#144 Nitrogen cycle

Nitrogen is essential for the formation of amino acids to make proteins. The nitrogen cycle describes the ways in which nitrogen is recycled.

Nitrogen cycle 

The element nitrogen is a very unreactive gas. Plants are not able to change it into nitrogen compounds, but it is needed to form proteins. Nitrogen compounds become available for plants in the soil in a number of ways, including:

• nitrogen-fixing bacteria (some plants – legumes such as peas, beans and clover – have roots with nodules that contain these bacteria, so the plant receives a direct source of nitrates)
• breakdown of dead plants and animals by decomposers (bacteria, fungi and invertebrates)
• the addition of artificial fertilizers, compost (decaying plant material) and manure (decaying animal waste – urine and faeces)
• lightning – its energy causes nitrogen to react with oxygen.

Plants absorb nitrates into their roots by active uptake. The nitrates are combined with glucose (from photosynthesis) to form protein. Proteins are passed through the food chain as animals eat the plants. When animals digest proteins the amino acids released can be reorganized to form different proteins.

Some soil bacteria - denitrifying bacteria- break down nitrogen compounds and release nitrogen back into the atmosphere. This is a destructive process, commonly occurring in waterlogged soil. Farmers try to keep soil well drained to prevent this happening – a shortage of nitrates in the soil stunts the growth of crop plants.

Nitrates and other ammonium compounds are very soluble, so they are easy leaches out of the soil and can cause pollution.

Farmer can increase the fertility (nitrogen compound concentration) of their soil by: 

• adding artificial fertilisers 
• adding manure or compost
• growing leguminous plants, then digging the roots (with their nodules) into the soil. 

Try this

The figure below shows the nitrogen cycle.

1. i) Name the main nitrogen-containing compound found both in plants and in animals.                                                                                             [1 mark]

   ii) Name one nitrogen-containing compound that is present in urine. [1 mark]

   iii) Name the type of organism that causes the changes at A.            [1 mark]

   iv) What atmospheric conditions bring about the change at B?          [1 mark]

2. Using the figure, explain why it is an advantage to have good drainage in most agricultural land.                                                                       [4 marks]

Answer

1. i) Proteins (or amino acids)

    ii) Urea or ammonia or uric acid.

    iii) Bacteria

    iv) Lightning or electrical storms.

2. Four points from:

• aerated soils allow the activity of useful bacteria
• to convert ammonium compounds in to nitrites
• and to convert nitrites into nitrates
• nitrates can be absorbed by plants
• to allow growth or formation of protein
• waterlogged soils encourage denitrifying bacteria
• which break down nitrates into nitrogen
• so there would be a shortage of nitrates for plants to absorb
• leading to poor growth.