Question 1
Define a biogeochemical cycle.
[2]
Question 2
A wetland carbon storage receives and releases .
Calculate the annual net change in the wetland carbon storage.
[1]
Identify whether the wetland is acting as a carbon store in equilibrium, a sink or a source.
[1]
Question 3
The carbon balances of three forests are shown in Figure 1.
Identify which forest acts as a carbon sink, which acts as a store in equilibrium and which acts as a carbon source.
[3]
Question 4
Distinguish between transfers and transformations in the carbon cycle, using one named example of each.
[4]
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Question 5
Outline how carbon in a mollusc can enter a long-term lithospheric store.
[3]
Question 6
Figure 2 shows four bacterial transformations in the nitrogen cycle.
Identify the nitrogen-cycle process represented by each arrow, A to D.
[4]
Question 7
Figure 1 shows annual carbon inputs and outputs for three forest ecosystems. Positive net balance indicates carbon accumulation.
Identify the forest condition in which carbon inputs and outputs are in equilibrium.
[1]
Calculate the annual net carbon balance of the young forest. State whether it acts as a carbon sink or source.
[2]
Explain why the burned forest acts as a carbon source, using evidence from Figure 1.
[3]
Question 8
Explain how increased atmospheric can harm reef-building corals.
[4]
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Question 9
Describe how two regenerative agricultural methods can promote soil as a carbon sink.
[4]
Question 10
Explain the production and atmospheric significance of methane released from a waterlogged rice paddy.
[4]
Question 11
A temperate forest reserve is managed for carbon sequestration and biodiversity. Figure 1 compares annual carbon flows at three stages in the history of the forest.
A fire occurred after a prolonged drought. Managers propose replanting native tree species, removing fire-damaged timber and suppressing all future fires.
Calculate the net annual change in carbon stored by the young growing forest.
[2]
Using Figure 1, classify the mature forest and the recently burned forest as a carbon store in equilibrium, a carbon sink or a carbon source.
[2]
Explain why the fire changed the forest from a carbon store to a carbon source.
[3]
Distinguish between a transfer and a transformation of carbon, using one forest example of each.
[2]
Evaluate the proposed management of the burned forest as a strategy for long-term carbon sequestration.
[5]
Question 12
Scientists studied ocean acidification in Bahía Esmeralda, a fictional subtropical bay containing coral reefs and a shellfish industry.
Atmospheric carbon dioxide concentration and mean surface-water pH in Bahía Esmeralda, 1995–2025.
| Year | Atmospheric / ppm | Mean surface-water pH |
|---|---|---|
| 1995 | 360 | 8.15 |
| 2000 | 371 | 8.13 |
| 2005 | 382 | 8.11 |
| 2010 | 395 | 8.09 |
| 2015 | 410 | 8.07 |
| 2020 | 418 | 8.06 |
| 2025 | 425 | 8.05 |
They also grew juvenile oysters for eight weeks under different seawater conditions.
Calculate the change in mean surface-water pH between 1995 and 2025.
[1]
Describe the relationship between atmospheric carbon dioxide concentration and surface-water pH shown in Figure 1.
[2]
Calculate the percentage decrease in oyster-shell mass increase between pH 8.15 and pH 7.90.
[2]
Explain how absorption of additional atmospheric carbon dioxide can reduce oyster-shell growth.
[3]
Evaluate the proposed measures for protecting the bay's marine animals from ocean acidification.
[5]
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Question 13
Four farming systems were monitored for ten years in the fictional Arroyo Plain. All plots began with the same soil organic-carbon stock.
Mean crop yield and annual fossil-fuel use for four farming systems in the Arroyo Plain.
| Farming system | Mean crop yield / | Fossil-fuel use / |
|---|---|---|
| No-till with winter cover crops | 6.2 | 42 |
| Crop rotation with reduced tillage | 6.5 | 55 |
| Annual monoculture with heavy tillage | 6.8 | 91 |
| Drained organic wetland soil | 7.1 | 105 |
Identify the farming system that produced the greatest increase in soil organic carbon.
[1]
Calculate the difference between the year-10 changes in soil organic carbon under no-till with cover crops and annual monoculture with heavy tillage.
[2]
Explain why no-till farming with cover crops promoted the soil as a carbon sink.
[3]
Explain why drainage of the organic wetland soil caused a large decrease in its carbon stock.
[2]
Evaluate whether the farmers should replace annual monoculture with no-till farming and cover crops.
[6]
Question 14
Figure 2 shows results from an experiment in which juvenile molluscs were grown in seawater of different pH values. Calcification at pH 8.1 was assigned an index value of 100.
Figure 3 summarizes the chemical pathway associated with ocean acidification.
State the calcification index at pH 7.8.
[1]
Calculate the percentage decrease in calcification index between pH 8.1 and pH 7.7.
[2]
Explain how increased fossil-fuel combustion may reduce biodiversity in a coral-reef ecosystem, using Figures 2 and 3.
