AP Environmental Science Quiz: The Carbon Cycle

What this quiz covers

This quiz focuses on The Carbon Cycle, giving you a quick way to practice the rules, question types, and explanations that matter most for AP Environmental Science.

How to use this quiz

Try each quiz question before looking at the correct answer. Use the explanations to review missed ideas, then come back to similar questions until the pattern feels familiar.

Question 1

1. Photosynthesis
2. Respiration/decomposition (correct answer)
3. Sedimentation into the lithosphere
4. Ocean absorption

Explanation: In lakes, the biosphere includes organic matter, atmosphere holds CO2. Respiration and decomposition break down organic carbon, releasing CO2 from biosphere to atmosphere (or water, then atmosphere). This dominates summer emissions. Photosynthesis fixes to biosphere, sedimentation buries to lithosphere, absorption is atm to water. Option B is correct for the biosphere-to-atmosphere flux.

Question 2

1. Decrease by 9 units
2. Increase by 3 units (correct answer)
3. Decrease by 1 unit
4. Increase by 11 units

Explanation: Key carbon reservoirs are the atmosphere (CO2), biosphere (tree biomass), lithosphere (fossil fuels), and ocean (dissolved carbon). Fluxes include photosynthesis removing CO2 from the atmosphere to the biosphere, respiration and decomposition returning it from biosphere to atmosphere, combustion transferring from lithosphere to atmosphere, and ocean absorption moving from atmosphere to ocean. Here, photosynthesis removes 25 units, respiration/decomposition adds 20, combustion adds 12, and ocean absorbs 4. Net change: -25 + 20 + 12 - 4 = +3 units increase in atmosphere. This happens because inputs (respiration and combustion) exceed outputs (photosynthesis and absorption). Option B correctly reflects this increase by 3 units, showing how urban greening helps but doesn't fully offset emissions.

Question 3

1. Biosphere
2. Ocean (correct answer)
3. Lithosphere
4. Atmosphere

Explanation: Reservoirs: ocean (dissolved inorganic carbon), atmosphere (CO2). In upwelling, deep ocean water brings dissolved carbon to the surface, where lower pressure and temperature cause CO2 release to the atmosphere. This flux is directly from ocean to atmosphere. The biosphere or lithosphere aren't immediate sources here. Option B correctly identifies the ocean as the source. This process highlights how physical ocean dynamics influence atmospheric carbon.

Question 4

1. Atmosphere
2. Ocean (especially deep ocean) (correct answer)
3. Lithosphere immediately, within months
4. Biosphere on land

Explanation: Carbon reservoirs include the ocean, which stores vast amounts of carbon, especially in deep waters as dissolved CO2 and sunken organic matter. High ocean productivity means more phytoplankton photosynthesis, fixing atmospheric CO2 into biomass that can sink to the deep ocean, increasing storage there. This process, known as the biological pump, transfers carbon from surface to deep ocean reservoirs over time. The atmosphere, lithosphere, and land biosphere do not directly gain from this sinking; instead, the deep ocean acts as a long-term sink. Choice B is correct, showing how marine ecosystems can sequester carbon away from the atmosphere. This mechanism helps regulate global CO2 levels.

Question 5

1. Atmospheric carbon decreases because soil microbes convert $CO_2$ into rock
2. Biosphere/soil carbon decreases and atmospheric $CO_2$ increases because respiration/decomposition flux to the atmosphere increases (correct answer)
3. Lithosphere carbon increases because plowing accelerates fossil-fuel formation
4. Ocean carbon decreases because less $CO_2$ dissolves in seawater

Explanation: Soil contains significant organic carbon in the biosphere reservoir, protected from rapid decomposition by soil structure and limited oxygen exposure. Deep plowing breaks up soil aggregates and increases oxygen penetration, accelerating microbial decomposition of organic matter. This enhanced decomposition increases the respiration flux from soil to atmosphere, releasing stored carbon as CO₂. Over time, soil carbon content decreases as organic matter is oxidized faster than it can be replenished. Simultaneously, atmospheric CO₂ increases due to the enhanced respiration/decomposition flux. Option B correctly identifies both outcomes: decreased biosphere/soil carbon and increased atmospheric CO₂ due to increased respiration/decomposition flux to the atmosphere.

