Nitrogen cycle
Laura Armstrong
Teacher
Contents
Recall Questions
This topic requires prior knowledge of biological molecules. You can test your knowledge on these below.
Where is most nitrogen found?
In the air as nitrogen gas.
What molecules contain nitrogen in living organisms?
Amino acids / proteins, nucleic acids (DNA and RNA) and ATP.
In what form do plants absorb nitrogen?
As nitrate ions (NO₃⁻) from the soil.
Explainer Video
The Nitrogen Cycle
The nitrogen cycle converts nitrogen gas into multiple chemical forms, helping move nitrogen through the atmosphere, soil, and living organisms.
Key Steps in the Nitrogen Cycle
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Nitrogen Fixation
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Nitrification
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Assimilation
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Ammonification
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Denitrification
Nitrogen Fixation
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Conversion of atmospheric nitrogen (N₂) into ammonia (NH₃) or ammonium ions (NH₄⁺).
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Done by:
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Nitrogen-fixing bacteria in root nodules (e.g. Rhizobium).
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Free-living nitrogen-fixing bacteria in soil.
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Nitrogen-fixing bacteria in root nodules live in the root nodules of legumes (e.g. peas, beans and clover).
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They fix atmospheric nitrogen into ammonium compounds that the plant can use in exchange for carbohydrates from the plant.
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They have a mutualistic relationship where both species benefit from their interaction.
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Plants with nitrogen-fixing bacteria in their root nodules are more likely to survive in soil with a low nitrate ion concentration.
Nitrification & Assimilation
Nitrification
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Conversion of ammonium ions (NH₄⁺) to nitrates (NO₃⁻) by nitrifying bacteria.
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Step 1: convert ammonium ions into nitrites.
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Step 2: convert nitrites into nitrates.
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- Requires aerobic conditions (oxygen in the soil) as these reactions are oxidation reactions.
Assimilation
- Nitrate ions (NO₃⁻) are absorbed by plants and used to synthesise organic nitrogen compounds (e.g. amino acids and nucleic acids).
Ammonification & Denitrification
Ammonification
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Decomposition of organic nitrogen-containing compounds (like urea, proteins, nucleic acids) found in dead plants and animals into ammonium ions (NH₄⁺).
- Carried out by saprobionts (decomposer microorganisms).
Denitrification
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Conversion of nitrate ions (NO₃⁻) back into nitrogen gas (N₂).
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Done by anaerobic denitrifying bacteria in waterlogged soils where the concentration of oxygen is low.
- Reduces nitrogen available to plants.
Summary Flow
Nitrogen gas → (Nitrogen fixation) → Ammonium ions → (Nitrification) → Nitrites → Nitrates → (Assimilation by plants) → Organic nitrogen in animals/plants → (Ammonification) → Ammonium ions → (Denitrification) → Nitrogen gas
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Key Terms
- Nitrogen fixation: Conversion of nitrogen gas into ammonia or ammonium ions.
- Ammonification: Breakdown of organic matter such as proteins in dead plants into ammonium ions.
- Nitrification: Conversion of ammonium ions into nitrates via nitrite.
- Denitrification: Reduction of nitrates to nitrogen gas under anaerobic conditions.
- Saprobiont: Microorganism that decomposes organic matter, releasing ammonium.
- Mutualistic bacteria: Nitrogen fixing bacteria found in root nodules that fix nitrogen in return for carbohydrates from the host plant.
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Exam Tip
Always name each step and link it to the type of organism involved (e.g. nitrifying vs denitrifying bacteria)
Remember that nitrification requires oxygen but denitrification occurs in anaerobic conditions – this distinction is often tested!
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Describe how nitrogen-containing compounds are made available to plants and returned to the atmosphere as part of the nitrogen cycle. (5 marks)
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Nitrogen-fixing bacteria convert nitrogen gas into ammonium ions.
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Nitrifying bacteria convert ammonium ions into nitrites and then nitrates.
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Plants absorb nitrates and use them to make amino acids/ proteins. (any nitrogen containing compound in a plant can be named here)
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Saprobionts convert protein (or other nitrogen containing compound) in dead plants into ammonium ions.
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Denitrifying bacteria convert nitrates in the soil into nitrogen gas.
Practice Question
Try to answer the practice question from the TikTok on your own, then watch the video to see how well you did!