5-4 Nutrient cycles

Question 1

What are the stages of the nitrogen cycle?

Answer 1

• Nitrogen fixation:
  o Atmospheric nitrogen gas is converted into nitrogen-containing compounds.
  o This biological nitrogen fixation is carried out by nitrogen-fixing bacteria such as Rhizobium.
  o The bacteria convert nitrogen into ammonia, which forms ammonium ions (in solution) that can then be used by plants.
  o These nitrogen-fixing bacteria are found inside the root nodules (small growths on the roots) of leguminous plants such as peas, beans and clover.
  o The bacteria have a symbiotic (mutually beneficial) relationship with these plants - the bacteria provide the plants with nitrogen-containing compounds and the plants provide the bacteria with organic compounds such as carbohydrates.
• Ammonification:
  o Nitrogen compounds in waste products (e.g., urine and faeces) and dead organisms are converted into ammonia by saprobionts (a type of decomposer including some fungi and bacteria)
  o This ammonia forms ammonium ions in the soil
• Nitrification:
  o The ammonium ions in the soil are converted by nitrifying bacteria into nitrogen compounds that can be used by plants, known as nitrates.
  o Initially, nitrifying bacteria such as Nitrosomonas convert ammonium ions into nitrites.
  o Different nitrifying bacteria such as Nitrobacter then convert these nitrites into nitrates.
• Denitrification:
  o Denitrifying bacteria use nitrates in the soil during respiration.
  o This process produces nitrogen gas, which returns to the atmosphere.
  o This process occurs in anaerobic conditions (when there is little or no oxygen available, such as in waterlogged soil)

Question 2

What are the stages of the phosphorus cycle?

Answer 2

• Phosphorus in rocks is slowly released into the soil and into water sources in the form of phosphate ions (PO₄³⁻) by the process of weathering (the slow breaking down and erosion of rocks over time)
• Phosphate ions are taken up from the soil by plants through their roots or absorbed from water by algae.
• Phosphate ions are transferred to consumers during feeding.
• Phosphate ions in waste products and dead organisms are released into the soil or water during decomposition by saprobionts.
• The phosphate ions can now be taken up and used once again by producers or may be trapped in sediments that, over very long geological time periods may turn into phosphorus-containing rock once again.

Question 3

Why are mycorrhizae important?

Answer 3

• They facilitate the uptake of water and inorganic ions in plants.
• They increase surface area and act as a sponge holding water and minerals.

Question 4

What is leaching?

Answer 4

• Leaching is the process by which mineral ions, such as nitrate, dissolve in rainwater and are carried from the soil to end up in rivers and lakes.
• As a result eutrophication occurs.
• This provides algae in waterways with enough nitrate ions to grow more rapidly than it otherwise would do.
• As a result this can block out light from other plants, causing decay and the use of oxygen in the waterway.
• This eventually leads to the death of the ecosystem.