Question 6

1. Atmospheric carbon increases because photosynthesis releases $CO_2$
2. Biosphere carbon increases because photosynthesis stores more carbon in biomass than is returned by respiration (correct answer)
3. Lithosphere carbon decreases because carbon is moved from rocks into trees
4. Ocean carbon decreases because less carbon is exchanged with the atmosphere

Explanation: The carbon cycle includes photosynthesis (atmosphere to biosphere flux) and respiration (biosphere to atmosphere flux). During reforestation, growing trees perform photosynthesis, removing CO₂ from the atmosphere and storing it as biomass in trunks, branches, roots, and leaves. Since the forest is not harvested or burned, this carbon remains stored in the biosphere reservoir. While trees do respire and release some CO₂, young growing forests typically have photosynthesis rates that exceed respiration rates, resulting in net carbon accumulation. Therefore, the biosphere carbon increases as more carbon is stored in biomass than is returned by respiration. The lithosphere is unaffected by tree growth on human timescales, and atmospheric carbon would decrease (not increase).

Question 7

1. Atmosphere, through respiration
2. Biosphere, through photosynthesis storing carbon in biomass (correct answer)
3. Lithosphere, through combustion converting biomass into coal
4. Ocean, through outgassing transferring carbon into seawater

Explanation: Trees remove CO2 from the atmosphere through photosynthesis and convert it into organic carbon compounds that form their biomass (wood, leaves, roots). This process transfers carbon from the atmosphere reservoir to the biosphere reservoir, specifically storing it in plant tissues. As trees grow over decades, they continuously accumulate carbon in their trunks, branches, and root systems, effectively sequestering atmospheric carbon in long-lived biomass. The biosphere reservoir increases directly through this photosynthetic carbon fixation and biomass accumulation. While trees do respire and return some CO2 to the atmosphere, healthy growing trees have net positive carbon storage, making urban forests effective carbon sinks that reduce atmospheric CO2 concentrations.

Question 8

1. Atmospheric carbon decreases by 6 units
2. Atmospheric carbon increases by 6 units (correct answer)
3. Atmospheric carbon is unchanged because oceans are always sinks
4. Atmospheric carbon decreases because combustion stops

Explanation: The ocean reservoir can release carbon to the atmosphere via outgassing, especially during heatwaves. An extra 6 units outgassed directly increases atmospheric carbon, with unchanged land fluxes. Choice B is correct; the atmosphere gains immediately. This shows how warming oceans can amplify CO2 levels. It's a feedback in climate change.

Question 9

1. Atmospheric CO$_2$ decreases because less carbon is fixed into biomass
2. Atmospheric CO$_2$ increases because less CO$_2$ is removed by photosynthesis (correct answer)
3. Atmospheric CO$_2$ decreases because ocean absorption stops
4. Atmospheric CO$_2$ stays constant because respiration removes CO$_2$

Explanation: In forests, the biosphere reservoir stores carbon in plant biomass, while the atmosphere holds CO2. Photosynthesis fluxes carbon from atmosphere to biosphere, reducing atmospheric CO2, whereas respiration returns it from biosphere to atmosphere. A 30% reduction in photosynthesis means less CO2 removal, while unchanged respiration continues adding CO2 at the same rate. This imbalance leads to a net increase in atmospheric CO2. Combustion and ocean fluxes are assumed constant, so the effect stems from diminished photosynthetic uptake. Option B accurately predicts this outcome, emphasizing the role of photosynthesis as a key sink.

