Essay on “Carbon Cycle” in Ecosystem

As compared to oxygen, the biological cycling of carbon is somewhat more direct. Only two main components, viz., CO₂ and organic carbon compounds, are involved in the carbon cycle, even though carbon occurs in several inorganic pools. Carbon exchange across water-air interfaces is a fairly slow process and this is why the carbon cycle occurs more or less independently in aquatic and terrestrial ecosystems.

Waldron and Ricklefs (1973) have at­tempted certain estimates of the size of carbon pools and the rates of carbon flux for the planet Earth and these are diagrammatically explained. They have also estimated that terrestrial ecosystems cycle about 12 per cent of the atmospheric carbon annually and that the cycling time of atmo­spheric CO₂ is about eight years.

Forests and woodlands of the world contain about 400-700 X 10⁹ tonnes of carbon. The amount of dry organic matter is estimated to be about double of this figure. This compares with 700 X 19⁹ tonnes of carbon existing in the form of carbon dioxide in the atmosphere. About one-third of the organic carbon occurs in forests, one-fourth in oceans, one-fourth in atmosphere, and the rest in grasslands, tundra, or other surface covers.

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Recent estimates indicate that burning of fossil fuels increases the carbon dioxide concentration of the atmosphere by about 0.64 ppm/annum. This is partly fixed by photosynthesis in forests and partly buffered in oceans. Burning of forests and vegetation is estimated to produce 1×10′ tons/annum of carbon.

The rapidly increasing industrialization and the predominantly coal- based technology of the present day world can lead to the release of CO₂ in the atmosphere, at a rate somewhat faster than can be naturally absorbed, e.g., by plants during photosynthesis.

Besides fossil fuels, two other important sources of carbon are deforestation and the oxidation of humus. Some recent estimates indicate a rise of about 12 per cent in the atmospheric CO₂ content during the period 1860 to 1980. The present level of CO₂ in the atmosphere is about 330- 350 ppm and this content is rising every year at the rate of about 1.5 to 1.6 ppm/year (see Hansen et al., 1981).

Many persons have implicated the increased C0₂ content of the atmosphere, as a significant contributing factor, to global climatic changes. An increase in the average global temperature by about 3K can potentially induce significant climatic changes of serious concern to farmers. According to Hansen et al., the C0₂ content of the atmosphere may reach 600 ppm in the next century.

Some recent estimates suggest that at an annual rate of increase in world fuel use of about 4.3 per cent, a doubling of the atmospheric C0₂ content could lead to a greenhouse effect with a concomitant 3°C increase in the oceans’ temperature by year A.D. 2035. The global carbon budgets are some­what unbalanced in the sense that whereas the source of C0₂ accounts for some 5.25 X 10′ tons C/yr, the sink accounts for only 4.6 X 10⁹ tons C/yr.

The difference between the two figures (= 0.65 X 10⁹ tons) represents the missing carbon in the chemical budget of the global CO₂ cycle (see Table 4.3). Likewise, the value of the missing carbon fraction for the biological budget of the global CO₂ cycle has been estimated to be 1.55 x 10⁹ tons C/yr (Walsh et al., 1981).

The annual loss of organic matter from continental shelf ecosys­tems is far greater than in open sea. According to Walsh et al., “if part of the loss of near shore primary production has increased in those coastal zones where anthropogenic inorganic nutrient supplies have been consistently in­creasing since the industrial revolution, the burial and diagenesis of this material in slope depocentres could represent the missing BMTs of carbon in global C0₂ budgets”.

Certain estimates made by Mopper and Degens (1979) indicate that the total flux of organic matter leaving the euphotic zone in the sea varies from about 4 g C/m²/yr to about 10 g C/m²/yr. The biological oxidation of particu­late organic carbon in the water columns amounts to about 2-4 g/C/m²/yr, whereas the corresponding estimate for the biological oxidation in the sedi­ment is about 0.5-3.0 gC/m²/yr. The total efficiency of recycling of C in the sea can sometimes reach 95 per cent.

The following activities (natural or man-made) are believed to cause a net accumulation (gain) of carbon in ecosystems: Phytomass increases in early tropical, secondary succession, and in temperate forests and forest plantations; accumulation of humus and litter in temperate forests; peat accumulation inpeatlands; and humus accumulation in grasslands and swamps in the boreal zone.

Likewise, the following activities are drought to lead to a net release (loss) of C (see Hampicke, 1980): Burning of tropical forests; loss of organic soil matter in tropical rain forests; ‘jhum’ cultivation (first year after clear­ance); destruction of litter and vegetation by ground fires in forests and grasslands; loss of humus in cultivated temperate steppe soils; and loss of humus from tropical podzols (see Hampicke, 1980).

The oceans play an important role in the global carbon cycle. According to Rothy (1980), about 150 x 10¹² kg of carbon have been released into the atmosphere as C0₂ through burning of coal, oil and gas during the last century, and out of this amount about 40 per cent is now present in the ocean.