AP Environmental Science Quiz: The Nitrogen Cycle

What this quiz covers

This quiz focuses on The Nitrogen 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. Assimilation by algae converting $\text{N}_2$ into amino acids
2. Denitrification by anaerobic bacteria converting $\text{NO}_3^-$ to $\text{N}_2$ (correct answer)
3. Nitrification by anaerobic bacteria converting $\text{NH}_4^+$ to $\text{NO}_3^-$
4. Fixation by decomposers converting organic N to $\text{NH}_4^+$

Explanation: Denitrification is the process in the nitrogen cycle where nitrate (NO₃⁻) is reduced to nitrogen gas (N₂) by anaerobic bacteria, such as Pseudomonas, in low-oxygen environments, effectively removing nitrogen from the soil. In a waterlogged wetland, the low-oxygen conditions inhibit aerobic processes and promote denitrifying bacteria, leading to decreased nitrate and increased N₂ release. These bacteria use nitrate as an alternative electron acceptor in respiration when oxygen is scarce, playing a key role in closing the nitrogen cycle by returning N₂ to the atmosphere. This explains the observations, as flooding creates the anaerobic habitat ideal for denitrification. Processes like nitrification require oxygen and would not thrive here, while assimilation or fixation do not produce N₂ gas. Thus, choice B correctly identifies denitrification by anaerobic bacteria.

Question 2

1. Nitrification by decomposer fungi converting $\text{NO}_3^-$ to $\text{NH}_4^+$
2. Denitrification by anaerobic bacteria converting $\text{NO}_3^-$ to $\text{N}_2$
3. Nitrogen fixation by symbiotic bacteria converting $\text{N}_2$ to $\text{NH}_4^+$ (correct answer)
4. Assimilation by plants converting $\text{NH}_4^+$ into atmospheric $\text{N}_2$

Explanation: The nitrogen cycle involves several key processes that transform nitrogen between different forms, making it available for organisms or returning it to the atmosphere. Nitrogen fixation is the process where atmospheric nitrogen gas (N₂) is converted into ammonium (NH₄⁺), a form usable by plants, primarily carried out by symbiotic bacteria like Rhizobia in the root nodules of legumes such as soybeans. In this scenario, the farmer planted soybeans in nitrogen-poor soil, and without fertilizer, the increase in soil ammonium is due to these bacteria fixing N₂ from the air into NH₄⁺. Symbiotic bacteria play a crucial role by living in mutualistic relationships with legume roots, providing fixed nitrogen to the plant while receiving carbohydrates in return. This explains the observed increase in ammonium, as the bacteria enhance soil fertility naturally. Other processes like nitrification or denitrification do not directly add new nitrogen to the soil from the atmosphere. Thus, choice C correctly identifies nitrogen fixation by symbiotic bacteria as the key process and agent.

Question 3

1. Reduced eutrophication because added nitrate immediately becomes atmospheric $\text{N}_2$ through fixation
2. Increased primary productivity because added reactive nitrogen (often $\text{NO}_3^-$ and $\text{NH}_4^+$) boosts assimilation by algae (correct answer)
3. Decreased algal growth because nitrification removes nitrogen from the ecosystem as $\text{N}_2$
4. No change in algal growth because nitrogen is never a limiting nutrient in freshwater lakes

Explanation: Human disruptions to the nitrogen cycle, such as fertilizer runoff, introduce excess reactive nitrogen (like NO₃⁻ and NH₄⁺) into aquatic systems, leading to eutrophication where algal growth explodes due to nutrient abundance. In this lake, farm runoff boosts assimilation by algae, which take up the nitrogen to form organic compounds, increasing primary productivity and causing blooms. Algae play a role by rapidly incorporating the added nitrogen, but excessive growth can deplete oxygen and harm ecosystems. This predicts increased productivity as the primary consequence, unlike reduced eutrophication or no change. Fixation or nitrification do not directly cause blooms from added reactive nitrogen. Thus, choice B best predicts the nitrogen-cycle-related outcome of this disruption.

