Nitrogen Cycle (Edexcel IGCSE Biology): Revision Note

The nitrogen cycle

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

• Nitrogen is cycled through ecosystems via the nitrogen cycle

Nitrogen fixation

• Nitrogen fixation makes nitrogen available to living organisms

• Nitrogen fixing bacteria convert N₂ gas into ammonium compounds, which are then converted into nitrates in the soil

• Nitrogen fixing bacteria include:

  • Free-living bacteria in the soil

  • Bacteria in the root nodules of leguminous plants (e.g. peas, beans, clover) which form a mutualistic relationship with the plant

• Nitrogen can also be fixed by lightning or during the production of chemical fertilisers

• After nitrogen fixation has occurred plants absorb nitrates from the soil to build plant proteins

Transfer of nitrogen between living organisms

• Animals feed on plants, 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 dead organisms and metabolic waste products is returned to the soil as ammonia by decomposers (bacteria and fungi)

  • Ammonia reacts with water in the soil to form ammonium ions

• Plants can’t absorb ammonium ions directly, so nitrifying bacteria convert ammonia 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:

ammonia → nitrites → nitrates

Denitrification

• Denitrifying bacteria convert nitrates back into N₂ gas, returning nitrogen to the atmosphere. This process is known as denitrification

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

• Farmers reduce denitrification by ploughing the soil to increase aeration

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