What you'll learn
The nitrogen cycle describes how nitrogen atoms move between the atmosphere, soil, living organisms and back again through interconnected processes. This topic appears regularly in CIE IGCSE Biology Paper 2 questions worth 4-6 marks, testing your understanding of bacterial roles, named processes and the cycle's importance to agriculture and ecosystems.
Key terms and definitions
Nitrogen fixation — the conversion of unreactive nitrogen gas (N₂) from the atmosphere into nitrogen-containing compounds that plants can absorb, carried out by nitrogen-fixing bacteria.
Nitrification — the conversion of ammonium compounds in the soil into nitrites and then nitrates by nitrifying bacteria, making nitrogen available to plants.
Denitrification — the conversion of nitrates in the soil back into nitrogen gas (N₂) by denitrifying bacteria, returning nitrogen to the atmosphere.
Decomposition — the breakdown of dead organic matter and waste products by decomposers (bacteria and fungi), releasing ammonium compounds into the soil.
Nitrogen-fixing bacteria — bacteria that convert atmospheric nitrogen into ammonia or ammonium compounds; includes free-living soil bacteria (e.g., Azotobacter) and mutualistic bacteria in root nodules (e.g., Rhizobium).
Root nodules — swellings on the roots of leguminous plants (beans, peas, clover) that house nitrogen-fixing bacteria in a mutualistic relationship.
Nitrifying bacteria — soil bacteria that oxidise ammonium compounds first to nitrites (by Nitrosomonas) then to nitrates (by Nitrobacter) in aerobic conditions.
Haber process — an industrial method of fixing atmospheric nitrogen to produce ammonia for artificial fertilisers, requiring high temperature and pressure.
Core concepts
Why nitrogen matters to living organisms
Nitrogen forms an essential component of biological molecules:
- Proteins — all amino acids contain nitrogen in their amino groups (-NH₂)
- DNA and RNA — nitrogenous bases (adenine, thymine, cytosine, guanine, uracil) form the genetic code
- ATP — the energy currency molecule contains nitrogen in its adenine base
- Chlorophyll — the photosynthetic pigment contains nitrogen atoms in its structure
Despite nitrogen gas (N₂) making up 78% of the atmosphere, most organisms cannot use it directly. The strong triple covalent bond between the two nitrogen atoms makes N₂ extremely unreactive. Plants can only absorb nitrogen in the form of nitrate ions (NO₃⁻) dissolved in soil water, which they take up through their roots by active transport. Animals obtain nitrogen by consuming plants or other animals and digesting their proteins.
Nitrogen fixation: getting nitrogen into ecosystems
Three processes convert atmospheric nitrogen into usable forms:
1. Biological nitrogen fixation (most important for CIE IGCSE)
Nitrogen-fixing bacteria possess the enzyme nitrogenase that breaks the N≡N triple bond:
- Free-living soil bacteria such as Azotobacter and Clostridium fix nitrogen independently in the soil, converting N₂ → NH₃ (ammonia)
- Mutualistic bacteria (Rhizobium species) live inside root nodules of leguminous plants (peas, beans, clover, soybeans)
The mutualistic relationship benefits both organisms:
- Bacteria receive carbohydrates (glucose) from the plant's photosynthesis
- Plant receives amino acids made from fixed nitrogen by the bacteria
- The plant can grow in nitrogen-poor soils without competing with other species
Root nodules appear as pink/red swellings because they contain leghaemoglobin, a protein similar to haemoglobin that maintains low oxygen levels (nitrogenase enzyme is deactivated by oxygen).
2. Lightning fixation
High-energy lightning provides enough energy to break N≡N bonds. Nitrogen reacts with oxygen in the atmosphere to form nitrogen oxides (NOₓ), which dissolve in rainwater and reach the soil as dilute nitric acid. This contributes a small percentage of fixed nitrogen.
