NUTRIENT CYCLES

• • A nutrient is a substance used by an organism to survive, grow and reproduce.
• Microorganisms play a vital role in recycling chemical elements such as phosphorus and nitrogen.
• • All nutrient cycles depend on saprobionts for decomposition.
  • Saprobionts are bacteria and fungi that break down dead material and absorb and use the resulting nutrients. 
  • They do this by secreting enzymes out of their cells, so that this matter can be digested externally. The simple nutrient products of this digestion can then be easily absorbed by the saprobionts.
  • This way of obtaining nutrients is called saprobiotic nutrition.
• • Ammonification: protein/DNA in dead material (e.g. dead organisms; faeces) decomposed by saprobionts to release ammonium ions.
• • Nitrification: ammonium ions are oxidised into nitrite ions and then into nitrate ions. Each stage of nitrification is carried out by different species of nitrifying bacteria.
  • After nitrification, nitrate ions can be absorbed by plant root hairs via active transport and used for growth e.g. making amino acids, nucleotides.
  • Ammonium ions can also be taken up by plant roots, but they are not as water soluble as nitrate ions, so nitrate is easier to absorb.
  • Nitrogen compounds then pass throughout the food web.
• • Nitrogen fixation: nitrogen fixing bacteria convert atmospheric nitrogen gas to ammonium ions. These bacteria live freely in the soil or in root nodules of leguminous plants (e.g. peas).
• • Ammonia, NH3, is produced initially by nitrogen-fixing bacteria.
  • However, ammonia dissolves in water immediately to form ammonium ions, NH4+.
  • So, ammonium is the best term to use: it is ammonium ions that undergo nitrification to nitrate.
• Denitrification

  • Denitrifying bacteria in the soil convert nitrate ions to nitrogen gas, which diffuses back to the atmosphere.
  • These bacteria grow well in anaerobic soils.
  • Waterlogging is a common cause of anaerobic soils, since the water displaces the air pockets from the soil.
  • Digging drainage ditches and ploughing soils add oxygen to the soil, reducing denitrification rates.
  • Nitrogen-fixing and nitrifying bacteria are both aerobic organisms, so these processes go faster in well-aerated soils.