A-LEVEL BIOLOGY AQA NOTES 
Nutrient Cycles

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Nutrient Cycles
  • There is a finite supply of nutrients on Earth, which are recycled within natural ecosystems.
  • ​The Nitrogen cycle:
  • The Phosphorus cycle:

​Microorganisms in Nutrient Cycles
  • Microorganism play a vital role in nutrient cycles
Microoragnism
Role
​Mycorrhizae
​Certain types of fungi associate with roots of plants to increase the surface area for absorption of water and mineral ions, including phosphate ions.
​Free-Living Nitrogen-Fixing Bacteria
​In the soil, they reduce nitrogen gas to ammonia.
​Mutualistic Nitrogen-Fixing Bacteria
​Use nitrogen gas to produce amino acids
​Saprobiontic organisms
Break down dead organism to release phosphate, ammonia or ammonium compounds
​Nitrifying bacteria
​Free living in soil, oxidise ammonium ions into nitrites and nitrites into nitrates
​Anaerobic denitrifying bacteria
​Use nitrates in respiration to produce nitrogen gas

​The Use of Fertilisers
  • Fertilisers can be used to provide plants with minerals, particularly nitrates, to support their growth
  • In agriculture systems, the harvesting of crops prevents the reintroduction of minerals to the soil
  • Natural ferilisers consist of dead and decaying remains of plants, animals and their waste
  • Artificial fertilisers are mined from rocks before being converted into different forms with their composition tailored for specific crops.
Effects of using fertilisers
Description
​Reduced species diversity
​Nitrogen-rich soils favour rapidly growing species
​Leaching (pollutes waterways)
Rainwater dissolves soluble nutrients (e.g. nitrates) and carries them deep into the soil and into waterways such as streams, rivers and lakes.
​Eutrophication
Nitrate levels increase in rivers and lakes due to leaching. The increased plant growth blocks light reaching the water underneath the surface, killing plants at a lower depth. The population of saprobiontic bacteria increase, respiring and reducing oxygen levels, killing other aerobic organisms like fish. 
  • Europhication