3. Industrial fixation (Haber process)
The Haber process produces ammonia (NH₃) for artificial fertilisers:
- N₂ + 3H₂ → 2NH₃
- Requires high temperature (450°C) and high pressure (200 atmospheres)
- Uses an iron catalyst
- Provides nitrogen fertilisers for agriculture but requires significant energy input
From organic nitrogen to nitrates: decomposition and nitrification
Decomposition
When organisms die or produce waste (urine, faeces), decomposer bacteria and fungi break down complex nitrogen-containing compounds:
- Proteins in dead organisms → amino acids
- Urea in urine → ammonia
- Amino acids are deaminated → ammonia (NH₃)
- Ammonia dissolves in soil water → ammonium ions (NH₄⁺)
Saprobionts (decomposers) secrete enzymes externally to digest organic matter, then absorb the soluble products. This process releases ammonium compounds into the soil.
Nitrification (two-stage process)
Nitrifying bacteria convert ammonium compounds into nitrates in aerobic conditions (they require oxygen):
Stage 1: Nitrosomonas bacteria oxidise ammonium ions to nitrites
- NH₄⁺ + O₂ → NO₂⁻ + H₂O + energy
Stage 2: Nitrobacter bacteria oxidise nitrites to nitrates
- NO₂⁻ + O₂ → NO₃⁻ + energy
Both bacterial groups are chemoautotrophs — they obtain energy from oxidising inorganic compounds rather than from light or organic food. The energy released allows them to synthesise their own organic compounds.
Nitrates (NO₃⁻) dissolve readily in soil water and are absorbed by plant roots through active transport. Plants incorporate nitrates into amino acids, then proteins, then DNA. When animals eat plants, nitrogen passes along food chains.
Denitrification: returning nitrogen to the atmosphere
Denitrifying bacteria live in waterlogged, anaerobic (oxygen-poor) soil conditions. They respire anaerobically, using nitrates as an alternative to oxygen:
- Nitrates (NO₃⁻) → nitrogen gas (N₂)
- Bacteria such as Pseudomonas and Thiobacillus reduce nitrates to gain energy
- Nitrogen gas diffuses out of the soil back into the atmosphere
Denitrification represents a loss of soil fertility. Farmers can reduce denitrification by:
- Improving drainage to increase oxygen levels in soil
- Avoiding overwatering and waterlogging
- Ploughing to aerate soil
Agricultural implications
Maintaining soil nitrogen levels:
Crop farming removes nitrogen from soil when harvested plants are taken away rather than allowed to decompose naturally. Farmers replenish nitrogen by:
- Crop rotation — planting legumes (beans, peas, clover) every few years to restore soil nitrogen through their root nodule bacteria
- Artificial fertilisers — applying ammonium nitrate or urea directly to soil
- Organic fertilisers — adding manure or compost that decomposers break down into ammonium compounds
Environmental concerns:
Excess fertiliser application causes problems:
- Leaching — nitrates dissolve in rainwater and drain into rivers and lakes
- Eutrophication — excess nitrates cause algal blooms that block light, killing aquatic plants; decomposing algae use up oxygen, suffocating fish
Worked examples
Example 1: Diagram interpretation (4 marks)
The diagram shows the nitrogen cycle. Identify the processes occurring at stages A, B, C and D.
[Diagram shows: Atmospheric N₂ → (A) → NH₃ in soil → (B) → NO₃⁻ → plant uptake → proteins in plants/animals → death/waste → (C) → NH₄⁺ compounds → back to B; also NO₃⁻ → (D) → N₂]
Model answer:
- A: nitrogen fixation [1]
- B: nitrification [1]
- C: decomposition [1]
- D: denitrification [1]
Examiner guidance: Use the precise process names. "Breaking down" instead of "decomposition" would likely lose the mark.
Example 2: Bacterial roles (5 marks)
Describe the role of bacteria in making atmospheric nitrogen available to plants. [5]
Model answer:
- Nitrogen-fixing bacteria convert atmospheric nitrogen/N₂ into ammonia/ammonium compounds [1]
- Examples include Rhizobium in root nodules or Azotobacter in soil [1]
- Nitrifying bacteria convert ammonium compounds into nitrates [1]
- Nitrosomonas converts ammonium to nitrite; Nitrobacter converts nitrite to nitrate [1]
- Plants absorb nitrates through roots (by active transport) [1]
Examiner guidance: Five marks requires five distinct points. Name specific bacteria and processes for full credit.