[4]
Question 15
A farm compared changes in soil organic carbon under three management systems. All plots contained the same soil organic carbon at the beginning of the study.
Identify the management system with the greatest loss of soil organic carbon over the study period.
[1]
Calculate the percentage change in soil organic carbon after 10 years under no-till farming with cover crops.
[2]
Explain the contrasting trends for no-till farming with cover crops and drained organic soil with heavy tillage.
[4]
Question 16
Dead plant material was incubated under different oxygen and moisture conditions. Figure 7 shows the resulting methane flux.
Figure 8 shows the later fate of methane released to the atmosphere.
Identify the treatment producing the highest methane flux.
[1]
Calculate how many times greater the methane flux is in the high-organic-matter anaerobic treatment than in the high-organic-matter oxygenated treatment.
[2]
Explain the methane flux pattern and its significance for the carbon cycle.
[4]
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Question 17
Explain why waterlogging can reduce the growth of ordinary plants while providing a competitive advantage to insectivorous plants such as sundews.
[4]
Question 18
Outline one advantage and one disadvantage of using ammonia produced by the Haber process as fertilizer.
[4]
Question 19
The fictional country of Montierra prepared an annual carbon budget and three proposals for reducing atmospheric carbon accumulation.
Comparison of three carbon-mitigation proposals available to Montierra.
| Proposal | Annual carbon effect / | Cost / billion currency units | Time frame | Other considerations |
|---|---|---|---|---|
| Renewable electricity | Avoids 16 | 2.4 | Operates for at least 25 years | Construction required |
| Reforestation | Absorbs 7 once forests are established | 0.9 | 12 years to reach stated uptake | Biodiversity, soil and water benefits; vulnerable to drought and fire |
| Artificial carbon capture | Stores 10 | 3.8 | May operate once installed | Uses 15% of power-station electricity output; leakage monitoring required |
| Funding constraint | Not applicable | Not applicable | Not applicable | Only two proposals can be funded at full scale |
For this question, assume that the annual carbon effects listed in the table are additive when proposals are combined; consider the other considerations qualitatively.
Calculate the percentage of Montierra's annual carbon emissions that is absorbed by the coastal ocean.
[2]
State whether each of the following is an organic or inorganic carbon store: terrestrial vegetation and atmospheric carbon dioxide.
[2]
Explain why atmospheric carbon continues to accumulate even though the ocean and terrestrial ecosystems act as carbon sinks.
[3]
Compare reforestation with artificial carbon capture as methods of carbon sequestration.
[3]
Evaluate which two proposals Montierra should fund to reduce net atmospheric carbon accumulation.
[5]
Question 20
Methane emissions and rice yields were measured under three water-management systems in the fictional Luma Delta. In the graph, negative water-table positions indicate levels below the soil surface and positive positions indicate levels above it.
Describe the relationship between water-table position and methane flux shown in the graph.
[2]
Calculate the percentage reduction in annual methane emissions when alternate wetting and drying replaces continuous flooding.
[2]
Explain why continuous flooding produces high methane emissions.
[3]
Explain why reducing methane emissions can slow atmospheric warming even though methane has a shorter residence time than carbon in many geological stores.
[3]
Evaluate which water-management system should be promoted in the Luma Delta.
[5]
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Question 21
Figure 5 compares the annual carbon balance of five mitigation scenarios. Net atmospheric addition equals emissions minus biological and artificial removals.
Figure 6 provides further information about two removal methods.
Calculate the percentage reduction in net atmospheric carbon addition achieved by the combined strategy compared with the baseline.
[2]
Explain why the combined strategy produces a lower net atmospheric addition than any single strategy shown.
[2]
Suggest why the removals shown may not be maintained over the long term.
[3]
Question 22
Figure 9 is a simplified nitrogen-cycle diagram in which three bacterial transformations are labelled A, B and C.
Figure 10 shows changes measured after an agricultural soil became waterlogged.
Identify the bacterial transformations labelled A, B and C in Figure 9.
[3]
Calculate the percentage decrease in soil nitrate concentration between day 0 and day 20.
[2]
Explain how waterlogging can reduce the growth of ordinary plants while providing an advantage to insectivorous plants.
[3]
Question 23
Figure 11 shows the sources of reactive nitrogen entering a river catchment during one year.
Annual sources of reactive nitrogen entering a river catchment.
| Source of reactive nitrogen | Annual input / units |
|---|---|
| Inorganic fertilizer | 48 |
| Livestock manure and excretion | 21 |
| Urban sewage | 14 |
| Aquaculture feed and excretion | 9 |
| Natural fixation and weathering | 8 |
Figure 12 shows nitrate concentrations at locations downstream through the catchment.
Calculate the percentage of total reactive nitrogen input that is anthropogenic.
[2]
Describe the downstream change in nitrate concentration shown in Figure 12.
[2]
Suggest three measures that could reduce nitrogen transfer to the estuary while maintaining food production in the catchment.
