2.3 Biogeochemical cyclesIB Environmental Systems and Societies HL: Subtopic test

1. 1
   In the Mekong Delta in Vietnam, rice is grown in paddy fields that are flooded for much of the growing season. The standing water keeps the soil waterlogged and starves it of oxygen. Measurements show that flooded paddies release more methane than fields growing other crops.

   (a)

   Which organisms produce the methane in the flooded paddy soil?

   [1 mark]

   • ANitrifying bacteria
   • BMethanogenic bacteria
   • CPhotosynthetic algae
   • DMycorrhizal fungi

   (b)

   What happens to methane that has been released into the atmosphere?

   [1 mark]

   • AIt stays there permanently
   • BIt is converted to nitrate within days
   • CIt is eventually oxidised to carbon dioxide, after a residence time of about 10 years
   • DIt is absorbed by the rocks of the lithosphere within a year

   (c)

   Suggest two ways in which the farmers' techniques could affect whether the agricultural system acts as a carbon store, source or sink.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   The white chalk cliffs at Dover in England are formed from calcium carbonate, mostly the tiny plates of marine algae and the shells of small sea organisms that lived tens of millions of years ago. The sea at the foot of the cliffs absorbs carbon dioxide from the air. Nearby, deep coal seams once lay beneath similar ancient swamps and seas.

   (a)

   In which store is most of the carbon held in the chalk cliffs?

   [1 mark]

   • AThe atmosphere
   • BThe biosphere
   • CThe oceans
   • DThe lithosphere

   (b)

   Which statement about the carbon dioxide absorbed by the sea is correct?

   [1 mark]

   • AIt is absorbed by dissolving and can be released again as a gas when it comes out of solution
   • BIt is absorbed only by photosynthesis
   • CIt can only leave the sea by being buried
   • DIt is absorbed by the sea floor and cannot be released

   (c)

   Explain how partially decomposed marine organisms can end up as oil and natural gas.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   On a wheat farm in eastern England, 180 kilograms of nitrogen per hectare were applied as ammonium nitrate fertiliser in one year. The crop took up 120 kilograms of nitrogen per hectare. Of the nitrogen not taken up by the crop, 40 kilograms per hectare were leached into the drainage ditches and 10 kilograms per hectare were lost to the atmosphere as nitrogen gas from waterlogged corners of the field.

   (a)

   Calculate the quantity of nitrogen from the fertiliser that remained in the soil, and state which process released nitrogen gas from the waterlogged corners of the field.

   [3 marks]

   (b)

   Explain the roles of bacteria in the nitrogen cycle in this field.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Before the Haber process was developed in the early twentieth century, the nitrogen available to crops was limited mainly by natural nitrogen fixation. Today synthetic nitrogen fertiliser made by the Haber process helps to feed a large share of the world's population. Burning fossil fuels, clearing forests and farming have also altered how carbon and nitrogen move through ecosystems. Scientists using the planetary boundaries model have concluded that the boundary for the nitrogen cycle has been crossed.

   (a)

   Compare how human activities have changed the carbon cycle and the nitrogen cycle.

   [6 marks]

   (b)

   Evaluate the extent to which global collaboration can bring the nitrogen cycle back within planetary boundaries.

   [6 marks]

   Total for question 4: 12 marks

End of questions