Example 3: Application question (6 marks)
A farmer notices poor crop growth in a waterlogged field. Explain why waterlogged soil reduces plant growth in terms of the nitrogen cycle. [6]
Model answer:
- Waterlogged soil lacks oxygen/is anaerobic [1]
- Nitrifying bacteria require aerobic conditions [1]
- Therefore less/no nitrification occurs, so fewer nitrates produced [1]
- Denitrifying bacteria are active in anaerobic conditions [1]
- They convert nitrates to nitrogen gas [1]
- Less nitrate available for plant uptake, so plants cannot make sufficient proteins/amino acids/DNA for growth [1]
Examiner guidance: This question requires explanation ("why"), not just description. Link each process to the consequence for the plants.
Common mistakes and how to avoid them
• Mistake: Stating "plants absorb nitrogen gas from the air through leaves." Correction: Plants can only absorb nitrogen as nitrate ions (NO₃⁻) through their roots from soil water. They cannot use atmospheric N₂ directly.
• Mistake: Confusing nitrification with nitrogen fixation, or using these terms interchangeably. Correction: Nitrogen fixation converts N₂ → NH₃ (from atmosphere to ammonia). Nitrification converts NH₄⁺ → NO₂⁻ → NO₃⁻ (ammonium to nitrate). These are completely different processes with different bacteria.
• Mistake: Writing "bacteria break down proteins into nitrates" without showing intermediate steps. Correction: The sequence is: proteins → amino acids → ammonia → ammonium ions (decomposition), then ammonium → nitrite → nitrate (nitrification). These are separate processes.
• Mistake: Claiming denitrification occurs in well-aerated soil. Correction: Denitrification requires anaerobic (waterlogged) conditions. In well-aerated soil, nitrification (not denitrification) occurs.
• Mistake: Forgetting to name specific bacteria in exam answers when the mark scheme requires them. Correction: Learn key examples: Rhizobium (nitrogen fixation in root nodules), Azotobacter (free-living nitrogen fixation), Nitrosomonas (ammonium to nitrite), Nitrobacter (nitrite to nitrate), Pseudomonas (denitrification).
• Mistake: Stating "lightning produces nitrates directly." Correction: Lightning produces nitrogen oxides (NOₓ) which dissolve in rain to form dilute nitric acid, eventually reaching soil as nitrates. Show the sequence, not just the start and end points.
Exam technique for Nitrogen cycle
• Command word "Describe" — state the processes and what happens, but explanation of why is not required. For nitrogen cycle questions, name the bacterial types and the conversions (e.g., "Nitrosomonas bacteria convert ammonium to nitrite"). Typically worth 3-4 marks.
• Command word "Explain" — provide reasons and consequences. Link bacterial activity to conditions (aerobic/anaerobic) and explain effects on plant growth or soil fertility. These questions often carry 5-6 marks and require cause-and-effect chains.
• Diagram completion questions — be precise with process names on arrows. Write "nitrogen fixation" not just "fixation"; "nitrification" not "nitrate production." One mark per correct term means spelling accuracy matters.
• Marks-per-point pattern — most nitrogen cycle questions award 1 mark per relevant biological point. A 6-mark question requires six distinct facts, not three facts explained in detail. Count your points before finishing.
Quick revision summary
Atmospheric nitrogen (N₂) enters ecosystems through nitrogen-fixing bacteria (Rhizobium in root nodules; Azotobacter in soil) that convert it to ammonia. Decomposers break down proteins from dead organisms and waste into ammonium compounds. Nitrifying bacteria (Nitrosomonas, then Nitrobacter) convert ammonium to nitrites then nitrates in aerobic conditions. Plants absorb nitrates through roots to make proteins. Animals obtain nitrogen by eating plants or other animals. Denitrifying bacteria in waterlogged soil convert nitrates back to N₂, completing the cycle. Farmers maintain soil nitrogen through crop rotation with legumes or adding fertilisers.