processes

• ionisation

  lightning and meteor trails provide energy for atmospheric nitrogen
• fixation

  some micro-organisms can chemically reduce nitrogen to ammonia

  -> may be bacteria living in the soil or in the root nodules of legumes
• food chains

  nitrogen passes between organisms as amino acids and proteins in food
• nitrification

  involves the oxidation of ammonium ions to nitrites then to nitrates by nitrifying bacteria in the soul
• denitrification

  chemical reduction of nitrates in soil to nitrogen reduces soil fertility

  -> by denitrifying bacteria

  -> occurs under anaerobic conditions
• leaching


  high solubility of nitrates means they easily leach out of soils into water bodies where they act as nutrients for algae
• root absorption 


  plants absorb nitrogen as soluble ions
• ammonification


  amino groups of proteins are released as ammonium ions by the action of bacteria as they decompose dead organic matter