1. 1
   A gardener builds a compost heap from grass cuttings and vegetable peelings. Over several weeks, the heap becomes warm and the organic matter breaks down into a dark, crumbly material. Micro-organisms living in the heap respire aerobically as they feed on the plant material.

   (a)

   Which gas is released into the atmosphere as decomposer micro-organisms respire aerobically in the compost heap?

   [1 mark]

   • ANitrogen
   • BOxygen
   • CMethane
   • DCarbon dioxide

   (b)

   Which term describes the group of organisms, such as bacteria and fungi, that break down the dead plant material in the compost heap?

   [1 mark]

   • AProducers
   • BPrimary consumers
   • CNitrifying bacteria
   • DDecomposers

   (c)

   Explain how the carbon originally taken up by the grass plants during photosynthesis ends up being released from the compost heap as carbon dioxide.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A farmer rotates his fields, growing a crop of clover (a legume) one year and wheat the next. He notices that the wheat grows better in fields that previously grew clover, even without adding fertiliser. Clover roots have small swellings (nodules) containing bacteria.

   (a)

   What is the name given to the type of bacteria found in the root nodules of the clover plant?

   [1 mark]

   • ADenitrifying bacteria
   • BNitrifying bacteria
   • CDecomposing bacteria
   • DNitrogen-fixing bacteria

   (b)

   Which molecule do these bacteria convert atmospheric nitrogen gas into, making it available for the clover plant?

   [1 mark]

   • ANitrates
   • BAmino acids directly
   • CNitrites
   • DAmmonium compounds

   (c)

   Explain why wheat grown in a field that previously grew clover has more nitrate available in the soil than wheat grown in a field without clover.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   After heavy rainfall, a field of wheat becomes waterlogged for several days. When the water drains away, the farmer notices that the wheat plants in the previously flooded area appear yellow and are growing poorly, despite nitrate fertiliser having been applied before the rain.

   (a)

   Explain, in terms of the nitrogen cycle, why the wheat plants in the waterlogged area are short of nitrate despite fertiliser being applied.

   [3 marks]

   (b)

   Describe how decomposers contribute nitrogen-containing compounds back into the soil from the remains of dead organisms and animal waste in this field, and explain how these compounds are converted into a form plants can absorb.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Since the start of the Industrial Revolution, humans have burned increasing amounts of coal, a fossil fuel formed from the compressed remains of plants that lived hundreds of millions of years ago and were never fully decomposed.

   (a)

   Using your knowledge of the carbon cycle, explain how carbon became locked up in the coal over millions of years, and describe how burning this coal today returns that carbon to the atmosphere.

   [6 marks]

   (b)

   Compare the roles of respiration, photosynthesis, decomposition and combustion in cycling carbon between living organisms and the atmosphere, and explain why burning fossil fuels disrupts the natural balance of this cycle.

   [6 marks]

   Total for question 4: 12 marks

End of questions