The carbon cycle refers to the phenomenon that carbon is exchanged in the biosphere, lithosphere, hydrosphere and atmosphere of the earth, and it circulates with the movement of the earth. 

Carbon Cycle Definition

Carbon CycleThe carbon cycle in the biosphere is mainly manifested in the fact that green plants absorb carbon dioxide from the atmosphere, convert it into glucose and release oxygen through photosynthesis with the participation of water, and the organism reuses glucose to synthesize other organic compounds. 

Organic compounds are passed through the food chain and become part of other organisms such as animals and bacteria. 

A part of the carbohydrate in the living body is oxidized into carbon dioxide and water by respiration as an energy source for metabolism of the organism, and releases the energy stored therein. 

During the carbon cycle, carbon dioxide in the atmosphere can be completely renewed in about 20 years. The vast majority of carbon in nature is stored in the earth’s crust rocks. The carbon in the rocks is decomposed by natural and artificial chemical processes and then enters the atmosphere and the ocean.

At the same time, the dead organisms and other various carbonaceous materials continue to deposit. The form of matter returns to the earth’s crust, thus forming part of the global carbon cycle. 

The earth’s biochemical cycle of carbon controls the migration of carbon between the surface or near-surface sediments and the atmosphere, the biosphere, and the ocean.

Introduction to carbon cycle

The two largest carbon pools on Earth are lithosphere and fossil fuels, which account for about 99.9% of the Earth’s total carbon. The carbon activity in these two libraries is slow and actually acts as a repository . 

There are also three carbon poolson Earth:

The atmospheric reservoir

The hydrosphere reservoir

The biological reservoir

The carbon in these three libraries is rapidly exchanged between biological and inorganic environments, with small and active capacity, and actually acts as a exchange library.

Carbon is mainly present in the form of carbonates in the lithosphere, with a total amount of 2.7×1016 t; in the form of carbon dioxide and carbon monoxide in the atmosphere in various forms in the hydrosphere. There are hundreds of bioorganic organics present in the biobank. The form of existence of these substances is regulated by various factors.

In the atmosphere, carbon dioxide is the main gas containing carbon and is the main form of carbon involved in the circulation of matter . In the biobank, the forest is the main absorber of carbon, and it has twice as much carbon as other vegetation types. The forest is the main reservoir of carbon in the biological reservoir, and the storage capacity is about 4.82×1011 t, which is equivalent to 2/3 of the atmospheric carbon content.

The rate at which plants, photosynthesizing microorganisms absorb carbon from the atmosphere through photosynthesis, is roughly equal to the rate at which carbon is released into the atmosphere through the respiration of organisms. Therefore, the amount of carbon dioxide in the atmosphere is quite comparable before being disturbed by human activities. stable.

Considering nature fires, plants and other carbon curing are more than carbon gasification caused by animals. Petroleum coal is a by-product of carbon solidification.

Basic process

The basic process of the natural carbon cycle is as follows: Carbon dioxide (CO2) in the atmosphere is absorbed by plants on land and in the ocean, and then returned to the atmosphere in the form of carbon dioxide through biological or geological processes and human activities.

Circulation between Biology and the Atmosphere

The green plant obtains carbon dioxide from the air, converts it into glucose through photosynthesis, and then synthesizes the carbon compound of the plant body, and passes through the food chain to become a carbon compound of the animal body. Plant and animal call

The suction action converts a part of the carbon ingested into carbon dioxide into the atmosphere, and the other part constitutes the organism’s body or is stored in the body. 

After the death of animals and plants, the carbon in the residue is also converted into carbon dioxide by the decomposition of microorganisms and finally discharged into the atmosphere. It takes about 20 years for the carbon dioxide in the atmosphere to circulate once.

A part (about one thousandth) of the animal and plant residues are buried by the sediment and become organic deposits before being decomposed. These sediments have been transformed into fossil fuels – coal, oil and natural gas – under heat and pressure over a long period of time . 

When they are burned during weathering or as a fuel, the carbon therein oxidizes to carbon dioxide and is released into the atmosphere. Human consumption of large amounts of fossil fuels has a major impact on the carbon cycle.

On the one hand, the carbon in the sedimentary rock decomposes into the atmosphere and the ocean due to various natural and artificial chemical actions; on the other hand, the death of the organism and various other carbonaceous substances are continuously returned to the earth’s crust in the form of sediments.