Question 4

1. Ammonium increases and nitrate decreases (correct answer)
2. Nitrate increases and ammonium decreases
3. Nitrogen gas increases because fixation speeds up
4. Organic nitrogen in dead matter immediately converts to nitrate without microbes

Explanation: Nitrifying bacteria perform nitrification, converting ammonium (NH4+) to nitrate (NO3-) in aerobic soils, a key step for nitrogen availability. Inhibiting these bacteria prevents this conversion, causing ammonium to accumulate and nitrate levels to drop. This immediate change reflects the blockage in the cycle, as in choice A. Nitrogen gas would not increase via fixation, which is unrelated, and organic nitrogen requires microbes for conversion, not bypassing them. Understanding bacterial specificity helps predict such shifts. Other processes like denitrification might later be affected, but the direct impact is on ammonium and nitrate forms.

Question 5

1. Fertilizer adds reactive N → increased algal growth → more decomposition → oxygen depletion (correct answer)
2. Fertilizer adds $\text{N}_2$ gas → denitrification stops → oxygen increases
3. Fertilizer reduces nitrification → nitrate decreases → algal blooms increase
4. Fertilizer increases fixation → less nitrogen available for algae → hypoxia decreases

Explanation: Human activities like agriculture disrupt the nitrogen cycle by adding excess reactive nitrogen through fertilizers, which can lead to environmental issues. Fertilizers introduce forms like ammonium or nitrate, boosting algal growth in downstream waters via eutrophication. When algae die, decomposition by bacteria consumes oxygen, causing hypoxic (low-oxygen) zones that harm aquatic life. This chain—added reactive N leading to algal blooms, decomposition, and oxygen depletion—best explains the impact, as in choice A. Other options incorrectly describe processes: fertilizers do not add N2 gas, reduce nitrification, or increase fixation in ways that decrease hypoxia. Bacteria in decomposition and denitrification play roles, but the primary driver is excess nutrient input fueling algal overgrowth.

Question 6

1. Denitrification (correct answer)
2. Nitrification
3. Assimilation
4. Fixation

Explanation: Soil conditions influence nitrogen cycle processes due to bacterial preferences for oxygen levels. In well-drained, oxygenated soils like Soil X, nitrification dominates, producing nitrate. Waterlogged, anaerobic soils like Soil Y favor denitrification, where bacteria convert nitrate to nitrogen gas (N2), leading to lower nitrate and higher N2 emissions. This explains the observations in Soil Y, making denitrification the most active process (choice A). Nitrification requires oxygen and would be limited in Soil Y, while assimilation and fixation do not directly cause N2 emissions or nitrate loss in this way. Bacterial adaptations to anaerobic environments drive denitrification as a key pathway for nitrogen loss.

Question 7

1. Nitrogen fixation by certain bacteria converting $\text{N}_2$ to $\text{NH}_4^+$ (correct answer)
2. Denitrification by bacteria converting $\text{N}_2$ to $\text{NO}_3^-$
3. Nitrification by fungi converting $\text{NO}_3^-$ to $\text{NH}_4^+$
4. Ammonification by plants converting $\text{NH}_4^+$ to organic N

Explanation: Nitrogen fixation is the process where certain bacteria, like Rhizobium in root nodules, convert atmospheric N2 into ammonium (NH4+), introducing new usable nitrogen into ecosystems. This is vital after events like fires that deplete soil nitrogen, as it rebuilds availability without relying on existing pools. Denitrification removes nitrogen by converting nitrate to N2, nitrification oxidizes ammonium to nitrate (not by fungi), and ammonification recycles organic N to ammonium but does not introduce new nitrogen. Choice A correctly identifies fixation's role in recovery. Bacteria symbiotically associated with plants enhance this process, emphasizing its importance for ecosystem resilience.

Question 8

1. Assimilation (correct answer)
2. Denitrification
3. Nitrification
4. Nitrogen fixation

Explanation: Assimilation describes the incorporation of inorganic nitrogen (like NO₃⁻ and NH₄⁺) into organic molecules such as amino acids and proteins by plants. In the hydroponic system, supplied nitrogen is taken up by roots and converted via enzymes like nitrate reductase. Plants are the primary agents, building biomass from these ions. This process is essential for growth and nitrogen cycling. Nitrification transforms between inorganics, not into organics. Thus, choice A correctly describes assimilation.

