Carbon Cycle PPT

Introduction

Carbon is a fundamental element of life on Earth. It is the backbone of macromolecules such as carbohydrates, proteins, lipids and nucleic acids. The carbon cycle is the continuous movement of carbon through the atmosphere, biosphere, hydrosphere, lithosphere and soils. This chapter explains the reservoirs of carbon, all major processes (both biological and geological), short- and long-term pathways, and how human activities alter the natural cycle.

Carbon Reservoirs (Carbon Pools)

Carbon is stored in several major reservoirs that exchange carbon with each other:

1. Atmosphere — Carbon exists mainly as carbon dioxide (CO₂) and methane (CH₄).
2. Biosphere — Living organisms (plants, animals, microbes) and dead organic matter.
3. Hydrosphere — Oceans, lakes and rivers store dissolved CO₂, bicarbonate (HCO₃⁻) and carbonate (CO₃²⁻).
4. Lithosphere — Sedimentary rocks (limestone, dolomite), fossil fuels (coal, oil, natural gas) and organic-rich sediments represent the largest long-term carbon stores.
5. Soils — Organic carbon in soil humus and detritus.

Major Processes of the Carbon Cycle (Stepwise)

1. Photosynthesis — Entry of Carbon into the Biosphere

Photosynthesis is the primary process which removes CO₂ from the atmosphere and fixes it into organic molecules. Green plants, algae and cyanobacteria use light energy captured by chlorophyll to convert carbon dioxide and water into glucose and other carbohydrates, releasing oxygen as a by-product. The general balanced equation is:

6CO₂ + 6H₂O —(light, chlorophyll)—> C₆H₁₂O₆ + 6O₂

Carbon fixed in plant tissues (cellulose, starch, lipids) becomes the base of food chains. Net primary production (NPP) is the amount of carbon assimilated by plants that remains after plant respiration; it determines the carbon available to herbivores and decomposers.

2. Respiration — Return of Carbon to Atmosphere

Cellular respiration is the oxidation of organic molecules to release energy (ATP) for cellular processes. All organisms respire — plants (in the dark and for maintenance), animals and microbes. The simplified reaction for aerobic respiration:

C₆H₁₂O₆ + 6O₂ —> 6CO₂ + 6H₂O + Energy (ATP)

Respiration returns CO₂ to the atmosphere and closes the short-term loop between producers and consumers. During periods of high respiration (e.g., decomposition bursts, wildfires) CO₂ flux to the atmosphere increases.

3. Decomposition — Recycling Organic Carbon

Decomposers (bacteria, fungi, detritivores) break down dead organic matter. In aerobic conditions, decomposition produces CO₂. In anaerobic environments (waterlogged soils, sediments, rice paddies, wetlands), decomposition often produces CH₄ (methane) and CO₂.

A portion of organic matter resists decomposition and accumulates as humus in soils, representing a stable soil carbon fraction that improves fertility and acts as a medium-term carbon sink (years to centuries).

Important: The balance between primary production and decomposition determines whether an ecosystem is a carbon sink or source.

4. Combustion — Rapid Oxidation of Stored Carbon

Combustion is the rapid chemical oxidation of organic matter producing CO₂, water and energy. Natural wildfires and controlled burning release carbon previously fixed in vegetation; however, the burning of fossil fuels (coal, oil, natural gas) releases carbon that was sequestered for millions of years and is a major anthropogenic source of atmospheric CO₂.

Example: Burning coal (predominantly carbon) in power plants converts solid carbon to CO₂ and releases large quantities of carbon dioxide in a short time.

5. Ocean–Atmosphere Exchange

Oceans absorb CO₂ directly from the atmosphere by diffusion at the air–sea interface. Dissolved CO₂ reacts with water to form carbonic acid (H₂CO₃), which dissociates into bicarbonate (HCO₃⁻) and carbonate (CO₃²⁻) ions:

CO₂(aq) + H₂O <—> H₂CO₃ <—> H⁺ + HCO₃⁻ <—> 2H⁺ + CO₃²⁻

Marine organisms (plankton, molluscs, corals) use carbonate ions together with calcium to form calcium carbonate (CaCO₃) shells and skeletons. When these organisms die, their calcareous remains contribute to sediments. Ocean uptake of CO₂ is temperature-dependent: colder waters hold more CO₂; warming reduces oceanic carbon uptake and can cause outgassing.

Note: Increased dissolved CO₂ leads to ocean acidification, lowering pH and affecting shell-forming organisms.

6. Sedimentation and Rock Formation — Long-Term Carbon Storage

Over geological timescales, the remains of marine organisms (shells, skeletons) and carbonate sediments lithify into sedimentary rocks such as limestone and chalk. Organic-rich sediments under high pressure and temperature become fossil fuels (coal, oil, natural gas). These processes remove carbon from the fast carbon cycle and store it in the lithosphere for millions of years.

These long-term stores are the source of fossil fuels; when extracted and burned, they reintroduce ancient carbon into the atmosphere.