Question 10

1. Decreased photosynthesis and increased combustion, both adding $CO_2$ to the atmosphere
2. Decreased photosynthesis (less $CO_2$ uptake) and continued respiration/combustion (continued $CO_2$ release), increasing net atmospheric $CO_2$ (correct answer)
3. Increased ocean absorption and decreased respiration, increasing net atmospheric $CO_2$
4. Increased photosynthesis and decreased combustion, increasing net atmospheric $CO_2$

Explanation: The carbon cycle involves photosynthesis (removing CO₂ from atmosphere) and respiration/combustion (adding CO₂ to atmosphere). During drought, plant stress causes photosynthesis to decrease markedly, reducing the flux of carbon from atmosphere to biosphere. However, decomposition by microbes continues, releasing CO₂ through respiration, and wildfires combust organic matter, rapidly releasing stored carbon to the atmosphere. The combination of decreased CO₂ uptake (less photosynthesis) and continued or increased CO₂ release (respiration and combustion) creates an imbalance. This shifts the net flux toward the atmosphere, causing atmospheric CO₂ to rise. Option B correctly identifies both processes: decreased photosynthesis reduces CO₂ uptake while continued respiration/combustion maintains CO₂ release.

Question 11

1. Atmospheric carbon decreases because photosynthesis increases
2. Atmospheric carbon increases because the ocean shifts from sink to source (correct answer)
3. Atmospheric carbon decreases because respiration slows
4. No change because the lithosphere is the largest reservoir

Explanation: The ocean reservoir holds dissolved CO2, while the atmosphere contains gaseous CO2. Fluxes between them include absorption (atmosphere to ocean) when the ocean acts as a sink and outgassing (ocean to atmosphere) when it acts as a source. Currently, the ocean absorbs 2 Gt/year, removing carbon from the atmosphere. If warming reduces solubility, shifting to a release of 1 Gt/year, this adds carbon to the atmosphere instead of removing it. The net effect is an increase in atmospheric carbon due to the ocean changing from sink to source. All else equal, this shift disrupts the balance, leading to higher atmospheric levels. Option B correctly describes this directional change.

Question 12

1. Increase by 20 units (correct answer)
2. Decrease by 20 units
3. Increase by 10 units
4. No net change

Explanation: Biosphere reservoir includes biomass and detritus. Fluxes: photosynthesis adds 200 from atm, respiration removes 180 to atm. Net biosphere change: +200 -180 = +20 units increase. Combustion and ocean absorption affect atmosphere but not directly biosphere here. Ignoring lithosphere transfers, the net is positive storage. Option A correctly indicates increase by 20 units.

Question 13

1. It won't matter because ocean absorption instantly removes all added CO$_2$
2. It matters because combustion increases the lithosphere → atmosphere flux faster than photosynthesis and ocean uptake can offset (correct answer)
3. It won't matter because respiration will decrease automatically
4. It matters only if the biosphere is larger than the lithosphere

Explanation: Though lithosphere is largest, combustion flux adds CO2 to atmosphere faster than photosynthesis (atm to bio) and ocean uptake (atm to ocean) can remove it. This imbalances the active cycle. Option B corrects the claim by focusing on flux rates. It emphasizes that reservoir size doesn't negate rapid additions.

Question 14

1. Decrease by 10 units
2. Increase by 10 units (correct answer)
3. Decrease by 35 units
4. Increase by 35 units

Explanation: Forest fluxes: photosynthesis removes 300 from atmosphere to biosphere, respiration adds 290 back, wildfire adds 25 from biosphere to atmosphere, ocean absorbs 5 from atmosphere. Net: -300 +290 +25 -5 = +10 increase in atmosphere. Choice B is correct. This shows disturbances like fires can turn sinks into sources. Ocean absorption mitigates some impact.

Question 15

1. Atmospheric carbon increases because less CO$_2$ dissolves in the ocean
2. Atmospheric carbon decreases relative to before because a source flux is reduced (correct answer)
3. Atmospheric carbon is unchanged because respiration dominates all fluxes
4. Atmospheric carbon decreases only if photosynthesis stops

Explanation: The lithosphere-to-atmosphere flux via combustion adds carbon to the atmosphere. Reducing this by 4 units decreases the input flux, leading to less atmospheric carbon accumulation compared to before. Other fluxes like photosynthesis (removal), respiration (addition), and ocean absorption (removal) remain unchanged. This reduction directly lowers net atmospheric increase. Option B correctly states atmospheric carbon decreases relative to before due to reduced source flux. It illustrates how mitigating emissions can help balance the cycle.

