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Quiz: Biogeochemical Cycles

Test your understanding of how matter cycles through Earth's living and nonliving systems with these review questions.

1. What is the key difference between the phosphorus cycle and the carbon, nitrogen, and water cycles?

  1. Phosphorus cycles faster than the other elements
  2. Phosphorus has no significant gaseous phase in its cycle
  3. Phosphorus is not required by living organisms
  4. Phosphorus is only found in marine ecosystems
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The correct answer is B. Unlike carbon, nitrogen, and water, phosphorus has no significant gaseous (atmospheric) phase. Phosphorus moves through rock, soil, water, and organisms but does not cycle through the atmosphere. This makes the phosphorus cycle fundamentally different and much slower, operating on geological timescales of thousands to millions of years.

Concept Tested: Phosphorus Cycle

2. Which organisms are primarily responsible for nitrogen fixation in terrestrial ecosystems?

  1. Fungi that form mycorrhizal associations with tree roots
  2. Nitrogen-fixing bacteria such as Rhizobium in legume root nodules
  3. Earthworms that break down dead organic matter in soil
  4. Photosynthetic algae that live on the soil surface
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The correct answer is B. Nitrogen fixation is performed by specialized bacteria that can break the strong triple bond in atmospheric nitrogen gas (N2). Rhizobium bacteria live symbiotically in root nodules of legumes like beans and clover, converting N2 into ammonia (NH3) or ammonium (NH4+) that plants can use. Other nitrogen-fixing bacteria include free-living soil bacteria like Azotobacter and aquatic cyanobacteria.

Concept Tested: Nitrogen Fixation

3. What is the process by which plants release water vapor through pores in their leaves?

  1. Evaporation
  2. Precipitation
  3. Transpiration
  4. Infiltration
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The correct answer is C. Transpiration is the process by which plants release water vapor through small pores called stomata in their leaves. A single large oak tree can transpire over 150,000 liters of water in a growing season. Transpiration is a major component of the hydrologic cycle and, combined with evaporation, is referred to as evapotranspiration.

Concept Tested: Transpiration

4. What happens when excess phosphorus from agricultural runoff enters a lake?

  1. The lake becomes more acidic, dissolving carbonate rocks on the lakebed
  2. Fish populations increase due to the additional nutrients available
  3. Algal blooms occur, and subsequent decomposition depletes dissolved oxygen
  4. The phosphorus binds to nitrogen gas and escapes into the atmosphere
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The correct answer is C. Excess phosphorus triggers eutrophication -- explosive algal growth on the water surface. When the algae die, decomposers consume them and use up dissolved oxygen in the process. This creates hypoxic "dead zones" where fish and other aquatic organisms cannot survive. The Gulf of Mexico dead zone, caused by nutrient runoff from farms, can exceed 15,000 square kilometers.

Concept Tested: Phosphorus Runoff

5. How does methane differ from carbon dioxide as a greenhouse gas?

  1. Methane traps more heat per molecule but breaks down faster in the atmosphere
  2. Methane is produced only by volcanic eruptions while carbon dioxide comes from respiration
  3. Methane has a longer atmospheric lifetime and lower warming potential than carbon dioxide
  4. Methane is produced only in oxygen-rich environments while carbon dioxide forms in all conditions
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The correct answer is A. Methane is approximately 80 times more potent than carbon dioxide at trapping heat over a 20-year period, but it has a much shorter atmospheric lifetime of about 12 years compared to 300-1000 years for CO2. Methane is produced when decomposition occurs in low-oxygen environments such as wetlands, landfills, rice paddies, and the digestive systems of livestock.

Concept Tested: Methane

6. What is carbon sequestration?

  1. The release of stored carbon from fossil fuels through combustion
  2. The conversion of carbon dioxide into methane by soil bacteria
  3. Any process that removes carbon from the atmosphere and stores it in a long-term reservoir
  4. The chemical weathering of carbonate rocks by acidic rainfall
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The correct answer is C. Carbon sequestration is any process that removes carbon from the atmosphere and locks it into a long-term reservoir. Natural examples include forests storing carbon in wood and soil, oceans dissolving CO2, and peat bogs accumulating carbon-rich organic matter. Over geological time, buried organisms become fossil fuels. Humans are currently releasing this sequestered carbon far faster than it was stored.

Concept Tested: Carbon Sequestration

7. In the nitrogen cycle, what is the role of denitrification?

  1. Converting atmospheric N2 into ammonia that plants can absorb
  2. Converting ammonium into nitrate through bacterial oxidation
  3. Converting nitrate back into N2 gas, returning nitrogen to the atmosphere
  4. Converting organic nitrogen in dead matter into ammonium through decomposition
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The correct answer is C. Denitrification is performed by anaerobic bacteria in waterlogged, low-oxygen soils. These bacteria convert nitrate (NO3-) back into nitrogen gas (N2), which escapes to the atmosphere. This process completes the nitrogen cycle and serves as a vital balancing mechanism that prevents toxic nitrate accumulation in soils and waterways.

Concept Tested: Denitrification

8. What is an aquifer and why is it ecologically important?

  1. A surface lake that stores rainwater for irrigation and drinking water
  2. A layer of permeable rock or sediment that stores and transmits groundwater
  3. A wetland area that filters pollutants before they reach the ocean
  4. A glacier that stores frozen freshwater at high elevations
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The correct answer is B. An aquifer is a layer of permeable rock, sand, or gravel that stores and transmits groundwater. Some aquifers are enormous -- the Ogallala Aquifer beneath the Great Plains holds enough water to cover all 50 states to a depth of about 0.5 meters. Aquifers are critical for drinking water and agriculture, but when pumped faster than they recharge, groundwater depletion becomes a serious ecological and human problem.

Concept Tested: Aquifer

9. Why does cutting down a tropical rainforest disrupt both the carbon cycle and the water cycle simultaneously?

  1. Trees block wind patterns that carry carbon dioxide across continents
  2. Trees store carbon in biomass and drive transpiration that contributes to local rainfall
  3. Trees absorb groundwater that would otherwise dissolve carbon in aquifers
  4. Trees produce methane that balances atmospheric carbon dioxide concentrations
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The correct answer is B. Trees store significant amounts of carbon in their trunks, branches, and roots. Cutting them releases that carbon through decomposition or burning, disrupting the carbon cycle. Simultaneously, trees drive transpiration, releasing water vapor that contributes to local cloud formation and precipitation. Removing forests reduces transpiration, potentially decreasing local rainfall and disrupting the hydrologic cycle.

Concept Tested: Carbon Cycle

10. What does nitrification accomplish in the nitrogen cycle?

  1. It breaks the triple bond in atmospheric nitrogen gas
  2. It returns nitrogen gas to the atmosphere from waterlogged soils
  3. It converts ammonium into nitrate, the form most easily absorbed by plant roots
  4. It releases nitrogen from dead organisms back into the soil
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The correct answer is C. Nitrification is a two-step bacterial process that converts ammonium (NH4+) to nitrate (NO3-). First, Nitrosomonas bacteria oxidize ammonium to nitrite (NO2-), then Nitrobacter bacteria oxidize nitrite to nitrate. Nitrate is the form of nitrogen most easily absorbed by plant roots, making nitrification a critical step in making fixed nitrogen available to plants.

Concept Tested: Nitrification