Nitrogen cycle

• Where is nitrogen found

  • Amino acids from which all proteins are made
  • In nitrogenous bases of DNA, RNA and ATP
  • In chlorophyll
• Parts of the nitrogen cycle

  • Nitrogen fixing
  • Decay and ammonification
  • Nitrification
  • Denitrification
• Nitrogen fixing

  Process where nitrates are returned to the soil
• methods of nitrogen fixing

  • Free living bacteria, Azotobacter, in the soil changes N2 to NH4+
  • Legumes - Peas/Beans/Clover contain Rhizobium bacteria which turn N2 to NH4+ to organic nitrogen.
  • Haber process - N2 to NH4+ and NO3- ions in fertilisers
  • Lightning, Where N2 is converted to nitrogen oxide
• Decomposition

  Saprotrophs like fungi and bacteria decay dead and waste material like urea and organic Nitrogen into NH4+ (ammonium ions), Nitrification can then occur.
• Nitrification - a 2 step process

  1. Ammonia is converted to nitrites by Nitrosomonas
  2. Nitrites are converted to nitrates by Nitrobacter
• Denitrification

  - Conversion of nitrates back to nitrogen by bacteria growing in anaerobic conditions, like Pseudomonas.

  - Process where nitrates are broken down, by denitrifying bacteria, and nitrogen is released into the atmosphere
• Rhizobium

  • Nitrogen fixing bacteria
  • Found on the root nodules of leguminous plants
• How do Rhizobiums work?

  1. Rhizobium lives in the centre of a root nodule
  2. It produces the enzyme nitrogenase, converting nitrogen to ammonium ions
  3. Nitrogenase works anaerobically so the rhizobium is surrounded by leghaemoglobin which absorbs oxygen
• Azotobacter

  • Nitrogen fixing bacteria
  • Free in the soil
  • Changes nitrogen in the soil to ammonium ions
• Nitrosomonas

  • Involved in the first stage of nitrification
  • Changes ammonia into nitrites
• Nitrobacter

  • Involved in the second stage of nitrification
  • Changes nitrites to nitrates
• Effect of human activities

  • Ploughing aerates the soil and provides the oxygen needed for aerobic respiration of the nitrification bacteria, Nitrosomonas and Nitrobacter.
  • Drainage also aerates the soil and provides a supply of oxygen too
  • Poorly drained, un-ploughed soil can cause denitrification as it creates anaerobic conditions.
  • Artificial fertilisers and natural fertilisers like manure add nitrates to soil. Nitrates can run off into water in ponds causing eutrophication.