Nutrient Cycles and Succession

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.

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The Carbon Cycle

Carbon is the backbone of every organic molecule. It cycles between the atmosphere, oceans, soil and living organisms through four main processes.

The Processes

1. Photosynthesis — plants, algae and cyanobacteria fix CO₂ into organic molecules, using light energy: $6\text{CO}_2 + 6\text{H}_2\text{O} \rightarrow \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2$6CO2​+6H2​O→C6​H12​O6​+6O2​
2. Respiration — all living organisms release CO₂ back to the atmosphere by breaking down organic molecules.
3. Decomposition — bacteria and fungi break down dead material, releasing CO₂.
4. Combustion — burning of wood, coal, oil and gas releases CO₂ from long-term stores.

Long-Term Carbon Stores

Carbon is stored on geological timescales in:

• Fossil fuels (coal, oil, natural gas).
• Carbonate rocks (limestone, chalk) formed from marine organisms.
• Deep ocean dissolved CO₂ and bicarbonate ions.
• Peat and permafrost.

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.

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The Nitrogen Cycle

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

1. Nitrogen Fixation

Converts N₂ to ammonia (NH₃) or ammonium (NH₄⁺). Carried out by:

• Rhizobium — lives symbiotically in root nodules of legumes (peas, beans, clover, alfalfa). It fixes atmospheric nitrogen using the enzyme nitrogenase (which is destroyed by oxygen — hence the pink pigment leghaemoglobin in root nodules, which binds free O₂).
• Azotobacter — free-living soil bacteria.
• Cyanobacteria — in water and soil; major nitrogen source in paddy rice fields.
• Lightning — converts a small but significant amount of N₂ to nitrate.
• Industrial fixation (Haber process) — man-made production of ammonia fertilisers; now rivals biological fixation in scale.

2. Ammonification (Putrefaction)

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⁺).

3. Nitrification

Ammonium ions are oxidised to nitrate by chemoautotrophic bacteria that use the released energy to fix carbon:

• Nitrosomonas — oxidises ammonium (NH₄⁺) to nitrite (NO₂⁻).
• Nitrobacter — oxidises nitrite (NO₂⁻) to nitrate (NO₃⁻).

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.

4. Denitrification

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.

Nitrogen Cycle Summary Table

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.

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Fertilisers and Nitrogen

Modern intensive agriculture depends on nitrogen fertilisers (ammonium nitrate, urea). Overuse causes: