Carbon Cycle
The carbon cycle is the biogeochemical process by which carbon atoms continuously move through the atmosphere, hydrosphere, lithosphere, and biosphere through processes such as photosynthesis, respiration, decomposition, combustion, and ocean absorption. Carbon exists in various forms — as CO₂ in the atmosphere, as carbonate in rocks, as organic molecules in living organisms, and dissolved in water. Human activities such as burning fossil fuels and deforestation have significantly accelerated the movement of carbon into the atmosphere, driving climate change.
Key Formula
CO2 + H2O → (light) → C6H12O6 + O2
LaTeX: \text{CO}_2 + \text{H}_2\text{O} \xrightarrow{\text{light}} \text{C}_6\text{H}_{12}\text{O}_6 + \text{O}_2
| Symbol | Meaning | Unit |
|---|---|---|
| CO₂ | Carbon dioxide absorbed from atmosphere | mol |
| H₂O | Water absorbed by plant roots | mol |
| C₆H₁₂O₆ | Glucose produced (organic carbon stored) | mol |
| O₂ | Oxygen released as byproduct | mol |
Worked Example
Problem
A forest ecosystem fixes 1200 g of carbon per m² per year through photosynthesis and releases 900 g of carbon per m² per year through ecosystem respiration. Calculate the net carbon sink value and express it in kg C per hectare per year.
Solution
Step 1: Net Ecosystem Production (NEP) = Gross Primary Production − Ecosystem Respiration. Step 2: NEP = 1200 g C/m²/yr − 900 g C/m²/yr = 300 g C/m²/yr. Step 3: Convert to kg/hectare: 1 hectare = 10,000 m². Step 4: 300 g/m²/yr × 10,000 m²/ha = 3,000,000 g/ha/yr = 3,000 kg/ha/yr.
Answer
Net carbon sequestration = 3,000 kg C per hectare per year (a net carbon sink)
Carbon Reservoirs and Their Approximate Sizes
| Reservoir | Carbon Content (Pg C) | Residence Time | Primary Form |
|---|---|---|---|
| Atmosphere | 860 | Years to decades | CO₂, CH₄ |
| Ocean (surface) | 900 | Decades | Dissolved CO₂, HCO₃⁻ |
| Ocean (deep) | 37,000 | Centuries–millennia | Dissolved inorganic carbon |
| Terrestrial biosphere | 2,300 | Years to centuries | Organic carbon in plants/soil |
| Fossil fuels | ~3,700 | Geological timescales | Coal, oil, natural gas |
| Lithosphere (rocks) | ~60,000,000 | Millions of years | Carbonate minerals (CaCO₃) |
Interactive Tools
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Related Terms
Nitrogen Cycle
The nitrogen cycle is the biogeochemical process by which nitrogen is converted between its various chemical forms as it circulates through terrestrial and aquatic ecosystems, including the atmosphere. Because nitrogen is a key component of amino acids, proteins, and nucleic acids, all life depends on it, yet atmospheric nitrogen (N₂) is largely inaccessible to most organisms without fixation. The cycle includes key processes: nitrogen fixation (N₂ → NH₃), nitrification (NH₃ → NO₂⁻ → NO₃⁻), assimilation, ammonification, and denitrification (NO₃⁻ → N₂).
Decomposer
Decomposers are organisms — primarily fungi and bacteria — that break down dead organic matter (detritus) into simpler inorganic compounds, releasing nutrients back into the soil, water, and atmosphere. This process of decomposition is essential for nutrient cycling, making elements like carbon, nitrogen, and phosphorus available again for producers such as plants and algae. Without decomposers, ecosystems would quickly become buried in dead material and nutrient reservoirs would be permanently locked away.
Water Cycle
The water cycle, also known as the hydrological cycle, is the continuous movement of water through Earth's systems — from the oceans to the atmosphere through evaporation, to the land through precipitation, and back to the oceans via runoff and groundwater flow. Driven primarily by solar energy and gravity, the water cycle regulates climate, freshwater availability, and supports all life on Earth. Key processes include evaporation, transpiration (evapotranspiration from plants), condensation, precipitation, infiltration, and surface runoff.
Carbon from Latin carbo meaning "charcoal or coal"; cycle from Greek kyklos meaning "circle or wheel". The modern understanding of the carbon cycle developed through 19th-century chemistry, with Joseph Priestley, Jan Ingenhousz, and others elucidating photosynthesis and respiration.