In this essay we will discuss about the carbon cycle with the help of a diagram.
Importance of Carbon:
It is estimated that about 99% of the total carbon lies in the geological components.
Carbon is essential constituent of all major organic compounds of protoplasm as carbohydrates, fats and nucleic acids. So carbon is generally considered the basis of life. Next to water, carbon is the most significant element, constituting 49 per cent of the dry weight of organisms.
Sources of Carbon:
In the biosphere, there are four sources of carbon:
1. Carbon dioxide present in air and that which is dissolved in water (1.3 – 5 x 1015 kg in hydrosphere) to form carbonic acid. Lime rocks also contributed to CO2 in water.
In atmosphere, 0.032% CO2 (i.e., 320 parts per million) is present. It constitutes only about 1 per cent of total global carbon. Each acre of earth’s surface has about 6 tonnes of carbon as CO2 in the atmosphere, total being 6 × 10″ kg in the atmosphere. Atmosphere contains about 700 × 10′ metric tonnes of CO2 while ocean water contains about 35,000 × 10′ metric tonnes of CO2 (i.e., 50 times than in atmosphere) and constitutes about 71 per cent of total quantity of global carbon.
2. Carbonates of earth’s crust derived from rocks which by chemical reactions give rise to carbon dioxide.
3. Fossil fuels like peat, coal and petroleum products. These are formed by partial decomposition of dead bodies of plants (yield coal) and animals (yield natural gas and petroleum). Peat is formed from the dead tissues of peat moss. Sphagnum. Lithosphere contains about 2.81 × 1021 kg of carbon.
4. Oceans where it remains stored as bicarbonates as limestone and marble rocks. Ocean absorbs CO2 from atmosphere, so acts as a global sink for CO2. Oceanic water can retain up to 50 times of CO2 level in the air so is capable of regulating CO2 in the atmosphere.
Dissolved CO2 occurs in the form of carbonic acid in a reversible form:
Because all these reactions are reversible, the direction of the reaction depends on the concentration of CO2 e.g. a local depletion of atmospheric CO2 would result in a net movement of CO2 into the atmosphere from the dissolved phase and vice versa.
Thus, the major reservoirs of carbon in the biosphere are atmosphere, ocean and fossil fuels.
Carbon cycle is simplest of all nutrient cycles. CO2 is required by green plants for the process of photosynthesis. The plants are able to fix carbon by the photosynthesis to the amount ranging from 4 to 9 × 1013 kg per year. Carbon present in lithosphere does not become available to organisms till it is burnt or changed chemically. During photosynthesis oxygen is released as by-product.
Glucose is used to synthesize other organic compounds like proteins, fats, nucleic acids, etc.
Carbon fixed by producers enters the food chain and passed to herbivores, carnivores, decomposers etc. (Fig. 12.22). Due to the process of photos5mthesis carbon contents of atmosphere and hydrosphere decreases. One hectare of healthy forest can produce 10 tonnes of oxygen and absorb 30 tonnes of carbon dioxide per year during photosynthesis.
(a) Carbon dioxide is released into atmosphere by respiration of producers and consumers.
(b) It is also released by decomposition of organic wastes and dead bodies by decomposers by the action of bacteria and fungi of decay.
(c) Another source of CO2 is burning of wood and fossil fuels.
(d) Volcanic eruptions and hot springs also release CO2, into the atmosphere.
(e) Weathering of carbonate containing rocks by the action of acids and excreted by micro-organisms and plant roots or treatment of carbonate minerals also add to CO2 in atmosphere.
So carbon cycling occurs through atmosphere, oceans and living and dead organisms. The carbon cycle is a perfect cycle in the sense that carbon is returned to atmosphere as soon as it is removed. The recycling of carbon is essentially a self regulating feedback system. However, human beings may upset the system by excessive use of fossil fuels and other activities like deforestation, massive burning of fossil fuels, etc.