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Water, Carbon and Nitrogen Cycles

The Nitrogen Cycle.

What is the Nitrogen Cycle?

The nitrogen cycle describes the movement of nitrogen
through the environment. Nitrogen exists in the soil,
in plants and animals, and in the air. Nitrogen moves
through the processes shown in the picture below. 

How does the Nitrogen Cycle work?

Although nitrogen gas (N₂) makes up 78% of the air, plants can
not use it for growth unless it is turned into nitrate (NO₃) in the soil.

Making nitrogen in the atmosphere into nitrate in the soil is
called fixing. Nitrogen fixing occurs naturally in four ways.
Nitrogen fixing also occurs with the Haber process - see below.

1. Nitrogen fixing bacteria
in the soil turn nitrogen in the soil into nitrate.

2. Nitrogen fixing bacteria on some plant root nodules
turn nitrogen into nitrate. Plants that have these
kinds of root nodules include peas, beans and clover.

3. Compounds containing ammonia occur in
animal excretion and in dead animals. The ammonia turns
into nitrite and there are nitrifying bacteria in the soil that turn
nitrite (NO₂) into nitrate (NO₃). They are called nitrifying
bacteria because they increase the amount of nitrate in the soil.

4. Lightning can cause chemical reactions in the atmosphere
that make nitrogen react with oxygen producing nitrous oxides.
Nitrous oxides are also made from petrol engine pollution.
The nitrous oxides can react with more oxygen and dissolve in
rain water to make dilute nitric acid (HNO_3(aq)) (see acid rain).
HNO₃ contains nitrate and so rain water
containing HNO₃ increases the amount of nitrate in the soil.

Most of the fixing of nitrogen occurs through the
natural processes described above. The Haber process for
making fertilisers accounts for about 30% of nitrogen fixing.
Nitrate in the soil is essential for plant growth.

There are denitrifying bacteria in the soil that turn
nitrate (NO₃) into nitrogen gas (N₂) and the nitrogen gas
goes back into the atmosphere. They are called
denitrifying bacteria because they decrease
the amount of nitrate in the soil. Compare this with
the nitrifying bacteria described above at number 3.

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