The Nitrogen Cycle | Environmental Chemistry | Chemistry | FuseSchool

Nitrogen is essential to life. Plants and animals need nitrogen to make proteins, which are the building blocks of cells.

Nitrogen makes up 78% of the air, however, neither plants nor animals can take nitrogen directly from the air because nitrogen is so unreactive.

Plants are able to take up nitrogen compounds as nitrates from the soil. Animals then eat these plants, thus providing them with a source of nitrogren.

In this video, we will look at the nitrogen cycle. This is the movement of nitrogen through the environment. Nitrogen is continually cycled through the air, soil and living things.

The process of nitrogen in the atmosphere being turned into nitrogen in the soils is called fixing. There are four different ways that nitrogen fixing occurs naturally:
(1) Nitrogen fixing bacteria in the soil, which turns nitrogen from the air into nitrates.
(2) Decomposers in the soils break down animal excretion and dead organisms, returning nitrogen back to the soil as ammonia. The ammonia turns into nitrite and then nitrifying bacteria in the soil turns this into nitrate.
(3) Lightning can cause chemical reactions in the atmosphere, resulting in nitrogen reacting with oxygen to produce nitrous oxide. The nitrous oxide reacts with more oxygen, and dissolves in rain water to make nitric acid. This rain water increases the amount of nitrate in the soil. Burning fossil fuels also adds nitrous oxide to the atmosphere, resulting in nitric acid, but this is, of course, not a natural way of nitrogen fixing.
(4) Many species of bluegreen algae (cyanobacteria) in the ocean can also fix nitrogen. This then provides sources of nitrogen to aquatic animals, and the nitrogen goes around a similar cycle to what happens on land.

The Haber Process (how fertilisers are made) makes up approximately 30% of nitrogen fixing. This is because nitrogen is essential for making proteins, proteins are essential for cell production, cell production is how growth occurs. Fertilisers are therefore used to increase nitrates in the soil to maximise plant growth. Watch our videos on the Haber Process to learn more about this.

Burning fossil fuels also adds nitrous oxide to the atmosphere, which dissolves in rain water to make nitric acid, which then adds nitrates to the soil. This upsets the balance of the natural nitrogen cycle, polluting ecosystems and altering the ecology of entire regions. Too much nitrogen in the soil makes it too acidic. The nitrogen also passes into rivers and lakes where it is considered a pollutant. In some conditions, such as in waterlogged soils, denitrifying bacteria in the soil break down nitrates and return nitrogen back to the air. This reduces the fertility of the soil.

We have learnt how the nitrogen cycle works. Nitrogen is essential to life, and so needs to be fixed in nitrates for plants and animals. Nitrogen is continually cycled through the air, soil, living things and water systems.



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