9.15: the nitrogen cycle

What is nitrogen used for?

The formation of amino acids to form proteins

What percentage of the air is nitrogen?

78%

In what form can nitrogen be used by plants?

As nitrates - not as the gas since it is chemically unreactive/inert

How is nitrogen converted from gas to the soil?

Nitrogen fixing bacteria in soil or root nodules of plants, lighting, and the Haber process to convert nitrogen gas and hydrogen into ammonia (which farmers use to make fertilisers) - ammonia is then converted to nitrates by nitrifying bacteria in the soil. Plants can absorb nitrates from the soil and use them to build proteins. Decomposers break down dead organisms, which results in nitrogen being returned to the soil as ammonia.

How is nitrogen converted from nitrates into nitrogen gas?

In some cases, denitrification happens when denitrifying bacteria breaks down nitrates and returns nitrogen into the atmosphere. This usually happens in waterlogged soil, when roots can’t access oxygen so respire anaerobically.

How can farmers increase the nitrate content of the soil?

Crop rotation and fertilisers

How do fertilisers help plants grow?

The most common artificial fertiliser is ammonium nitrate and ensures that the plant gets enough nitrates required for growth

What are natural fertilisers?

Manure or compost

What is crop rotation?

Peas, beans and clover have nitrogen fixing bacteria in their roots, so will increase the nitrates in the soil, thus increasing the fertility of the soil. Crop rotation involves changing which crop grows in which field, ensuring that a leguminous plant is grown in each field every few years. This ensures that the levels of nitrates in the soil don’t decrease too quickly, and each plant has enough nitrates to grow well.

How is nitrogen taken up by animals?

Animals eat plants so digest some nitrates

How are nitrates released back to the soil from animals?

Through an animal excreting, breathing, sweating, its urine or death and subsequent decomposition