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Ecosystems recycle chemical elements through biogeochemical cycles, and develop over time through the process of succession. OCR A-Level Biology A specification 6.3.1 (d)–(e) requires you to describe the carbon and nitrogen cycles in detail, name the microorganisms involved, and explain how communities change through primary and secondary succession to a climax community.
Key Definitions:
- Nutrient cycle — the circulation of an element between living organisms and the abiotic environment.
- Decomposer — a heterotroph (usually bacterium or fungus) that breaks down dead organic matter.
- Fixation — conversion of an element from an unusable form (e.g. N₂) into a biologically useful form (e.g. NH₃).
- Succession — the change in community structure over time as one community replaces another.
- Pioneer species — the first species to colonise a new area.
- Climax community — the stable, self-perpetuating community at the end of a successional sequence.
Carbon is the backbone of every organic molecule. It cycles between the atmosphere, oceans, soil and living organisms through four main processes.
Carbon is stored on geological timescales in:
Burning fossil fuels transfers carbon from these long-term stores to the atmosphere at a rate that natural processes cannot offset, causing the rise in atmospheric CO₂ from around 280 ppm (pre-industrial) to over 420 ppm today. This is the primary driver of anthropogenic climate change.
Although nitrogen gas (N₂) makes up 78% of the atmosphere, most organisms cannot use it directly because the triple bond in N₂ is extraordinarily strong. Four groups of microorganisms transform nitrogen between forms plants and animals can use.
flowchart TD
A[N2 atmospheric nitrogen] -->|Nitrogen fixation Rhizobium Azotobacter| B[NH3 ammonia]
C[Dead plants animals urine faeces] -->|Ammonification saprotrophs| B
B -->|Nitrification Nitrosomonas| D[NO2- nitrite]
D -->|Nitrification Nitrobacter| E[NO3- nitrate]
E -->|Plant uptake| F[Plant proteins]
F -->|Feeding| G[Animal proteins]
G --> C
F --> C
E -->|Denitrification Pseudomonas anaerobic| A
Converts N₂ to ammonia (NH₃) or ammonium (NH₄⁺). Carried out by:
Decomposer bacteria and fungi (saprotrophs) break down proteins and other nitrogen-containing molecules in dead organisms, faeces and urine, releasing ammonia into the soil. Ammonia dissolves as ammonium ions (NH₄⁺).
Ammonium ions are oxidised to nitrate by chemoautotrophic bacteria that use the released energy to fix carbon:
Both require oxygen, so nitrification only happens in well-aerated soils. Plants absorb nitrate through their roots (active transport) and use it to make amino acids, nucleotides and chlorophyll.
Under anaerobic conditions (waterlogged soils, landfill, anaerobic sediments), Pseudomonas and similar bacteria reduce nitrate back to N₂ gas, which escapes to the atmosphere. This is a loss from the ecosystem's usable nitrogen pool — bad for farmers, because fertiliser is wasted.
Waterlogging encourages denitrification and discourages nitrification, which is why farmers drain fields.
| Process | Microorganism | Input | Output | Aerobic/anaerobic |
|---|---|---|---|---|
| Nitrogen fixation | Rhizobium, Azotobacter, cyanobacteria | N₂ | NH₃ / NH₄⁺ | Aerobic (in nodules) |
| Ammonification | Saprotrophs | Proteins / urea | NH₃ / NH₄⁺ | Aerobic |
| Nitrification step 1 | Nitrosomonas | NH₄⁺ | NO₂⁻ | Aerobic |
| Nitrification step 2 | Nitrobacter | NO₂⁻ | NO₃⁻ | Aerobic |
| Denitrification | Pseudomonas | NO₃⁻ | N₂ | Anaerobic |
Exam Tip: OCR loves naming. Learn the four genus names (Rhizobium, Nitrosomonas, Nitrobacter, Pseudomonas) and what each does. Most marks in nitrogen-cycle questions go to students who give the right genus for the right process.
Modern intensive agriculture depends on nitrogen fertilisers (ammonium nitrate, urea). Overuse causes:
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