Nutrient cycles

• Name the general stages in the phosphorus cycle

  1. Weathering
  2. Runoff
  3. Assimilation
  4. Decomposition
  5. Uplift
• Why is the phosphorus cycle a slow process?

  • Phosphorus has no gas phase, so there is no atmospheric cycle
  • Most phosphorus is stores as phosphate ions in rocks
• What happens during weathering and runoff?

  Phosphate compounds from sedimentary rocks leach into surface water and soil
• Explain the significance of phosphorus to living organisms

  • Plants convert inorganic phosphate into biological molecules e.g DNA, ATP, NADP
  • Phosphorus is passed to consumers via feeding
• What happens during uplift?

  Sedimentary layers from oceans (formed by the bodies of aquatic organisms) are brought up to land over many years
• How does mining affect the phosphorus cycle?

  Speeds up uplift
• Name the 4 main stages of the nitrogen cycle

  1. Nitrogen fixation
  2. Ammonification
  3. Nitrification
  4. Denitrification
• Why can't organisms use nitrogen directly from the atmosphere?

  N2 is very stable due to strong covalent triple bond
• What happens during atmospheric fixation of nitrogen?

  1. high energy of lightning breaks N2 into N
  2. N reacts with oxygen to form nitrogen dioxide
  3. Nitrogen dioxide dissolves in water to form Nitrate
• Outline the role of bacteria in nitrogen fixation

  • Mutualistic nitrogen-fixing bacteria in nodules of legumes & free-living bacteria in soil
  • Use the enzyme nitrogenase to reduce gaseous nitrogen into ammonia
• Outline the role of bacteria in ammonification

  1. Saprobionts feed on and decompose organic waste containing nitrogen
  2. NH3 released
  3. NH3 dissolves in water in soil to form NH4 +
• Outline the role of bacteria in denitrification

  Anaerobic denitrifying bacteria convert soil nitrates back into gaseous nitrogen
• Explain the significance of nitrogen to living organisms

  Plant roots uptake nitrates via active transport & use them to make biological compounds

  • amino acids
  • NAD/NADP
  • nucleic acids
• Outline the role of mycorrhizae

  Mutualistic relationship between plant and fungus increases surface area of root system = increase uptake of water and mineral ions
• Give 3 benefits of planting a different crop on the same field each year

  • Nitrogen-fixing crops e.g legumes make soil more fertile by increasing soil nitrate content
  • Different crops have pathogens
  • Different crops use different proportions of certain ions
• Name the 2 categories of fertiliser and state the purpose of using fertiliser

  • Organic: decaying organic matter & animal waste
  • Inorganic: minerals from rocks, usually containing nitrogen, phosphorus, potassium
  • To increase gross productivity for higher yield
• At a certain point, using more fertiliser no longer increases crop yield. Why?

  • A factor unrelated to the concentration of mineral ions limits the rate of photosynthesis
  • so rate of growth cannot increase any further
• Outline 2 main environmental issues caused by the use of fertilisers

  1. Leaching: nitrates dissolve in rainwater and 'runoff' into water sources
  2. Eutrophication: water source becomes putrid as a result of algal bloom
• What happens during eutrophication?

  1. Aquatic plants grow exponentially since nitrate level is no longer a limiting factor
  2. Algal bloom on water surface prevents light reaching the bottom and plants die
  3. Oxygen levels decrease as population of aerobic saprobionts increases to decay dead matter, so fish die
  4. Anaerobic organism reproduce exponentially and produce toxic waste which makes water putrid
• How can the risk of eutrophication be reduced?

  • Sewage treatment marshes on farms
  • Pumping nutrient-enriched sediment out of water
  • Using phosphate-free detergent
• Process of nitrification

  • Ammonium ions that are released from ammonification are converted into nitrates by bacteria into the soil
  • nitrifying bacteria converts NH4+ ions to nitrites
  • another bacteria then convert nitrites to nitrates
• Process of ammonification

  • when an organism dies or produces waste, it is decomposed by saprobionts (decomposers like bacteria or fungi)
  • this releases the nutrients contained inside
  • saprobionts decompose the organisms biomass by extracellular digestion and inorganic ammonium ions are released into the soil
  • ammonification is a by-product of saprobiont nutrition
• Process of denitrification

  • denitrifying bacteria convert nitrates in the soil back into atmospheric nitrogen
  • this takes place in anaerobic conditions