The Nitrogen Cycle (WJEC GCSE Biology)

Higher Tier Only

• Nitrogen is present as N₂ gas in the atmosphere and within biological molecules, e.g. proteins, in the tissues of living organisms
• Nitrogen is cycled through ecosystems by the processes of the nitrogen cycle

Uptake of nitrogen by living organisms

• N₂ in the atmosphere is made available to living organisms by the process of nitrogen fixation
• Nitrogen fixation is carried out by nitrogen-fixing bacteria which convert N₂ gas into ammonium compounds; these compounds are converted into nitrates in the soil
  • Nitrogen-fixing bacteria can be free-living in the soil or they can live within root nodules of legume plants, e.g. peas, beans and clover
• Nitrogen gas can also be fixed by lightning when it strikes the earth, or during the production of chemical fertilisers
• After nitrogen fixation has occurred plants absorb the nitrates in the soil and use the nitrogen to build plant proteins

Transfer of nitrogen between living organisms

• Animals feed on plants and digest the proteins in the plant tissues, providing nitrogen to build animal proteins
• Nitrogen may then be passed from one consumer to another up the food chain in the same way

Release of nitrogen from living tissues

• Nitrogen from living organisms is returned to the soil in the form of ammonia by the action of decomposers such as bacteria and fungi
  • When animals and plants die the proteins inside their tissues are broken down by the action of decomposers and returned to the soil in the form of ammonia
  • Waste, i.e. urine and faeces, from animals contains urea, which is converted into ammonia by the action of the bacterial enzyme urease
• The plants can’t absorb ammonia so nitrifying bacteria convert the ammonia to nitrates which can then be taken up again by plants
  • The conversion of ammonium compounds to nitrates is known as nitrification

Returning nitrogen to the atmosphere

• Nitrates in the soil can be converted back into nitrogen gas (N₂) by the action of denitrifying bacteria
  • This process is known as denitrification
• Denitrifying bacteria are active in anaerobic conditions, e.g. in waterlogged or compacted soil
  • Farmers can decrease the activity of denitrifying bacteria by ploughing the soil to increase aeration

Nitrogen cycle diagram

 The nitrogen cycle involves nitrogen fixation, decomposition, nitrification and denitrification

Factors affecting the nitrogen cycle

• Because so many processes within the nitrogen cycle are carried out by microorganisms the cycle can be affected by factors that affect microorganism activity, e.g.
  • Temperature
    • This affects the rate at which enzyme-controlled reactions can occur
  • Oxygen availability
    • Aerobic bacteria rely on oxygen for respiration
    • Low oxygen availability may lead to an increase in the activity of anaerobic bacteria, e.g. denitrifying bacteria
  • pH
    • This affects the rate of enzyme-controlled reactions as extreme pH levels can cause denaturation
  • Water
    • Water is needed by living organisms, so the rate of microbial activity increases in soil where moisture is present
  • The presence of heavy metals in the soil
    • Heavy metals, e.g. mercury and lead, can be toxic to the metabolism of microorganisms
• These factors are known to influence the rate at which decomposition occurs in compost heaps and landfill sites

Practical: demonstrating urease activity

• Urease enzymes break down urea to release ammonia and carbon dioxide:

Urea + water → ammonia + carbon dioxide

• It is possible to demonstrate the activity of urease enzymes in the classroom by testing for the presence of ammonia
  • Ammonia has a high pH so a pH indicator can be used
  • Soya beans contain urease enzymes

Apparatus

• Fertiliser containing urea
• Acidic solution, e.g. citric acid
• Alkaline solution, e.g. sodium bicarbonate
• Red cabbage indicator solution
• Soya bean extract
• Distilled water
• Dropping pipettes
• 6 test tubes
• Test tube rack

Method

1. Add 2 ml indicator solution into 3 test tubes
2. Add a few drops of each of 3 solutions of different known pH levels into each of the 3 test tubes as follows: 
   • • Acidic solution
     • Alkaline solution
     • Distilled water
   • These 3 test tubes will act as a visual reference so that any pH changes taking place during the investigation can be seen clearly
3. Add 2 ml indicator solution into the 3 remaining test tubes
4. Add the following combinations of solutions into each of the 3 test tubes
   • • Urea fertiliser solution and soya bean extract
     • Urea fertiliser solution alone
     • Soya bean extract alone
   • The first of these three tubes will demonstrate urease activity while test tubes 5 and 6 are experimental controls that demonstrate the need for both the urea solution and the urease enzyme to be present
5. Observe any colour change in the three test tubes
   • We would expect the tube containing urease and soya bean extract to change colour from blue to green, indicating a change from a neutral to an alkaline pH
   • This is because soya beans contain urease enzymes which convert urea in the fertiliser solution into ammonia
   • Ammonia causes the solution to become alkaline

Safety

• Be aware of any soya bean allergies
• Wear eye protection to avoid contact between eyes and solutions of high and low pH
• Rinse skin after any contact with test solutions