Carbon Cycle: Steps, Examples, Significance, Human Impacts

The carbon cycle is a biogeochemical cycle where different forms of carbon compounds are cycled through the Earth’s various systems like the atmosphere, biosphere, hydrosphere, and geosphere.

  • The carbon cycle determines the flow of carbon between reservoirs, from the atmosphere to the living beings, and back to the atmosphere in a cyclic process.
  • Various factors like living beings influence the carbon cycle, change in climate, ocean chemistry, and even geochemical activity.
  • Carbon is an essential component of living beings as well as non-living things in the form of organic and inorganic compounds.
  • It is the essential element for various biomolecules like carbohydrates, amino acids, and nucleotides.
  • The primary source of carbon on Earth is CO2 which either remains in the air or dissolved in water.
  • The cycling of carbon from one form to another occurs between reservoirs by different processes like photosynthesis, respiration, and combustion.
  • The principal reservoirs of carbon include the atmosphere, the oceans, and three land compartments consisting of land plants, their detritus, and soils, collectively called the terrestrial biosphere.
  • Components like rivers, lakes, and animals are not considered significant reservoirs, but they perform as a part of the pathways linking the terrestrial biosphere to the oceans and the atmosphere.
  • The lithosphere that stores very large amounts of carbon in limestone and shale forms the fourth reservoir, but it is not considered as important as it interacts very slowly with the other reservoirs.
  • As the Earth is a closed system, the amount of carbon on the Earth doesn’t change, but the concentration of carbon in a reservoir might change depending on the carbon cycle process.
  • The carbon cycle is an important natural phenomenon as it balances the amount of carbon in different forms to make the Earth a hospitable habitat.
  • The carbon cycle can be classified as a fast and slow carbon cycle depending on the time required for the movement of carbon compounds from one reservoir to another.

The Slow Carbon Cycle

  • The slow carbon cycle is defined by the cyclic movement of carbon compounds between the atmosphere, land, and the ocean that usually takes between 100 to 200 million years.
  • The slow carbon cycle constitutes the lithosphere reservoir where the carbon is stored in the rocks and soil, which then slowly cycles to the atmosphere and hydrosphere.
  • This cycle moves about 1011-1013 grams of carbon in a year through different reservoirs.
  • The slow carbon cycle begins with the formation of carbonic acid when the atmospheric carbon combines with the water.
  • The acid then dissolves the rocks causing the release of carbon and calcium compounds and their flow into rivers, finally moving into the oceans.
  • The calcium with carbon compounds forms calcium carbonate, which forms shells and sediments in the form of layers at the bottom of the ocean.
  • The carbon stored in the ocean then moves to the atmosphere by processes like volcanoes, which causes the release of carbon dioxide into the atmosphere.
  • The balancing of carbon amounts in a different reservoir in the slow carbon cycle takes a few thousand years.

The Fast Carbon Cycle

  • The fast carbon cycle is the movement of carbon through the living component of the Earth or the biosphere that usually occurs within a short period of time.
  • The fast carbon cycle operates on a daily basis with the exchange of gases between living beings that influence the changes in the carbon storage for decades.
  • The cycle moves more than 1015 to 1017 grams of different carbon forms in a year.
  • Living beings are the major reservoirs of carbon in the fast carbon cycle as carbon forms an essential component of life in the form of biomolecules.
  • The movement of carbon in the fast carbon cycle begins with the uptake of carbon dioxide by green plants from the atmosphere.
  • Plants then convert the carbon dioxide into organic form by the process of photosynthesis.
  • The organic carbon then moves through different living forms like animals before releasing into the atmosphere as carbon dioxide by respiration.
  • Another form of release of carbon dioxide occurs during decomposition, where different microorganisms degrade the organic compound.
  • The fast carbon cycle is closely tied to plants as they are imperative for the conversion of inorganic carbon into its organic form.
  • The concentration of carbon in the different reservoir during the fast carbon cycle changes within a short period of time, allowing a comparatively shorter time for balance.
Carbon Cycle Steps

Figure: Carbon Cycle. Image Source: OpenStax.

