Biogeochemical (Nutrient) Cycles

• Movement of nutrients between biotic (living) and abiotic (air, water, soil) components.
• Nutrients are recycled, unlike energy, which is lost as heat.
• Major elements: C, N, O, H, P → ~97% of living matter.

Types

Carbon Cycle (Gaseous Cycle)

• Carbon is present mainly as CO₂ in the atmosphere.
• Essential for photosynthesis → carbohydrates.
• Short-term cycle: Photosynthesis ↔ Respiration ↔ Decomposition.
• Long-term cycle: Fossil fuels, carbonate rocks, deep-sea sediments.
• Human role: Burning fossil fuels releases stored CO₂.

Adds CO₂: Respiration, decay, volcanic action
Removes CO₂: Photosynthesis

Nitrogen Cycle (Gaseous Cycle)

• Nitrogen is essential for proteins, amino acids, DNA, and chlorophyll.
• Atmospheric N₂ cannot be directly used by plants.

Main Steps

1. Nitrogen Fixation:
   N₂ → NH₃ / NH₄⁺ (by bacteria, lightning, industries)
2. Nitrification:
   NH₃ → NO₂⁻ (Nitrosomonas) → NO₃⁻ (Nitrobacter)
3. Ammonification:
   Organic waste → NH₃
4. Denitrification:
   NO₃⁻ → N₂ (Pseudomonas)

Excess fertilisers → Eutrophication, acid rain, groundwater pollution.

Phosphorus Cycle (Sedimentary Cycle)

• No gaseous phase (key difference from C & N cycles).
• Reservoir: Phosphate rocks (earth’s crust).
• Released by weathering → rivers → oceans.
• A major cause of eutrophication in freshwater bodies.
• Returns to land through geological uplift (very slow).

Sulphur Cycle (Sedimentary Cycle)

• Reservoir: Soil, rocks, fossil fuels.
• Exists as sulphates, sulphides, and organic sulphur.
• Has a minor gaseous phase (H₂S, SO₂).
• Sources: Volcanic eruptions, fossil fuel combustion, decomposition.
• SO₂ + rainwater → acid rain (H₂SO₄).
• Plants absorb sulphur as sulphates → proteins.

Key Pointers

• Gaseous cycles: Carbon, Nitrogen
• Sedimentary cycles: Phosphorus, Sulphur
• Main eutrophication nutrient: Phosphorus
• Acid rain contributors: Nitrogen oxides & Sulphur dioxide
• Fastest cycle: Gaseous
• Slowest cycle: Phosphorus