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Science

Biological compounds

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The nitrogen cycle

Seventy-nine per cent of the air around us is nitrogen. Living things need nitrogen to make proteins, but they cannot get it directly from the air because nitrogen gas is too unreactive to be used to make new compounds within an organism.

Plants can take up and use nitrogen when it is in a more reactive form - for example, in nitrates or ammonium salts. Changing nitrogen into a more reactive substance is called nitrogen fixation.

Nitrogen fixation

Nitrogen fixation happens in three different ways:

  • The energy in a lightning bolt can split nitrogen molecules in the air, allowing each nitrogen atom to react with oxygen to form nitrogen oxides. The rain washes these oxides to the ground, where they form nitrates.

  • The Haber Process is used by industry to produce ammonia from nitrogen. Ammonia is then used to make the fertiliser that farmers spread on the soil to feed their crops.

  • Nitrogen-fixing bacteria found in both the soil and root nodules of leguminous plants fix nitrogen into a form that can be used by plants.

When plants are eaten by animals, the nitrogen compounds are passed on.

Nitrogen compounds are returned to the soil by excretion and egestion from animals, or when plants and animals die and decay.

The nitrogen compounds returned in this way are changed back to nitrogen gas by denitrifying bacteria which live in the soil. This completes the cycle, so that the percentage of nitrogen in the air remains constant.

This slideshow illustrates the processes involved.

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