Question 16

1. It reduces the lithosphere → atmosphere carbon flux, slowing atmospheric CO$_2$ accumulation (correct answer)
2. It increases respiration, raising atmospheric CO$_2$
3. It shifts carbon from the ocean to the atmosphere
4. It converts atmospheric CO$_2$ into fossil fuels

Explanation: Reservoirs in the carbon cycle are the lithosphere (fossil fuels), atmosphere (CO2), biosphere (organisms), and ocean (dissolved carbon). Replacing coal with wind reduces the combustion flux from lithosphere to atmosphere, decreasing CO2 emissions. This slows atmospheric CO2 accumulation since inputs from combustion drop while removals like photosynthesis and ocean absorption continue. Choice A accurately describes this effect, unlike B (which incorrectly suggests increased respiration), C (no ocean-to-atmosphere shift), or D (wind doesn't convert CO2 to fuels). This transition exemplifies sustainable energy's role in balancing the carbon cycle. It reduces reliance on fossil carbon stores.

Question 17

1. Decrease by 15 units (correct answer)
2. Decrease by 5 units
3. Increase by 5 units
4. Increase by 15 units

Explanation: Carbon reservoirs in this marine context include the ocean (dissolved CO2 and other inorganic forms), biosphere (organic carbon in phytoplankton biomass), and atmosphere (gaseous CO2). Fluxes involve photosynthesis moving carbon from the ocean to the biosphere as phytoplankton fix dissolved CO2 into organic matter, respiration returning carbon from the biosphere to the ocean as organisms break down biomass, and outgassing transferring carbon from the ocean to the atmosphere due to warming reducing solubility. Here, photosynthesis removes 50 units from the ocean, respiration adds back 40 units, and 5 units are lost to the atmosphere. The net change in ocean carbon is -50 + 40 - 5 = -15 units, a decrease. This occurs because the bloom's carbon fixation exceeds respiration returns, but outgassing further depletes the ocean reservoir. Option A correctly identifies this decrease by 15 units, illustrating how biological and physical processes interact in the carbon cycle.

Question 18

1. Ocean absorption may increase (more CO$_2$ dissolves), slowing but not stopping atmospheric increase (correct answer)
2. Ocean absorption must drop to zero immediately
3. Ocean absorption reverses because photosynthesis stops on land
4. Ocean absorption converts CO$_2$ into fossil fuels in decades

Explanation: The ocean reservoir buffers atmospheric CO2 by increasing absorption as concentrations rise, slowing accumulation. Choice A describes this partial buffering. Others misrepresent ocean dynamics. This highlights the ocean's role in the cycle. However, it leads to acidification.

Question 19

1. Atmosphere
2. Lithosphere (via sediment burial) (correct answer)
3. Atmosphere (via respiration)
4. Ocean to atmosphere (via outgassing)

Explanation: Wetland restoration boosts biosphere carbon via growth, with burial transferring it to lithosphere sediments for long-term storage. Choice B is correct; not atmosphere or direct ocean fluxes. This enhances carbon sequestration. It's a nature-based climate solution.

Question 20

1. Atmospheric carbon decreases because the ocean stops absorbing CO$_2$
2. Atmospheric carbon increases because input exceeds removal (correct answer)
3. Biosphere carbon decreases because photosynthesis increases
4. Lithosphere carbon increases because more fossil fuels form

Explanation: Carbon cycle reservoirs include the lithosphere (fossil sources) and biosphere (plants). The diagram shows combustion adding more carbon to the atmosphere than photosynthesis removes, leading to increased atmospheric carbon as inputs exceed removals. Choice B captures this trend, unlike A (ocean absorption doesn't stop), C (photosynthesis would decrease biosphere if it increased, but it's unchanged), or D (lithosphere decreases from combustion). This illustrates anthropogenic impacts on the cycle. Over time, it contributes to global warming.