Return nitrate to atmospheric nitrogen
Denitrifying microorganisms convert nitrate ions into nitrogen gas, returning nitrogen to the atmosphere.
Section 19.3 · Nutrient cycles
Extended only (Supplement); Biology 0610, 2026–2028. Use the stated specimen features and evidence. Diagrams are schematic. Written practice is self-checked
Before you begin
- Use the food-web pathway and distinguish matter cycling from energy flow.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What is the direction of denitrification?
What you will learn
- Describe denitrification direction and product
- Identify denitrifying microorganisms as a nitrogen-cycle role
Denitrification closes the soil-to-air route
Denitrifying microorganisms convert nitrate ions in soil into nitrogen gas, N₂, which returns to the atmosphere. This is the denitrification step. It moves nitrogen out of the plant-available soil nitrate pool and back into atmospheric nitrogen.
Conditions with little oxygen can favour denitrification, but the key syllabus outcome is nitrate to nitrogen gas. Do not reverse the arrow or confuse it with nitrification, which changes ammonium compounds into nitrate.
Name microbial roles without unnecessary detail
The four microorganism roles in the syllabus are decomposition, nitrification, nitrogen fixation and denitrification. A process diagram can label the generic microorganism group by its role; individual bacterial names are not required.
Use the chemical starting material to choose the role: protein to ammonium is decomposition; ammonium to nitrate is nitrification; atmospheric N₂ into compounds is fixation; nitrate to N₂ is denitrification.
Pause and explain
Which microbial role returns nitrogen to the atmosphere?
Worked example
Nitrate ions in soil are converted by microorganisms to nitrogen gas. Name the process and destination.
Show the worked solution
- Nitrate is the starting compound.
- Nitrogen gas is the product.
Answer This is denitrification; nitrogen returns to the atmosphere as N₂.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Complete: soil nitrate ions → ____ by _____.
Give me a hint
State the gas and process.
Compare my reasoning
- Nitrate is converted to nitrogen gas.
- This is denitrification.
Soil nitrate ions → atmospheric nitrogen gas (N₂) by denitrification.
Look for these in your work
- Product correct
- Process correct
A diagram shows N₂ in air becoming nitrate by fixation and nitrate returning to N₂. What process labels the return arrow?
Give me a hint
Identify the direction from soil nitrate to air.
Compare my reasoning
- Nitrate → N₂ is denitrification.
- Fixation moves atmospheric nitrogen into compounds.
Label the return arrow denitrification; fixation is the opposite atmospheric-entry route.
Look for these in your work
- Direction correct
- Fixation contrasted
A plant has nitrate available before denitrification. Explain how denitrification can affect this soil nitrogen pool.
Give me a hint
State the chemical conversion, not an unsupported population result.
Compare my reasoning
- Microorganisms convert nitrate into nitrogen gas.
- Nitrogen gas returns to the atmosphere.
Denitrification converts nitrate ions into atmospheric nitrogen gas, reducing the nitrate in the soil pool available for plant uptake.
Look for these in your work
- Chemical change stated
- Soil availability linked
Common mistakes
- Drawing denitrification as nitrate formation
- Calling it nitrogen fixation
- Claiming plants absorb N₂ made by denitrification directly
What can you explain now?
What is the direction of denitrification?
Compare with the explanation
Nitrate ions → atmospheric nitrogen gas
The nitrogen returns from soil to atmosphere.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Describe the denitrification arrow and list the four microorganism roles named in the syllabus.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.