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1. Nitrogen gas is converted to nitrate compounds
by nitrogen-fixing bacteria in soil or root nodules. Lightning also
converts nitrogen gas to nitrate compounds. Ammonia is
converted to nitrates by nitrifying bacteria in the soil.
2. Plants absorb nitrates from the soil and use these to build
3. Urea and egested material are broken down by decomposers.
This results in nitrogen being returned to the soil as ammonia.
4. Decomposers also break down the bodies of dead organisms
resulting in nitrogen being returned to the soil as ammonia.
5. In some conditions denitrifying bacteria in the soil break down
nitrates and return nitrogen to the air. This is usually in
waterlogged soil. Improving drainage reduces this effect, making
the soil more fertile.
1. Carbon enters the atmosphere as carbon dioxide from
respiration and combustion.
2. Carbon dioxide is absorbed by producers to
make carbohydrates in photosynthesis.
3. Animals feed on the plant passing the carbon compounds along
the food chain. Most of the carbon they consume is exhaled
as carbon dioxide formed during respiration. The animals and
plants eventually die.
4. The dead organisms are eaten by decomposers and the carbon in
their bodies is returned to the atmosphere as carbon dioxide. In
some conditions decomposition is blocked. The plant and animal
material may then be available as fossil fuel in the future for
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