3.5.4 Nutrient cycles

• Nitrogen is essential for making proteins and nucleic acids.
• The 4 stages of the nitrogen cycle include:

  • Nitrogen fixation
  • Ammonification
  • Nitrification
  • Denitrification
• Nitrogen fixation:

  • Reducing nitrogen gas (NH2) into ammonia (NH3), which dissolves in water to form ammonium ions (NH4+).
  • Carried out by nitrogen-fixing bacteria which live in the root nodules of legumes.
• The relationship between nitrogen fixing bacteria and the plant is known as mutualistic, as it is beneficial to both organisms.
• Ammonification (decomposition):

  • Saprobionts (decomposers) break down organic nitrogen-containing compounds (e.g., proteins, DNA, urea) into ammonium ions (NH4+).
  • Firstly, proteins are broken down into amino acids with the use of extra cellular protease enzymes
  • amino acids are further broken down to remove amino groups with the use of deaminase enzymes
  • This releases nitrogen back into the soil.
• Nitrification:

  • Nitrifying bacteria convert ammonium into nitrites (NO2-), then into nitrates (NO3-) in an oxidation reaction.
  • Aerobic process (requires oxygen).
  • Nitrates can be absorbed by plants for growth.
• Denitrification:

  • Denitrifying bacteria convert nitrates (NO3-) back into nitrogen gas (N2-), which is released into the atmosphere.
  • Anaerobic process (happens in waterlogged soils with low oxygen).
• The stages of the Phosphorous cycle:

  • Weathering and erosion
  • Absorption by producers
  • Consumption
  • Decomposition
  • Sedimentation
• Weathering and erosion:

  • phosphate ions (PO43-) in rocks are released into the soil and water by weathering and erosion.
• Absorption by producers:

  • plants absorb phosphate ions from soil with the help of mutualistic mycorrhizal fungi in their roots
  • incorporate phosphate ions into biological molecules
• Consumption:

  • phosphates are transferred through food chains when organisms consume other organisms.
• Decomposition:

  • Saprobionts break down dead organisms and release phosphate ions back into the soil.
• Sedimentation:

  • Phosphates return to rocks when dissolved phosphate ions settle into sediments at the bottom of oceans.
• Mycorrhizae are a type of symbiotic relationship (where each organism benefits from the other) formed between fungi and the roots of plants.
• Mycorrhizae benefit a plant by increasing surface area of the roots – used to increase the absorption of water and mineral ions (phosphate ions) from the soil.
• Plants benefit mycorrhizae by providing organic compounds, such as glucose.
• Fertilisers are used to replace minerals such as phosphorous and nitrates in the soil to improve plant growth.
• Natural fertilisers are organic material (e.g. dead plants, animals or waste). The decomposition of the material leads to ions being replaced.
• Artificial fertilisers are inorganic material. They are concentrated solutions of lost minerals that are sprayed onto the plants.

  • They are faster than natural fertilisers, but have higher environmental risks.
• Artificial fertilisers cause a greater risk of leaching. This is when fertilisers are washed into rivers and lakes. This can lead to eutrophication.
• Eutrophication:

  1. Chemical fertilisers leach into water sources.
  2. The fertiliser accumulates, and is absorbed and utilised by algae. The excess of nitrates will lead to an algae bloom.
  3. Algae grows rapidly on the surface of the water, blocking any light.
  4. Plants cannot photosynthesise and will die.
  5. Many organisms rely on the plants for food and shelter, so when these die so do they.
  6. Dead organic matter provides more nutrients to the saprobionic bacteria. The bacteria respire aerobically, so use up the oxygen.
  7. This causes fish and other aquatic organisms to die