Carbon Cycle

The carbon circulates within biosphere in two cycles. In gaseous cycle it is present as free gas in the form of carbon dioxide (CO₂) and as a gas dissolved in the waters of sea and land (HCO₃⁻). In non-gaseous or solid cycle, it is present as biomass (carbohydrates i.e. - CH₂O compounds) in living/dead matter and as hydrocarbon compounds (petroleum, coal, natural gas) and as mineral carbonates in carbonate rocks (limestone, corals, i.e. - CaCO₃ compounds).

This carbon enters the biotic world through the action of autotrophy by the process of photosynthesis. The carbon then again returns back to the atmosphere and water by (I) respiration (CO₂); (ii) decay (producing CO₂ if oxygen is present methane (cH₄ if it is not).

The total amount of carbon in solid phase normally remains stored as it is. The carbon dioxide assimilated by plants is stored in the woody tissues of plants. This is called as organic reservoir of carbon. Forests are significant reservoirs of biological carbon of the biosphere (Carbon sink). The respiration by the biota releases CO₂ back into the atmosphere. Carbon is returned to the atmosphere at the same rate as it is removed, but in recent years the carbon concentration in the atmosphere has been increasing because of deforestation and burning of organic matter such as woods.

Simultaneously carbon from sedimentary (solid) phase is released into atmosphere mainly due to increased burning of fossil fuels and partly by weathering of rocks and volcanic eruption. In normal carbon cycle, the uptake and return of CO₂ occurs very slowly over a longer geological time scale. But the carbon cycle is being disturbed due to anthropogenic activities i.e. burning of fossil fuels, wood fuels and deforestation (forests are largest consumers of carbon dioxide). The gradual increase in concentration of carbon dioxide in the atmosphere would intensify green house effect of the atmosphere, resulting into global into global warming and climatic changes.