BIOGEOCHEMICAL CYCLES

Biogeochemical Cycles

Biogeochemical cycles describe how essential nutrients such as carbon, nitrogen, water, phosphorus, and sulphur circulate through living organisms and the physical environment. These cycles maintain ecological balance and support life.

Types of Biogeochemical Cycles

Gaseous Cycles

• Reservoir in the atmosphere or oceans.
• These are generally perfect cycles.
• Examples: Carbon cycle, Nitrogen cycle, Oxygen cycle.

Sedimentary Cycles

• Reservoir in Earth's crust as sediments.
• These cycles are relatively imperfect because nutrients often get locked in sediments.
• Examples: Phosphorus cycle, Sulphur cycle, Calcium cycle.

Gaseous Cycles

Carbon Cycle

The atmosphere and oceans are major carbon reservoirs. Carbon moves through the ecosystem via:

• Photosynthesis – Plants absorb CO₂ to make food and release oxygen.
• Respiration – All living organisms release CO₂ by breaking down food.
• Decomposition – Microorganisms break down dead matter and release CO₂.
• Combustion – Burning fossil fuels, biomass and wood emits CO₂.
• Long-term storage – Carbon becomes trapped in sediments or ocean carbonates.

Human Impact: Industrialization, deforestation and fossil fuel use have increased CO₂ levels, leading to global warming.

Nitrogen Cycle

Atmospheric nitrogen (78%) cannot be used directly by most organisms. It must be converted to usable forms.

Nitrogen Fixation

• Atmospheric fixation (lightning)
• Industrial fixation (Haber process)
• Bacterial fixation (Rhizobium, Azotobacter, Nostoc)

Nitrification

• Nitrosomonas / Nitrococcus convert ammonia → nitrite
• Nitrobacter converts nitrite → nitrate

Assimilation

• Plants absorb nitrates to make proteins and nucleic acids.
• Animals get nitrogen by eating plants.

Ammonification

• Decomposers break down dead organisms into ammonia.

Denitrification

• Anaerobic bacteria convert nitrates back into nitrogen gas (N₂).

Water Cycle (Hydrological Cycle)

Water circulates through:

• Evaporation
• Transpiration
• Condensation
• Precipitation
• Runoff and infiltration

Solar energy and gravity drive the water cycle.

Sedimentary Cycles

Phosphorus Cycle

Phosphorus is stored in rocks as phosphates. Weathering releases phosphates into soil and water. Plants absorb them, animals consume them, and decomposers return phosphorus to the soil. Some phosphorus flows into oceans and becomes locked in sediments. Geological uplift exposes these rocks again.

Human Impacts: Fertilizer runoff and deforestation increase phosphate pollution and cause eutrophication.

Sulphur Cycle

Sulphur is found in rocks, ocean sediments and fossil fuels. Natural sources include volcanic eruptions, sea spray, decomposition and bacterial activity. Human activities release sulphur dioxide (SO₂) and hydrogen sulphide (H₂S).

In the atmosphere, these gases form sulphuric acid, causing acid rain. Sulphur returns to soil and is absorbed by plants, completing the cycle.

These biogeochemical cycles ensure the continuous supply of essential nutrients and maintain ecological stability.

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Subject: Environment

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