The nitrogen cycle and the phosphorus cycleAQA A-Level Biology: Subtopic test

1. 1
   In a woodland, fungi and bacteria in the soil feed on dead leaves, dead animals and faeces. Over time the nutrients in this dead material are released and become available again to the trees.

   (a)

   How do the fungi and bacteria that feed on the dead material digest it?

   [1 mark]

   • AThey ingest it into food vacuoles and digest it inside the cell
   • BThey make sugars from the dead material by photosynthesis
   • CThey secrete enzymes onto the dead material and absorb the soluble products
   • DThey form a mutualistic relationship with the roots of trees

   (b)

   What is meant by ammonification?

   [1 mark]

   • AConversion of ammonium ions to nitrite ions
   • BConversion of nitrogen-containing organic compounds in dead material and waste into ammonia and ammonium ions
   • CConversion of nitrogen gas to ammonia
   • DConversion of nitrate ions to nitrogen gas

   (c)

   Explain why the activity of fungi and bacteria in the woodland soil is essential for the trees to grow.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A farmer notices that wheat grows poorly and has yellow leaves in a part of a field where the soil is badly drained and waterlogged for much of the year. The farmer has applied ammonium fertiliser to the whole field, but the wheat in the rest of the field grows well.

   (a)

   Which process converts nitrate ions to nitrogen gas?

   [1 mark]

   • ADenitrification
   • BNitrification
   • CAmmonification
   • DNitrogen fixation

   (b)

   Why are denitrifying bacteria more active in waterlogged soil?

   [1 mark]

   • AThey need a high concentration of oxygen
   • BThey are photosynthetic and need water for light
   • CThey cannot survive in dry soil
   • DThey are anaerobic and waterlogged soil contains little oxygen

   (c)

   Explain why the wheat in the waterlogged part of the field shows signs of nitrogen deficiency even though ammonium fertiliser was added.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Pine seedlings were grown for six months in sterilised soil with a very low concentration of phosphate ions. Some seedlings were inoculated with a mycorrhizal fungus and the others were not. The mean dry mass of the inoculated seedlings was 2.4 g and the mean dry mass of the uninoculated seedlings was 0.9 g.

   (a)

   Calculate the percentage increase in mean dry mass of the inoculated seedlings compared with the uninoculated seedlings. Give your answer to the nearest whole number.

   [3 marks]

   (b)

   Explain how the mycorrhizal fungus increased the growth of the pine seedlings in the soil with a low phosphate concentration.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A farm includes a field where clover, a legume, is grown in rotation with wheat without artificial nitrogen fertiliser, and an adjoining woodland where nutrients are recycled naturally. The wheat grows well in the field the year after the clover.

   (a)

   Explain how the nitrogen in the atmosphere becomes available to the wheat in the year after the clover, naming the bacterial processes involved.

   [6 marks]

   (b)

   Describe how phosphate is recycled in the woodland, and state one way in which the phosphorus cycle differs from the nitrogen cycle.

   [6 marks]

   Total for question 4: 12 marks

End of questions