Nitrogen Cycle (Edexcel IGCSE Biology)

• 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 by 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 ammonia by the action of decomposers, such as bacteria and fungi
• The plants can’t absorb ammonia so nitrifying bacteria convert it to nitrates, which can then be taken up again by plants
  • The conversion of ammonium compounds to nitrates is known as nitrification

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