Nitrogen cycle

• What is ammonification?

  This is when nitrogen compounds from dead organisms are turned into ammonia by saprobionts through extracellular digestion, this then goes on to form ammonium ions.
• What is nitrification?

  This is when ammonium ions in the soil are changed into nitrite ions and then further into nitrate ions. This process is carried out by nitrifying bacteria to release energy using oxygen.
• What is nitrogen fixation?

  This is when nitrogen gas in the atmosphere is turned into nitrogen-containing compounds. Free living nitrogen fixing bacteria produce ammonia which they use to produce amino acids and nitrogen containing compounds are released when they decay. Mutualistic nitrogen fixing bacteria living in root nodules produce ammonium used for amino acids in the plant and they obtain carbohydrates from the plant.
• What is denitrification?

  This when nitrates in the soil are converted into nitrogen gas by anaerobic denitrifying bacteria present in waterlogged soil.
• Root Nodules of Legumes:

  Leguminous crop plants have mutualistic nitrogen-fixing bacteria in nodules on their roots, they convert nitrogen in the air into ammonia which they use to produce amino acids required for growth.
• When the proteins or amino acids of legume are ploughed into the soul, saprobiotic microorganisms convert protein into ammonium ions then to nitrate by nitrifying bacteria. Nitrates are then absorbed by plants and used to produce proteins for growth.
• How does aeration, e.g. ploughing, increase nitrification?

  The soil oxygen concentration increases so there is more oxidation of ammonia compounds to nitrite ions and then nitrate ions (more nitrification). This results in a higher nitrate concentration in the soil so the soil is more fertile.
• How does aeration, e.g. ploughing, decrease denitrification?

  There is less anaerobic respiration by denitrifying bacteria so less nitrate ions are converted into nitrogen and less nitrates are removed from the soil.
• Crop Rotation and increasing fertility:

  Different crop species use different minerals from the soil, so when crop rotation is applied there’s a variation in which minerals are removed from the soil year to year. Leading to higher crop yields.
• Crop Rotation and increasing fertility:

  Growing legume crops which are associated with nitrogen fixing bacteria will replenish and increase nitrogen containing ions in the soil. Leading to higher crop yields.
• Crop Rotation and increasing fertility:

  Different crop species have different pest species and thus rotation reduces any increases in pest populations. Leading to higher crop yields.
• During an experiment tomato seeds were planted into sterilised soil.

  Give two reasons why the soil was sterilised.

  • To kill fungi (f)
  • To kill pests (p)
• During an experiment which compared the growth of different tomato plants the soil used had the recommended concentration of fertiliser.

  Suggest two reasons why.

  • To allow comparison between plants as all soil had the same fertility.
  • So that the pH of the soil was not effected.
• Farming cattle for humans to eat is less efficient than farming crops because of energy transfer.

  Explain why.

  Cattle have far higher respiratory losses than plants because they have to move around far more.

  Energy is also lost across the trophic levels.