Drill 35 ·
AP Biology: Unit 8, Biogeochemical Cycles (Drill 35) is a practice drill. It contains 5 original questions created by Brian Stewart, a Barron's test prep author with over 20 years of tutoring experience.
Practice interpreting carbon and nitrogen flux data in this AP Biology drill on biogeochemical cycles. Analyze ecosystem nutrient movement before and after a disturbance, apply carbon cycle calculations, and evaluate claims about nitrogen cycling processes.
| Flux Pathway | Carbon Before Burn (kg/ha/yr) | Carbon After Burn (kg/ha/yr) | Nitrogen Before Burn (kg/ha/yr) | Nitrogen After Burn (kg/ha/yr) |
|---|---|---|---|---|
| Photosynthesis (atmosphere to plant) | 4,200 | 1,100 | -- | -- |
| Plant respiration (plant to atmosphere) | 1,800 | 420 | -- | -- |
| Decomposer respiration (soil to atmosphere) | 940 | 210 | -- | -- |
| Nitrification (NH4+ to NO3-) | -- | -- | 180 | 44 |
| Denitrification (NO3- to N2) | -- | -- | 38 | 91 |
| Nitrogen fixation (N2 to NH4+) | -- | -- | 12 | 67 |
Question 1. In the nitrogen cycle, which process converts atmospheric nitrogen (N2) into a form usable by plants?
Explanation: Nitrogen fixation converts atmospheric N2 into NH4+, a form plants can absorb directly or that can be further converted to NO3- via nitrification. It is carried out by free-living bacteria such as Azotobacter and by symbiotic bacteria such as Rhizobium in legume root nodules. A describes nitrification, which acts on ammonium already in the soil and does not fix atmospheric N2. B describes denitrification, which removes fixed nitrogen from the ecosystem. D describes decomposition, which releases organic nitrogen as ammonium but does not fix atmospheric N2.
Question 2. Before the burn, what is the net carbon flux between the forest ecosystem and the atmosphere, considering only photosynthesis and total respiration (plant plus decomposer)?
Explanation: Net carbon flux = photosynthesis - (plant respiration + decomposer respiration). Before the burn: 4,200 - (1,800 + 940) = 4,200 - 2,740 = 1,460 kg C/ha/yr absorbed. The ecosystem is a net carbon sink. A (460) subtracts only decomposer respiration from plant respiration rather than comparing both respiration terms to photosynthesis. C is incorrect -- photosynthesis (4,200) exceeds total respiration (2,740) by a substantial margin. D (2,740) is total respiration alone, not net flux.
Question 3. After the burn, denitrification increases from 38 to 91 kg N/ha/yr while nitrogen fixation increases from 12 to 67 kg N/ha/yr. Which explanation best accounts for both changes occurring simultaneously?
Explanation: After the burn, reduced plant cover means less nitrogen uptake, allowing NO3- to accumulate in soil and providing substrate for denitrifying bacteria, which are anaerobic and favor conditions that may promote anaerobic microsites in disturbed soils. Simultaneously, the loss of established vegetation creates open conditions colonized by pioneer nitrogen-fixing species such as legumes and free-living bacteria. A is incorrect -- denitrification is anaerobic; increased oxygen would inhibit, not stimulate, denitrifying bacteria. C overstates the burn effect -- soil nitrogen is not entirely converted to N2. D is incorrect -- fixation and denitrification are independent processes; fixation produces NH4+, not NO3-, and does not directly fuel denitrification.
Question 4. Carbon fixed by photosynthesis can reach the soil carbon pool through multiple pathways. Which sequence correctly traces one such pathway?
Explanation: Carbon fixed in photosynthesis is incorporated into plant biomass. When leaves, roots, and stems die and fall as litter, decomposers break down organic matter and incorporate carbon into soil organic matter. B correctly traces this pathway. A is incorrect -- herbivore respiration releases carbon to the atmosphere, not to the soil. C is incorrect -- plant respiration releases CO2 directly to the atmosphere; it does not enter a dissolved organic carbon pool en route to soil organic matter. D incorrectly introduces nitrification, a nitrogen cycle process, into a carbon pathway.
Question 5. A forest ecologist argues that the controlled burn converted this ecosystem from a long-term carbon sink to a carbon source. Which combination of data from the table most directly evaluates this argument?
Explanation: Net carbon flux after the burn = 1,100 - (420 + 210) = 1,100 - 630 = +470 kg C/ha/yr. The ecosystem remains a net carbon sink after the burn, though a weakened one. This directly challenges the ecologist's argument rather than supporting it, making C the best answer because it uses the data accurately to evaluate the claim. A introduces nitrogen recovery but does not address carbon flux. B correctly calculates both values and the ratio, but the decrease in the respiration-to-photosynthesis ratio (0.65 to 0.57) indicates respiration consumes a smaller share of photosynthesis after the burn -- further undermining the source argument rather than supporting it. D incorrectly claims net balance is unchanged.