Nutrient cycles

• Nitrogen cycle


  1. Ammonification

  2. Nitrification

  3. Denitrification

  4. Nitrogen fixation
• Ammonification


  Saprobionts feed on & decompose organic waste containing nitrogen (e.g. urea, proteins, nucleic acids), releasing NH3 (ammonia) which dissolves in water in soil to form NH4+ ions (ammonium)
• Nitrification


  Nitrifying bacteria convert ammonia to nitrite ions, NO2- (oxidation reaction) & then convert these into nitrate ions, NO3-

  
  

  Most plants can take up nitrate through their roots.
• Denitrification


  Anaerobic denitrifying bacteria convert soil Nitrate ions, back to nitrogen gas, N2, (during respiration)
• Nitrogen fixation


  Nitrogen-fixing bacteria convert atmospheric nitrogen gas into ammonia; forms ammonium ions that can be used by plants
• Organisms can't use nitrogen directly from the atmosphere as N2 is very stable due to strong covalent triple bond
• Phosphorus cycle


  1. Weathering

  2. Assimilation

  3. Decomposition

  4. Uplift
• Weathering


  Phosphorus in rocks is slowly released into the soil & the water sources in the form of phosphate ions
• Assimilation


  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.
• Decomposition


  Phosphate ions in waste products & dead organisms are released into the soil or water during decomposition by saprobionts
• Uplift


  Sedimentary layers (phosphorus-containing rock) from oceans (formed by the bodies of aquatic organisms) are brought up to land over many years
• Mining affects the phosphorus cycle as it speeds up uplift
• Mycorrhizae


  Mutualistic relationship between plant & fungus, increases surface area of root system = increases uptake of water & ions (nitrate /phosphates) - increase in crop yield, act as a sponge holding water & minerals; a plant can better resist drought = defend itself
• Fertilisers prevent development of mycorrhizae
• 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 different pathogens
  • Different crops use different proportions of certain ions
• Fertiliser


  • Organic: decaying organic matter & animal waste
  • Inorganic: minerals from rocks, usually containing nitrogen, phosphorus, potassium
• At a certain point, using more fertiliser no longer increases crop yield because a factor unrelated to the concentration of mineral ions limits the rate of photosynthesis, so rate of growth cannot increase any further
• Environmental issues caused by the use of fertilisers

  • Leaching: nitrates dissolve in rainwater and 'runoff' into water sources, leading to eutrophication
  • Eutrophication: water source becomes putrid as a result of algal bloom
• Eutrophication


  1. Aquatic plants grow exponentially since nitrate level is no longer a limiting factor

  2. Algal bloom on the water surface prevents light from reaching the bottom and plants die

  3. Oxygen levels decrease as the population of aerobic saprobionts increases to decay dead matter, so fish die

  4. Anaerobic organisms reproduce exponentially & produce toxic waste which makes water putrid
• How the risk of eutrophication is reduced


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