Fix atmospheric nitrogen
Nitrogen fixation by lightning and bacteria transfers atmospheric nitrogen into nitrogen compounds that can enter the soil cycle.
Cross-referenced to the 2028–2029 0653 section using original teaching from the related single-science courses. A 0653-specific objective-by-objective and Core/Supplement review is in progress. Written practice is self-checked and does not demonstrate supervised laboratory competence or award an official Cambridge grade.
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 does nitrogen fixation do in the cycle?
What you will learn
- Describe fixation by lightning and bacteria
- Identify nitrogen fixation as a microorganism role
Atmospheric nitrogen must enter compounds
Most atmospheric nitrogen is nitrogen gas, N₂. Plants cannot use this atmospheric gas directly to build amino acids and proteins. Nitrogen fixation changes atmospheric nitrogen into nitrogen-containing compounds; lightning and some bacteria carry out fixation in the syllabus model. Lightning forms nitrogen compounds that can reach soil in rain, while nitrogen-fixing bacteria make compounds from N₂.
Do not claim that all bacteria fix nitrogen or that plants directly absorb atmospheric N₂. The syllabus does not require naming individual bacterial genera, so use generic terms such as nitrogen-fixing bacteria.
Follow fixed nitrogen into the soil route
Fixed nitrogen compounds become available in the soil cycle. Nitrate ions can then be absorbed by plants. The bacterial fixation role and lightning route are distinct ways atmospheric nitrogen enters the cycle.
A diagram may simplify intermediate compounds. Label the starting source as atmospheric N₂ and the process as fixation; do not label the arrow nitrification or denitrification.
Sources for this lesson
Pause and explain
Which two fixation agents are included in the syllabus?
Worked example
A cycle diagram shows atmospheric N₂ being changed by bacteria into nitrogen compounds. Name the process and the bacterial role.
Show the worked solution
- The source is atmospheric nitrogen gas.
- Bacteria change it into nitrogen compounds.
Answer This is nitrogen fixation by nitrogen-fixing bacteria.
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.
A farmer asks how atmospheric nitrogen can enter soil nitrogen compounds. Give both syllabus routes.
Give me a hint
Name lightning and the bacterial route.
Compare my reasoning
- Lightning forms nitrogen compounds that reach soil.
- Nitrogen-fixing bacteria convert atmospheric N₂ into compounds.
Lightning and nitrogen-fixing bacteria can fix atmospheric nitrogen into nitrogen compounds that enter the soil cycle.
Look for these in your work
- Both routes stated
- Atmospheric source included
A plant is growing in nitrate-containing soil. Does that show it absorbed N₂ directly from the air? Explain.
Give me a hint
Distinguish plant nitrate uptake from nitrogen fixation.
Compare my reasoning
- Plants absorb nitrate ions from soil.
- Fixation changes atmospheric nitrogen into compounds.
No. The plant absorbs nitrate ions from soil; lightning or nitrogen-fixing bacteria first transfer atmospheric nitrogen into compounds in the cycle.
Look for these in your work
- Absorption described
- Fixation separated
A learner labels an arrow atmospheric N₂ → nitrate “denitrification.” Correct it.
Give me a hint
Check whether nitrogen is entering or leaving soil compounds.
Compare my reasoning
- Denitrification returns nitrate to nitrogen gas.
- Atmospheric nitrogen entering compounds is fixation.
Relabel it nitrogen fixation, because nitrogen is moving from atmospheric N₂ into nitrogen compounds; denitrification runs in the opposite direction.
Look for these in your work
- Arrow direction checked
- Correct process named
Common mistakes
- Saying plant roots absorb atmospheric N₂ directly
- Claiming every bacterium fixes nitrogen
- Naming a specific bacterial genus as required
What can you explain now?
What does nitrogen fixation do in the cycle?
Compare with the explanation
Transfers atmospheric nitrogen into nitrogen compounds
This makes nitrogen available for later soil and plant routes.
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Make it stick
Define nitrogen fixation and name the two routes in the Cambridge 0610 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.