Nitrogen Cycle Quiz
Questions: 16 · 10 minutes
1. How can lightning add biologically available nitrogen to ecosystems?
It causes plants to release proteins directly into the atmosphere.
It replaces microbial decomposition by turning dead tissue into ammonium.
It converts soil nitrate exclusively into atmospheric nitrogen gas.
It helps form reactive nitrogen compounds that can be deposited in rain.
2. A chemical inhibits nitrifying microorganisms in a well-aerated soil. Which immediate change is most likely?
Ammonium will accumulate because its oxidation has slowed.
Nitrate will be converted directly into plant proteins without uptake.
Nitrogen gas will be fixed more rapidly by every plant species.
Organic nitrogen will stop entering the soil in dead material.
3. What transformation occurs during biological nitrogen fixation?
Nitrate is converted into nitrogen gas and returned to the atmosphere.
Atmospheric nitrogen gas is converted into ammonia or related ammonium compounds.
Organic nitrogen in dead material is converted into ammonium.
Ammonium is oxidized first to nitrite and then to nitrate.
4. Which sequence correctly describes the main chemical changes during nitrification?
Ammonium → nitrite → nitrate
Nitrate → nitrite → ammonium
Nitrogen gas → ammonium → organic nitrogen
Organic nitrogen → nitrate → nitrogen gas
5. Plant residues rich in organic nitrogen are mixed into moist, aerated soil. Which pathway is most likely as decomposition proceeds?
Organic nitrogen → nitrogen gas → plant protein
Organic nitrogen → nitrate → dead plant material, with no microbial steps
Organic nitrogen → nitrite → atmospheric nitrogen through assimilation
Organic nitrogen → ammonium through ammonification, followed by possible nitrification
6. Which process commonly converts nitrate into nitrogen gases under oxygen-poor conditions?
Assimilation
Ammonification
Nitrification
Denitrification
7. When decomposers convert organic nitrogen in dead organisms into ammonium, what process is occurring?
Ammonification
Denitrification
Nitrogen fixation
Assimilation
8. Heavy rain washes excess nitrate fertilizer from farmland into a lake. What is the most likely ecological consequence?
Aquatic plants will lose the ability to assimilate all forms of nitrogen.
Nitrogen fixation will cease permanently throughout the lake.
The nitrate will immediately become harmless atmospheric nitrogen without microbial activity.
Nutrient enrichment may stimulate algal growth and contribute to oxygen depletion.
9. In wastewater treatment, water containing nitrate enters an oxygen-poor tank supplied with an energy source for microbes. Which outcome is intended?
Denitrifying microbes convert nitrate into nitrogen gases that leave the water.
Plants assimilate all nitrate into roots growing inside the tank.
Microbes convert nitrate into ammonium through standard nitrification.
Lightning reactions fix additional atmospheric nitrogen into the water.
10. What happens during nitrogen assimilation by a plant?
The plant releases nitrogen gas through its leaves.
The plant converts organic nitrogen directly into atmospheric nitrogen.
The plant takes up inorganic nitrogen and incorporates it into organic molecules.
The plant changes nitrate into nitrite and releases both into the soil.
11. A rabbit builds proteins using nitrogen obtained from grass. How did that nitrogen enter the rabbit most directly?
The rabbit fixed nitrogen gas from the air in its lungs.
The rabbit absorbed nitrate directly through its skin.
The rabbit consumed organic nitrogen compounds in its food.
Denitrifying bacteria transferred nitrogen gas into its muscles.
12. Researchers add ammonium to warm, oxygen-rich soil and later detect increased nitrate. Which process most directly caused the increase?
Assimilation
Denitrification
Nitrification
Nitrogen fixation
13. Why is atmospheric nitrogen gas not directly usable by most plants, despite being abundant in the air?
It cannot enter ecosystems until animals release it during respiration.
Its strong chemical bond must first be broken through fixation into reactive forms.
It is usable only after plants convert nitrate into nitrogen gas.
Its molecules are too large to occur in soil or water.
14. Why can legumes such as peas often improve nitrogen availability when associated with the appropriate root-nodule bacteria?
Their roots perform denitrification and release nitrate into the soil.
The bacteria fix atmospheric nitrogen into forms that can enter biological pathways.
The bacteria convert plant proteins into atmospheric nitrogen gas.
Their leaves absorb nitrate directly from atmospheric nitrogen gas.
15. A waterlogged soil has little oxygen. Its nitrate concentration falls while nitrogen gases are released. Which process best explains these observations?
Nitrogen fixation by root-associated microbes
Denitrification by anaerobic microorganisms
Assimilation by rapidly growing plants
Nitrification by aerobic microorganisms
16. A farmer repeatedly harvests crops and removes all plant material from a field. Without fertilizer or nitrogen-fixing inputs, what is the likely long-term effect?
All soil nitrogen is immediately converted into atmospheric nitrogen.
The field gains unlimited nitrate because harvesting accelerates fixation.
The field loses nitrogen because harvested biomass exports organic nitrogen.
Nitrogen remains unchanged because plants do not contain organic nitrogen.