5.4 nutrient cycles

• nitrogen fixation:

  • conversion of nitrogen gas into nitrogen-containing compounds
  • free-living nitrogen-fixing bacteria reduce gaseous nitrogen to ammonia, which they then use to produce amino acids
  • mutualistic nitrogen-fixing bacteria live in the root nodules of legume plants and convert gaseous nitrogen into nitrogen-containing compounds in producers (plants)
• decomposition and ammonification:

  • ammonification is the conversion of ammonia from organic nitrogen-containing compounds (urea, nucleic acids etc)
  • this is done by saprobionts which feed on feaces and decaying organisms, releasing ammonium ions into the soil
• nitrification:

  • nitrifying bacteria in the soil release energy by oxidising ammonium ions into nitrite ions
  • other nitrifying bacteria oxidise nitrite ions to nitrate ions
  • these are both aerobic processes
• aerating the soil increases

  • O2 content
  • no. of nitrifying bacteria
  • rate fo oxidation of ammonium ions to nitrate ions
  • the growth rate of plants
• denitrification:

  • denitrifying bacteria in the soil reduce nitrate ions to inert nitrogen gas
  • this is an anaerobic process
  • waterlogged soil contains less nitrifying bacteria and more denitrifying bacteria
  • this reduces nitrate ion concentration and plant growth rates
• phosphorous is a component of

  • nucleotides
  • phospholipid bilayer
  • ATP
• phosphorous exists mainly as phosphate ions (PO4^3-) in sedimentary rock
• there is no gas phase in the phosphorous cycle
• phosphate ions in plants -- feeding + digestion --> phosphate ions in animals -- excretion + decomposition --> phosphate ions in wastes + remains -- deposition --> phosphates in rocks -- erosion + use of fertilisers --> dissolved phosphate ions in oceans, lakes + soils -- absorption --> phosphate ions in plants

  phosphate ions in animals -- excretion --> dissolved phosphate ions in oceans, lakes + soils

  phosphate ions in wastes + remains -- erosion --> dissolved phosphate ions in oceans, lakes + soils

  dissolved phosphate ions in oceans, lakes + soils -- sedimentation --> phosphates in rocks
• NPK fertilisers contain Nitrogen, Potassium, and Phosphorous
• fertilisers allow addition of minerals to soils, replacing lost minerals
• natural (organic) fertiliser

  • consists of dead and decaying remains of plants and animals as well as animal wastes (manure)
  • more expensive
  • slow acting
  • less prone to leaching
• artificial (inorganic) fertiliser

  • mined from rocks and deposits and industrially converted to fertiliser
  • less expensive
  • quicker release of nutrients
  • prone to leaching and therefore can lead to eutrophication
• mineral ions are lost from agricultural land as plants uptake ions from the soil and are then removed by the farmer before they can return the ions to the soil in a nutrient cycle
• using nitrogen rich fertilisers results in lower biodiversity as plant species that are favoured by nitrogen rich soils will outcompete those that are not
• leaching: rain dissolves the mineral ions from inorganic fertilisers and washes it down deep into the ground where it is then washed in to rivers and lakes via underground water systems
• eutrophication

  1. inorganic fertilisers added to soil
  2. leaching occurs3. fertilisers in the water cause plants + algae in water to grow -> overgrown algae blocks out the sunlight4. no sunlight = no photosynthesis for underwater plants = death of those plantsno photosynthesis + respiration of saprobionts decomposiong dead plants producing CO2 = lower O2 levels in water = no respiration for fish = death of fish