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Carbon Cycle Steps

The carbon cycle consists of different geochemical reservoirs that store carbon on the Earth and the pathways that transport carbon between them. The movement of carbon through different reservoirs in the Earth occurs via different steps, all of which work together to maintain a balance on the planet. The following are the steps involved in the carbon cycle process;

1. Entry of carbon dioxide into the atmosphere

  • The first step in the carbon cycle is the entry of carbon dioxide, an inorganic form of carbon, into the atmosphere.
  • Initially, this process occurred through natural processes like volcanic and asteroid activity, which resulted in the release of carbon dioxide into the atmosphere.
  • In the slow carbon cycle, the carbon present in the lithosphere (limestone and other sediments in the oceans) is released into the atmosphere by volcanic activity.
  • Activities like the use of fossil fuels and combustion also cause a release of inorganic carbon into the atmosphere.
  • Nowadays, however, carbon dioxide is also released by metabolic processes like respiration, decomposition, or by the burning of woods or fossil fuels by humans.
  • Carbon dioxide is the starting point of the carbon cycle which is then followed by processes that convert the inorganic form into an organic form.

2. Carbon dioxide intake by Producers

  • The carbon dioxide present in the atmosphere enters the terrestrial biosphere by means of green plants and algae that take up CO2 during the process of photosynthesis.
  • Producers are a group of living beings capable of converting the inorganic form of carbon (CO2) into the organic form (carbohydrates).
  • Photosynthesis is an essential step of the carbon cycle, primarily responsible for the movement of carbon from the atmosphere to the biosphere.
  • Besides, in the case of the oceanic reservoir, the atmospheric carbon dioxide dissolves into the water to be utilized by various underwater producers.
  • In the presence of energy (solar or chemical), the inorganic carbon dioxide is converted into an energy-rich organic form.

3. Movement of carbon compounds in the food chain

  • The organic form of carbon produced by producers then moves through the food chain as consumers feed on producers, resulting in the movement of carbon from one form of life to another.
  • The organic form of carbon also undergoes changes as it moves through the consumers.
  • These consumers utilize these compounds to produce energy and release carbon dioxide into the atmosphere by the process of respiration.
  • The consumers convert some of the carbon into biomolecules and store them for living processes.
  • Some part of it, however, gets converted into carbon dioxide and is released into the atmosphere to maintain a balance of the carbon concentration in different reservoirs.

4. Release of carbon by decomposition

  • Eventually, as all the plants and consumers die, their dead bodies are feed upon by different decomposers.
  • These decomposers convert the organic form of carbon into the inorganic form so that a cycle is formed.
  • These decomposers include microorganisms like fungi and some bacteria that feed on the complex organic compounds and convert them into a simpler inorganic form.
  • Some of the inorganic carbon remains in the soil to form sediments, eventually resulting in the lithosphere while the rest is released into the atmosphere in the form of carbon dioxide and other gases.
  • The carbon remaining in the soil eventually forms fossil fuels that are then combusted to release carbon into the atmosphere as well.

Carbon Cycle Examples

Along with the carbon cycle, there are many parallel systems that are indirectly involved in the transport of carbon through different reservoirs, to maintain a relative balance. There are different reservoirs in different ecosystems, all of which can absorb, the release of transport carbon from one form to another.

1. Atmosphere

  • The atmosphere serves mainly as a passive reservoir for carbon.
  • It stores carbon-containing gases that have escaped from the other two reservoirs as a result of their volatility.
  • Carbon dioxide is the most important form of carbon in the atmosphere, which is release either via respiration or by combustion.
  • The carbon present in the atmosphere moves through different systems either by absorption by producers or by mixing with water or by dissolving in water.
  • The atmosphere plays an important role in the carbon cycle as it works as a reservoir of carbon in order to maintain a balance in the ecosystem.
  • It is, however, considered a passive reservoir as the changes in the concentration of carbon in the atmosphere occurs rapidly.