Question 9

1. Removal of denitrifying bacteria from the stream by predation
2. Runoff and leaching of reactive nitrogen added as fertilizer (correct answer)
3. Increased nitrogen fixation by stream fish
4. Decreased nitrification because fertilizers contain only $\text{N}_2$

Explanation: Human activities like heavy fertilization disrupt the nitrogen cycle by adding excess reactive nitrogen, which can leach or run off into nearby water bodies during storms. In the cornfield, applied fertilizer (often containing NO₃⁻) is mobilized by rainwater, directly increasing stream nitrate concentrations. This runoff represents a key anthropogenic input, bypassing natural cycling rates. Bacteria or fish do not significantly contribute here; it's the physical transport of added nitrogen. Decreased nitrification is irrelevant as fertilizers provide reactive forms. Thus, choice B identifies runoff and leaching as the direct disruption.

Question 10

1. Denitrification (correct answer)
2. Nitrification
3. Ammonification
4. Assimilation

Explanation: Denitrification produces gaseous forms like nitrous oxide (N2O) from nitrate under low-oxygen, wet conditions, with bacteria using nitrate as an electron acceptor. This explains emissions after fertilizer use, as excess nitrate fuels the process (choice A). Nitrification produces nitrate but not gases, ammonification releases ammonium, and assimilation stores nitrogen. Wet soils promote anaerobic bacteria for denitrification. N2O's greenhouse gas role highlights environmental concerns.

Question 11

1. Denitrification converting $\text{NO}_3^-$ to $\text{N}_2$ (correct answer)
2. Nitrification converting $\text{NH}_4^+$ to $\text{NO}_3^-$
3. Assimilation converting proteins to $\text{NH}_4^+$
4. Fixation converting $\text{NO}_3^-$ to $\text{N}_2$

Explanation: Denitrification occurs when anaerobic bacteria reduce nitrate (NO₃⁻) to N₂ gas in low-oxygen soils, leading to nitrogen loss from the ecosystem. In compacted, saturated soil, limited oxygen diffusion creates anaerobic conditions that favor denitrifying bacteria like Pseudomonas. These bacteria use NO₃⁻ in respiration, explaining the decrease in nitrate levels. This process is common in waterlogged agricultural fields, reducing available nitrogen for crops. Nitrification, being aerobic, would decrease under these conditions. Thus, choice A accurately indicates denitrification as the increased process.

Question 12

1. Nitrification (correct answer)
2. Ammonification
3. Nitrogen fixation
4. Denitrification

Explanation: The biofilters enhanced nitrification, the two-step aerobic oxidation of ammonium to nitrate performed by chemoautotrophic bacteria. In the well-aerated environment, Nitrosomonas bacteria oxidize NH₄⁺ to NO₂⁻ (nitrite), then Nitrobacter oxidize NO₂⁻ to NO₃⁻ (nitrate). These bacteria derive energy from these oxidation reactions to fix CO₂. The high oxygen availability in the biofilters is crucial because nitrification is strictly aerobic. This process is commonly used in wastewater treatment to convert toxic ammonia to less toxic nitrate, though the nitrate still needs management to prevent eutrophication. Ammonification produces ammonium, nitrogen fixation produces ammonium from N₂, and denitrification would remove nitrate, not produce it.

