Nitrogen Cycle (Edexcel IGCSE Biology) : Revision Note

The nitrogen cycle

• Nitrogen is present as N₂ gas in the atmosphere and within biological molecules, e.g. proteins, in the tissues of living organisms

• Nitrogen is cycled through ecosystems in the processes of the nitrogen cycle

Uptake of nitrogen by living organisms

• N₂ in the atmosphere is made available to living organisms by the process of nitrogen fixation

• Nitrogen fixation is carried out by nitrogen-fixing bacteria which convert N₂ gas into ammonium compounds; these compounds are converted into nitrates in the soil

  • Nitrogen gas can also be fixed by lightning when it strikes the earth, or during the production of chemical fertilisers

• After nitrogen fixation has occurred plants absorb the nitrates in the soil and use the nitrogen to build plant proteins

Transfer of nitrogen between living organisms

• Animals feed on plants and digest the proteins in the plant tissues, providing nitrogen to build animal proteins

• Nitrogen may then be passed from one consumer to another up the food chain in the same way

Release of nitrogen from tissues

• Nitrogen from living organisms is returned to the soil in the form of ammonium ions by the action of decomposers, such as bacteria and fungi

• Plants can’t absorb ammonium ions so, in order for the nitrogen to be made available again to plants, nitrifying bacteria convert ammonium ions into nitrites, and then into nitrates, which can be taken up again by plants

  • The conversion of ammonium compounds to nitrates is known as nitrification, and can be summarised as follows:

Ammonium ions → nitrites → nitrates

Returning nitrogen to the atmosphere

• Nitrates in the soil can be converted back into nitrogen gas by the action of denitrifying bacteria

  • This process is known as denitrification

• Denitrifying bacteria are active in anaerobic conditions, e.g. in waterlogged or compacted soil

  • Farmers can decrease the activity of denitrifying bacteria by ploughing the soil to increase aeration

The nitrogen cycle involves nitrogen fixation, decomposition, nitrification and denitrification