety of
species on Earth. It is most commonly
thought of as a variety of different species in
an ecosystem. Biodiversity is not the same
everywhere on Earth.
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As a rule, biodiversity is reduced by extreme
conditions (e.g. arctic, Antarctica, deserts,
etc.)
Very high levels of nutrients also cause a
reduction in biodiversity. This is because
they benefit a few species but harm the
majority.
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In fresh-water ecosystems, biodiversity has
been reduced by both acidification and
eutrophication.
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Read Pages 302 & 303
#8, #9, #10, #11
1. Why do living things need nitrogen?
2. Describe the role of nitrogen-fixing bacteria
in the nitrogen cycle.
3. What is nitrification?
4. What is denitrification?
5. Describe two ways of adding nitrogen to the
soil without using artificial fertilizers.
6. Give one example of the impact of artificial
fertilizers on
a. The atmosphere
b. Lakes
7. THINKING CRITICALLY Make a concept
map of how farming might impact a local
fishery. Identify the environmental concerns
and who would be affected. How could both of
these industries exist successfully together?