7. Volcanic Emissions and Rock Weathering

Carbon stored in the lithosphere returns to the atmosphere through volcanic eruptions and metamorphism (release of CO₂) as rocks are subjected to heat and pressure. Weathering of carbonate rocks by weakly acidic rainwater dissolves carbonates and transports dissolved bicarbonates to the oceans:

CaCO₃ + CO₂ + H₂O —> Ca(HCO₃)₂

Weathering is a slow but important natural process that balances carbon fluxes over geological timescales.

8. Human Influence (Anthropogenic Disturbances)

Since the Industrial Revolution, human activities have strongly altered the carbon cycle:

• Fossil fuel combustion: Extraction and burning of coal, oil and gas release large quantities of CO₂ previously stored for millions of years.
• Deforestation and land-use change: Removal of forests reduces carbon uptake via photosynthesis and often releases stored carbon when biomass is burned or decomposed.
• Agriculture and livestock: Rice paddies, wetlands drainage and ruminant digestion produce methane (CH₄), a potent greenhouse gas.
• Industrial processes: Cement production releases CO₂ from decarbonation of limestone.

These changes increase atmospheric greenhouse gases, strengthen the greenhouse effect, and drive global climate change. The current imbalance shows the atmosphere is accumulating more CO₂ annually than natural sinks (oceans, forests) can remove.

Pathways: Short-Term vs Long-Term Carbon Cycles

Carbon cycles operate on different timescales:

1. Short-term cycle (days to decades) — Photosynthesis, respiration, decomposition, ocean-atmosphere exchange. This cycle controls seasonal and annual CO₂ fluxes and ecosystem carbon budgets.
2. Long-term cycle (thousands to millions of years) — Sedimentation, rock formation, fossil fuel formation, uplift, weathering and volcanism. These processes regulate atmospheric CO₂ over geological time and influence climate trends on long timescales.

Significance of the Carbon Cycle

• Climate regulation: The carbon cycle controls atmospheric CO₂, a key greenhouse gas that affects global temperature.
• Support of life: Carbon forms organic molecules needed for growth, energy and reproduction of organisms.
• Energy flow: Photosynthesis captures solar energy and respiration releases it, powering trophic interactions.
• Ocean chemistry: The carbonate-bicarbonate system buffers seawater pH and enables shell formation in marine organisms.
• Soil fertility: Soil organic carbon improves soil structure, water retention, and nutrient availability.
• Carbon sequestration: Forests, soils and oceans act as sinks that partially mitigate anthropogenic CO₂ emissions.

Suggested Diagram (for notes/exams)

Example: Carbon Flow in a Forest Ecosystem

Consider a temperate forest: each year the ecosystem's primary productivity fixes a certain mass of carbon (NPP). Some example statements:

• If Gross Primary Productivity (GPP) = 12 t C ha⁻¹ yr⁻¹ and autotrophic respiration R_a = 4 t C ha⁻¹ yr⁻¹, then Net Primary Productivity (NPP) = GPP − R_a = 8 t C ha⁻¹ yr⁻¹.
• If heterotrophic respiration (decomposers) = 7 t C ha⁻¹ yr⁻¹, then net ecosystem production (NEP) = NPP − heterotrophic respiration = 1 t C ha⁻¹ yr⁻¹ (a small net sink).
• Long-term sequestration requires transfer of carbon into stable pools (e.g., soil humus, woody biomass accumulation or burial), not rapid recycling.

These numbers are illustrative — real values vary widely by ecosystem, climate and management.

Impacts of Anthropogenic Changes

The sustained rise of atmospheric CO₂ (measured at observatories like Mauna Loa) correlates with industrialization and fossil fuel use. Higher CO₂ and other greenhouse gases lead to:

1. Global warming and changes in precipitation patterns.
2. Ocean acidification, harming calcifying organisms (corals, shellfish).
3. Shifts in ecosystem productivity and species distributions.
4. Increased frequency of extreme weather events, droughts and heatwaves, affecting carbon uptake.

Mitigation strategies include reducing fossil fuel use, afforestation/reforestation, improving agricultural practices to increase soil carbon, and technological carbon capture and storage (CCS). Natural sinks (forests, wetlands, oceans) play a central role but can be weakened by climate change.

Summary

The carbon cycle is a complex, multi-timescale system that maintains life and climate on Earth. It links biological processes (photosynthesis, respiration, decomposition) with geochemical processes (sedimentation, weathering, volcanism) and human activities. Understanding the flow of carbon and the size and rates of fluxes between reservoirs is essential for ecology, biogeochemistry and climate science.

Further reading & references (suggested)

1. Chapin, F. S., Matson, P. A., & Mooney, H. A. (2011). Principles of Terrestrial Ecosystem Ecology.
2. Schlesinger, W. H., & Bernhardt, E. S. (2013). Biogeochemistry: An Analysis of Global Change.
3. IPCC Assessment Reports — chapters on carbon cycle and climate (for advanced reading).