Question 13

1. Denitrification by anaerobic bacteria converting $\mathrm{NO_3^-}$ to $\mathrm{N_2}$
2. Nitrification by chemoautotrophic bacteria converting $\mathrm{NH_4^+}$ to $\mathrm{NO_3^-}$ (correct answer)
3. Nitrogen fixation by Rhizobium converting $\mathrm{N_2}$ to $\mathrm{NH_4^+}$
4. Assimilation by plants converting $\mathrm{NO_3^-}$ into animal proteins

Explanation: The scenario describes ammonium (NH₄⁺) converting to nitrate (NO₃⁻) in soil without plant uptake, which is characteristic of nitrification. Nitrification is a two-step aerobic process where chemoautotrophic bacteria oxidize ammonia for energy: first Nitrosomonas converts NH₄⁺ to NO₂⁻ (nitrite), then Nitrobacter converts NO₂⁻ to NO₃⁻. These bacteria are autotrophs that use the energy from this oxidation to fix carbon dioxide. The lightning initially created the ammonium through atmospheric nitrogen fixation. Since plants aren't actively growing, assimilation isn't occurring, and the other processes either go in the wrong direction (denitrification removes nitrate) or produce the wrong product (nitrogen fixation produces ammonium, not nitrate).

Question 14

1. Denitrification (correct answer)
2. Nitrogen fixation
3. Nitrification
4. Assimilation

Explanation: Denitrification occurs in low-oxygen sediments, where bacteria use nitrate as an electron acceptor, producing nitrogen gas (N2) in anaerobic respiration. This matches the observations (choice A). Nitrogen fixation requires energy to convert N2 to ammonium, nitrification needs oxygen, and assimilation is uptake by organisms. Estuarine conditions favor denitrifying bacteria. This process contributes to nitrogen loss from aquatic systems.

Question 15

1. Nitrification
2. Denitrification (correct answer)
3. Nitrogen fixation
4. Assimilation

Explanation: Denitrification is depicted by the arrow from nitrate (NO3-) to nitrogen gas (N2), as anaerobic bacteria perform this reduction, releasing N2 to the atmosphere. This process balances nitrogen levels in ecosystems. Nitrification goes from ammonium to nitrate, fixation from N2 to ammonium, and assimilation incorporates nitrate into biomass. Choice B matches the diagram's bacterial process. Understanding these arrows helps visualize nitrogen flows. Denitrifying bacteria use nitrate in low-oxygen environments.

Question 16

1. Denitrification $\rightarrow$ nitrogen fixation $\rightarrow$ assimilation, causing oxygen to rise
2. Assimilation of added $\mathrm{NO_3^-}$ by algae $\rightarrow$ ammonification during decomposition, increasing oxygen
3. Assimilation of added $\mathrm{NO_3^-}$ by algae $\rightarrow$ ammonification by decomposers, increasing biological oxygen demand (correct answer)
4. Nitrogen fixation by algae $\rightarrow$ nitrification to $\mathrm{N_2}$, decreasing decomposition

Explanation: The sequence connecting fertilizer runoff to hypoxia involves assimilation followed by ammonification with increased biological oxygen demand. First, excess nitrate from fertilizer runoff is assimilated by algae, fueling rapid growth and algal blooms. When these algae die, they sink and are decomposed by bacteria through ammonification, breaking down organic nitrogen compounds. This decomposition process requires oxygen, dramatically increasing the biological oxygen demand (BOD) in the water. As decomposers consume available oxygen faster than it can be replenished, dissolved oxygen levels plummet, creating hypoxic (low oxygen) or anoxic (no oxygen) zones. These "dead zones" cannot support most aquatic life. The other options incorrectly describe the sequence or suggest processes that would increase rather than decrease oxygen.

Question 17

1. Ammonification converting organic nitrogen into $\text{NH}_4^+$ (correct answer)
2. Denitrification converting $\text{NO}_3^-$ into $\text{N}_2$
3. Nitrification converting $\text{NO}_3^-$ into $\text{NH}_4^+$
4. Fixation converting $\text{NH}_4^+$ into $\text{N}_2$

Explanation: Ammonification, also known as mineralization, is the process where decomposer microbes break down organic nitrogen compounds in dead matter, such as proteins and nucleic acids, into ammonium (NH₄⁺). In a forest floor with accumulating leaf litter, microbes like bacteria and fungi release enzymes to decompose the organic material, increasing soil NH₄⁺ levels. These decomposers are vital for recycling nitrogen, making it available again for plants and other organisms in the ecosystem. This process occurs as part of the decay cycle, directly explaining the rise in ammonium from litter breakdown. In contrast, denitrification or nitrification transform inorganic forms and do not produce NH₄⁺ from organics, while fixation involves atmospheric N₂. Therefore, choice A properly identifies ammonification as the occurring process.

