The Nitrogen Cycle

Why is nitrogen an important molecule for organisms?

All living organisms need a source of nitrogen in order to synthesise molecules such as DNA, chlorophyll and proteins.

Are plants and animals able to use nitrogen gas?

No.

Atmospheric nitrogen is inert and unavailable to plants or animals for use.

How do plants obtain nitrogen to use?

By absorbing nitrate ions or ammonium ions through their roots by active transport.

What are the 4 main processes of the nitrogen cycle?

• Ammonification

• Nitrification

• Denitrification

• Nitrogen fixation

What happens in ammonification?

Decomposers, such as bacteria and saprophytic fungi, break down dead organisms and faeces / urine which contain nitrogenous compounds into ammonium ions.

What happens in nitrifcation?

To nitrify means to add nitrites and nitrates to the soil.

• The ammonium ions formed in ammonification are converted into nitrites and then, nitrates.

• This process involves nitrifying bacteria.

What 2 nitrifying bacteria are involved in nitrification?

• Nitrosomonas- converts ammonium ions to nitrites ions.

• Nitrobacter- converts nitrites ions to nitrate ions.

What happens in nitrogen fixation?

• Nitrogen fixation is the conversion of atmospheric nitrogen gas into ammonium.

• This is carried out by nitrogen fixing bacteria.

What are the 2 nitrogen fixing bacteria involved in nitrogen fixation?

• Azotobacter- found free living in soil.

• Rhizobium- occurs inside the roots nodules of legumes such as clover and beans. Nitrogen gas is converted into ammonium ions; these are then converted to organic acids and then amino acids.

What is denitrification?

• Nitrates in the soil is converted back to atmospheric nitrogen gas.

• This occurs as a result of the actions of denitrifying bacteria such as Pseudomonas.

What conditions are needed for denitrification?

Anaerobic conditions i.e. where no oxygen is present.

The most common cause of anaerobic conditions in nature is water-logging, where air spaces in soil are filled with water. Water-logged soils are therefore often deficient in nitrates.

What non-biological processes have impacted the nitrogen cycle?

• The application of agricultural fertilisers adds nitrogen to the soil.

• Lightning adds a small amount of nitrogen to the soil.

• Leaching of minerals removes nitrogen from the soil.

Describe how human activities can improve the circulation of nitrogen in agricultural soil.

• Ploughing fields- improves soil aeration. This favours:

Aerobic organisms, such as free-living nitrogen fixers, enhances the formation of ammonium ions in the soil.

Nitrifying bacteria and therefore enhances the conversion of ammonium into nitrites and nitrates.

• Draining land