OCR Biology B2 Processes


The Nitrogen Cycle

Nitrogen cycle 

  • 1.       Nitrogen fixation-Atmospheric nitrogen is converted into nitrates by nitrogen-fixing bacteria attached to the root nodules of leguminous plants. A small amount of nitrogen is ‘fixed’ by lightning because it provides temperatures high enough for atmospheric nitrogen and oxygen to react.
  • 2.       The nitrates are taken up by plants and are used to make plant proteins, animals then eat the plants as their source of nitrates. When plants and animals die or excrete, the remaining organic matter is broken down by decomposition and decay, releasing ammonia.
  • 3.       Nitrification-Nitrifying bacteria in the soil convert ammonium compounds from decomposed matter back into nitrates.
  • 4.       Denitrification-In some conditions (usually in waterlogged soil), denitrifying bacteria break down nitrates in the soil and convert them back into nitrogen gas.

NB: Exposure to heavy rain may deplete the soil of nitrates by washing them into nearby rivers and lakes.

NB: Fast-growing crops reduce the levels of nitrates in the soil because they use them up at a quicker rate.

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  • 1.       Fertilisers are washed off fields into rivers and lakes.
  • 2.       The nitrate and phosphate levels of the rivers increase, causing algae to grow rapidly on the surface of the water in a process known as ‘algal bloom’.
  • 3.       The algae on the surface block sunlight and prevent it from reaching plants at the bottom of the river.
  • 4.       Without sunlight these plants cannot photosynthesise and therefore die. Oxygen production in the river is stopped.
  • 5.       Aerobically respiring bacteria use up the remaining oxygen in the river, causing other water-dwelling organisms (such as fish) to die. 
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