Question 18

1. Ammonification by plant roots converting organic N to $\text{NH}_4^+$
2. Nitrification by aerobic bacteria converting $\text{NH}_4^+$ to $\text{NO}_3^-$ (correct answer)
3. Nitrogen fixation by cyanobacteria converting $\text{N}_2$ to $\text{NO}_3^-$
4. Denitrification by aerobic bacteria converting $\text{NO}_3^-$ to $\text{N}_2$

Explanation: Nitrification is a two-step process in the nitrogen cycle where ammonium (NH₄⁺) is oxidized first to nitrite (NO₂⁻) by bacteria like Nitrosomonas, and then to nitrate (NO₃⁻) by bacteria like Nitrobacter, requiring aerobic conditions. In well-aerated garden soil, adding ammonium-based fertilizer provides substrate for these aerobic bacteria, leading to a decrease in NH₄⁺ and an increase in NO₃⁻ over time. These microorganisms are essential because they convert nitrogen into a form (nitrate) that is more mobile and often preferred by plants for uptake. The well-aerated environment favors nitrification by supplying the oxygen needed for the bacteria's metabolism. In contrast, processes like denitrification occur under anaerobic conditions and would reduce nitrate levels instead. Therefore, choice B accurately describes nitrification by aerobic bacteria as the responsible process.

Question 19

1. Ammonification by decomposers converting organic nitrogen to $\mathrm{NH_4^+}$
2. Nitrification by aerobic bacteria converting $\mathrm{NH_4^+}$ to $\mathrm{NO_3^-}$
3. Denitrification by anaerobic bacteria converting $\mathrm{NO_3^-}$ to $\mathrm{N_2}$ (correct answer)
4. Nitrogen fixation by cyanobacteria converting $\mathrm{N_2}$ to $\mathrm{NH_4^+}$

Explanation: The anoxic (oxygen-poor) conditions in waterlogged sediments create the perfect environment for denitrification, where anaerobic bacteria use nitrate (NO₃⁻) as an alternative electron acceptor in respiration when oxygen is unavailable. These denitrifying bacteria convert NO₃⁻ through a series of steps (NO₃⁻ → NO₂⁻ → NO → N₂O → N₂), ultimately producing nitrogen gas that escapes to the atmosphere. This process removes bioavailable nitrogen from the ecosystem and is important for reducing excess nitrate from agricultural runoff. The low oxygen conditions are essential because denitrifying bacteria only use this pathway when O₂ is limited. Nitrification requires oxygen, nitrogen fixation produces ammonium not gas, and ammonification produces ammonium from organic matter.

Question 20

1. Ammonification (mineralization) by decomposers converting organic nitrogen to $\mathrm{NH_4^+}$ (correct answer)
2. Assimilation by nitrifying bacteria converting $\mathrm{NH_4^+}$ to $\mathrm{NO_3^-}$
3. Denitrification by plants converting $\mathrm{NO_3^-}$ to $\mathrm{N_2}$
4. Nitrogen fixation by fungi converting $\mathrm{NO_3^-}$ to $\mathrm{NH_4^+}$

Explanation: Ammonification (also called mineralization) is the decomposition process where bacteria and fungi break down organic nitrogen compounds in dead plant and animal matter into inorganic ammonium (NH₄⁺). Decomposers release enzymes that break down proteins into amino acids, then deaminate these amino acids to release ammonia, which becomes protonated to NH₄⁺ in soil. This process is essential for recycling nitrogen from dead organic matter back into forms available for plant uptake. The warm, wet conditions described are ideal for microbial activity and decomposition. Assimilation goes in the opposite direction (inorganic to organic), denitrification produces N₂ gas not ammonium, and nitrogen fixation requires specialized bacteria not typically found in leaf litter decomposition.