[3]
Question 24
Explain why biogeochemical cycles are essential for the sustainability of ecosystems.
[4]
Explain how carbon is transferred and transformed as it moves through a terrestrial food web and its associated soil.
[7]
Using named examples, evaluate the effectiveness of ecosystem restoration as a strategy for increasing carbon sequestration.
[9]
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Question 25
Figure 3 compares a proposed planetary-boundary value for intentionally fixed nitrogen applied to agriculture with the estimated anthropogenic input.
Calculate the percentage by which the estimated anthropogenic input exceeds the planetary-boundary value.
[2]
Outline why global collaboration is required to reduce this overshoot.
[2]
Question 26
The fictional Río Claro catchment contains forest, intensive farmland, a town and coastal fish farms. Water quality was measured at five sites.
Calculate the percentage increase in nitrate concentration between sites A and E.
[2]
Describe the relationship between nitrate concentration and dissolved-oxygen concentration across the five sites.
[2]
Explain two ways in which agriculture changes nitrogen flows in the catchment.
[4]
Explain how aquaculture can increase the inorganic nitrogen store in the coastal inlet.
[3]
Evaluate measures that could reduce disruption of the nitrogen cycle in the Río Claro catchment.
[5]
Question 27
Researchers compared clover, a legume with nitrogen-fixing root nodules, with a grass species in soils containing different nitrate concentrations. They also studied a nearby waterlogged bog.
Identify process X in Figure 3.
[1]
Describe the effect of soil nitrate concentration on the biomass of the grass.
[2]
Explain the difference in biomass between clover with active nodules and grass under low-nitrate conditions.
[3]
Explain why the relationship between clover and its nitrogen-fixing bacteria is mutualistic.
[2]
Explain why waterlogging reduces the availability of nitrate to ordinary plants in the bog.
[3]
Evaluate whether planting legumes would be an effective method of increasing plant production in nitrogen-poor soils.
[4]
Question 28
Figure 13 compares the proposed planetary-boundary value for intentionally fixed nitrogen used in agriculture with the estimated current anthropogenic input. The values shown are for the boundary and for the current anthropogenic input.
Figure 14 summarizes selected consequences and management considerations.
Calculate the percentage by which the estimated anthropogenic nitrogen input exceeds the planetary-boundary value.
[2]
Explain why the data provide evidence of increased risk to Earth systems.
[3]
Discuss why international collaboration is required to reduce the nitrogen input while protecting food security.
[3]
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Question 29
Distinguish between an organic carbon store and an inorganic carbon store, giving one example of each.
[4]
Explain how increased fossil-fuel combustion changes carbon flows between the lithosphere, atmosphere and oceans, and may affect marine ecosystems.
[7]
Using named examples, evaluate strategies for reducing human disruption of the carbon cycle and its effects on the oceans.
[9]
Question 30
Explain how the balance of carbon inputs and outputs determines whether an agricultural system acts as a carbon sink, a store in equilibrium or a carbon source.
[3]
Explain how agricultural practices and the use of harvested products can alter carbon storage in an agricultural system.
[8]
Using named examples, discuss whether increasing carbon storage should be the main objective of sustainable agricultural management.
[9]
Question 31
Outline how carbon in marine organisms may enter a long-term lithospheric store.
[4]
Explain how different environmental conditions can cause dead organic matter to enter fossil-fuel stores, release methane or release carbon dioxide.
[7]
Using named examples, evaluate the importance of reducing methane emissions as part of managing human disruption of the carbon cycle.
[9]
Question 32
Distinguish between organic and inorganic nitrogen stores in an ecosystem, giving two examples of each.
[4]
Explain how bacteria connect atmospheric, soil and biological nitrogen stores and may provide a competitive advantage to leguminous plants.
[7]
Using named examples, evaluate whether biological nitrogen fixation and nutrient recycling can replace inorganic nitrogen fertilizers while maintaining food security.
[9]
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Question 33
Global food production depends on industrial nitrogen fixation, but large quantities of reactive nitrogen are lost to the environment.
Calculate how many times greater the current anthropogenic nitrogen input is than the proposed planetary-boundary value.
[2]
Describe how cereal yield and nitrogen loss change as fertilizer application increases.
[3]
Explain one advantage and two disadvantages of using the Haber process to supply agricultural fertilizer.
[3]
Explain why crossing the nitrogen planetary boundary increases the risk of large-scale environmental change.
[3]
Evaluate the need for global collaboration to bring the nitrogen cycle back within the planetary boundary while maintaining food security.
[5]
Question 34
Explain how the Haber process alters the nitrogen cycle and supports crop production.
[3]
Analyse how deforestation, intensive agriculture, aquaculture and urbanization can alter nitrogen stores and flows from land to aquatic systems.
[8]
Using named examples, discuss the extent to which global collaboration can bring the nitrogen cycle back within planetary boundaries without reducing food security.
[9]
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