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Nutrient cycles: nitrogen, phosphorus and eutrophication questions

Why nitrogen matters and why nitrogen gas is useless to plants; the four bacterial processes of the nitrogen cycle, which are nitrogen fixation, ammonification, nitrification and denitrification, with the organisms, the substrates, the products and the oxygen requirement of each; saprobionts and mycorrhizae; the phosphorus cycle and its lack of a gaseous stage; natural and artificial fertilisers; leaching; and eutrophication as a sequence ending in bacterial respiration and oxygen depletion.

6 original questions · 22 marks · the nutrient cycles: nitrogen, phosphorus and eutrophication notes · Ecology, populations and environmental change

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• 5 marks · Describe

  Describe the sequence of events by which nitrate leaching from a fertilised field leads to the death of fish in a lake.

  Mark scheme

  1. B1 nitrate ions are very soluble and are not held by the negatively charged clay and humus particles, so rain washes them down through the soil into the water, relieving the shortage of the nutrient that was limiting the algae
  2. B1 algae and cyanobacteria at the surface multiply rapidly to form an algal bloom, because they already had light, carbon dioxide and warmth and now have the nutrient too
  3. B1 the bloom forms a dense mat that blocks the light, so submerged rooted plants and deeper algae photosynthesise below the rate at which they respire, and die
  4. B1 those dead plants, and the bloom itself once it exhausts the nutrient and dies back, are a vast supply of dead organic matter on which saprobiotic bacteria feed, so the bacterial population grows enormously
  5. B1 those bacteria respire aerobically and take dissolved oxygen out of the water faster than it can dissolve back in from the air, so the dissolved oxygen concentration falls and fish and invertebrates needing well-oxygenated water suffocate

• 4 marks · Compare

  Compare nitrogen fixation with nitrification, referring to what each starts with, what each produces, the organisms responsible and the part oxygen plays.

  Mark scheme

  1. B1 fixation starts with nitrogen gas from the atmosphere, whereas nitrification starts with ammonium already present in the soil, so only fixation brings nitrogen into the ecosystem
  2. B1 fixation reduces nitrogen gas to ammonia, which takes up a hydrogen ion to become ammonium, whereas nitrification oxidises ammonium to nitrite and then nitrite to nitrate
  3. B1 fixation is carried out by Rhizobium in the root nodules of legumes, by free-living soil bacteria such as Azotobacter and by cyanobacteria, whereas nitrification is carried out by Nitrosomonas and then Nitrobacter
  4. B1 nitrogenase is destroyed by oxygen, so fixation happens where oxygen is kept away and leghaemoglobin mops it up inside a nodule, whereas nitrification requires oxygen because both of its steps are oxidations

• 4 marks · Explain

  Explain why a well-drained, ploughed soil holds more nitrate than a compacted, waterlogged one.

  Mark scheme

  1. B1 ploughing and draining keep air in the spaces between the soil particles, so oxygen is available
  2. B1 nitrification is carried out by Nitrosomonas and Nitrobacter and both of its steps are oxidations requiring oxygen, so ammonium is oxidised through nitrite to nitrate and nitrate accumulates
  3. B1 in a compacted or waterlogged soil the spaces are filled with water and oxygen is absent, so nitrification stops and no further nitrate is produced
  4. B1 denitrifying bacteria such as Pseudomonas then respire using nitrate as the final electron acceptor in place of oxygen, converting it to nitrogen gas that leaves the soil, so the nitrate already there is lost as well

• 4 marks · Calculate

  A 25-hectare field receives ammonium nitrate, NH₄NO₃, at 200 kg per hectare; take its relative formula mass as 80 and the relative atomic mass of nitrogen as 14. Over the following months a stream draining the field carries 8.0 × 10⁷ litres of water, and its nitrogen concentration is on average 5.6 mg per litre higher than upstream of the field. Calculate the mass of nitrogen applied and calculate what percentage of it reached the stream.

• 3 marks · Suggest

  Phosphate is usually the nutrient in shortest supply in fresh water, and the phosphorus cycle has no gaseous stage. Suggest why stripping phosphate from effluent at a sewage works reduces algal blooms in the river below it, and suggest why phosphate carried out to sea is not replaced on any useful timescale.

• 2 marks · Name

  Name the genus of bacteria that oxidises ammonium to nitrite, and name the genus that oxidises nitrite to nitrate.

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