2. Biosphere

  • The biosphere consists of living components of the Earth and the ocean carbon reservoir, resulting in a terrestrial and ocean biosphere.
  • The terrestrial biosphere also acts as a sink for excess atmospheric CO2 because plants are sensitive to the amount of CO2 in the air.
  • When plants secure adequate light and nutrients in controlled experiments, they grow faster in ambient air containing elevated concentrations of CO2.
  • The living beings are of different groups with some absorbing carbon dioxide from the environment while others release carbon dioxide back to the atmosphere.
  • Producers utilize carbon dioxide to form carbohydrates which are then transferred to consumers.
  • The decomposers finally convert the organic compound into inorganic forms by the process of mineralization.
  • The oceanic carbon reservoir contains a biospheric component comprising living marine plants and animals and their detritus, but it also stores a very considerable quantity of inorganic carbon as dissolved bicarbonate and carbonate salts.
  • The ocean also acts as a sink for excess atmospheric CO2 accumulating in the atmosphere, because of the ability of seawater to absorb CO2 gas.

3. Lithosphere

  • The lithosphere acts as a reservoir of inorganic carbon in the form of soil and rocks.
  • The lithosphere contains large amounts of carbon in limestone and shale, which take part in the slow carbon cycle.
  • Volcanic activity causes the release of carbon dioxide into the atmosphere, helping in maintaining a balance.
  • The Earth’s crust also helps reduce carbon dioxide by burying dead plans and animals increasing the carbon content on the biosphere.

Carbon Cycle Significance

  • The carbon cycle is important as it balances the amount of carbon on Earth which allows the planet to remain hospitable for life.
  • Carbon is an essential element for all living beings, and its movement through different systems helps to understand different biological factors and factors that influence them.
  • Carbon dioxide is an essential gas of the atmosphere that protects the Earth from the sun’s harmful rays. However, the increased amount of carbon dioxide causes the insulation of Earth, increasing the temperature of the Earth. Understanding the movement of carbon dioxide through different ecosystems helps to understand the climate and the onset of global warming.
  • The carbon cycle is also tied to the availability of other elements and compounds like oxygen and sulfur.
  • The carbon cycle enables the flow of energy through the food chain. Carbon compounds carry the solar energy trapped from producers to consumers to decomposers.
  • The decomposition or the mineralization process works as a natural waste removal system.
  • Different carbon-containing gases like carbon dioxide and methane also affect the Earth’s climate.

Human impacts on the Carbon Cycle

  • Human activities like burning fossil fuels, change in land use, and using limestone is responsible for the transfer of a tremendous amount of carbon back to the atmosphere, affecting the carbon cycle.
  • The increase in carbon dioxide in the atmosphere increases the greenhouse effect causing noticeable warming of the earth’s surface. The increase in Earth’s temperature affects all forms of life and other components like oceans and glaciers.
  • Deforestation and urbanization results in the loss of green plants throughout the world, which causes a further increase in the carbon dioxide amount in the atmosphere.
  • Agriculture causes a change in the land structure and vegetation, which directly and indirectly affects the overall carbon cycle.
  • The use of artificial fertilizers and deeper ploughing causes an increase in mineralization of soil and the reduction of soil organic matter.
  • Industrial society, along with a rapidly rising world population and increasing mechanization of agriculture and forestry, modifies extensive areas of previously undisturbed landscape and thus alter the terrestrial biosphere.

References and Sources

  • Heimann, M. (Ed.). (1993). The Global Carbon Cycle.DOI:10.1007/978-3-642-84608-3.
  • Baldocchi, D. (2005). The carbon cycle under stress. Nature, 437(7058), 483–484.DOI:10.1038/437483a.
  • Keller DP, Lenton A, Littleton EW, Oschlies A, Scott V, Vaughan NE. The Effects of Carbon Dioxide Removal on the Carbon Cycle. Current Climate Change Reports. 2018;4(3):250-265. DOI:10.1007/s40641-018-0104-3
  • Bramryd T (1980). Effects on the Carbon Cycle due to Human Impact on Forest Ecosystems. Biogeochemistry of Ancient and Modern Environments. Springer, Berlin, Heidelberg.

About Author

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Anupama Sapkota

Anupama Sapkota has a bachelor’s degree (B.Sc.) in Microbiology from St. Xavier's College, Kathmandu, Nepal. She is particularly interested in studies regarding antibiotic resistance with a focus on drug discovery.

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