Forms part of the global carbon cycle. Although the biological cycle of carbon has a great impact on the environment of the earth, from the perspective of geological time in millions of years, the geochemical cycle of slowly changing carbon is the most important controlling factor of the earth’s environment.


Exchange between Atmosphere and Ocean

Carbon dioxide can enter the sea from the atmosphere or from the sea. This exchange occurs at the interface of gas and water and is enhanced by the action of wind and waves. 

The amount of carbon dioxide flowing in these two directions is roughly equal, the amount of carbon dioxide in the atmosphere increases or decreases, and the amount of carbon dioxide absorbed by the ocean increases or decreases.

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Formation and Decomposition of Carbonaceous Salts

Carbon dioxide in the atmosphere dissolves in rainwater and groundwater to become carbonated. Carbonic acid converts limestone into soluble bicarbonate and is transported by the river to the ocean.

The carbonate and bicarbonate content in the seawater is Saturated. With the new input of carbonate, an equivalent amount of carbonate is deposited. 

Through different diagenesis processes, limestone, dolomite and carbonaceous shale are formed. 

Under chemical and physical effects (weathering), these rocks are destroyed and the carbon contained is released into the atmosphere in the form of carbon dioxide. 

Volcanic eruptions can also re-add carbon from a portion of the organic carbon and carbonate to the carbon cycle. 

The destruction of carbonaceous rocks has little effect on the cycle in a short period of time, but it is important for the balance of carbon in millions of years.

Human Activity

When humans burn fossil fuels to get energy, they produce a lot of carbon dioxide. From 1949 to 1969, carbon dioxide production was estimated to increase by 4.8% per year due to burning fossil fuels and other industrial activities. 

The result is an increase in the concentration of carbon dioxide in the atmosphere. This destroys the original balance of nature and may lead to climate anomalies. 

A small portion of the carbon dioxide produced by fossil fuel combustion and discharged into the atmosphere can be dissolved by seawater, but the increase in dissolved carbon dioxide in seawater can cause changes in the acid-base balance and carbonate dissolution equilibrium in seawater.

Incomplete combustion of fossil fuels produces a small amount of carbon monoxide. Natural processes also produce carbon monoxide. 

Carbon monoxide remains in the atmosphere for a short period of time, mainly by microorganisms in the soil, and can also be converted to carbon dioxide by a series of chemical or photochemical reactions.


The Role of Forest Ecosystems

The role of forest ecosystems in the carbon cycle When humans realized that the increase in greenhouse gases, especially carbon dioxide, would warm the global temperature and bring about a series of serious ecological problems, the carbon cycle was studied. 

The forest ecosystem, as a large carbon sink that absorbs carbon dioxide and releases oxygen , plays a very important role in the carbon cycle. 

The global forest area is 4.161 billion hectares, of which tropical, temperate and cold zones account for 32.9%, 24.9% and 42.1% respectively. 

The carbon in the upper part of the global terrestrial ecosystem is 562 Gt, and the carbon content in the aboveground part of the forest ecosystem is 483 Gt, accounting for 86%. 

The global terrestrial ecosystem has a carbon content of 1,272 Gt, while the forest has a carbon content of about 927 Gt, accounting for 73% of the world’s soil carbon.

The role of forest ecosystems in the carbon cycle depends on the following aspects:


The biomass of forest ecosystems stores a large amount of carbon.

For example, according to the carbon content of plant biomass, which is 45% to 50%, nearly half of the biomass of the entire forest ecosystem is carbon. The biomass of forests is most closely related to its growth stage.

Generally, forests can be divided into young forests , middle-aged forests , near-mature forests, mature forests and over-ripe forests according to their ages .

The accumulation rate of carbon is in the middle-aged forest ecosystem. The largest and mature forest/over-ripe forest, in which the accumulation rate of carbon is the largest in the middle-aged forest ecosystem, while the mature forest/over-ripe forest basically balances its absorption and release of carbon due to its biomass growth. 

Estimating the potential for carbon uptake from the age structure of forests is a major aspect of determining the carbon sink function of forest ecosystems.

The structure of forests in China is mostly young forests and middle-aged forests. Therefore , the potential of plants to fix atmospheric carbon in China’s forest ecosystems is great.

According to Wang Xiaoke and other estimates, the potential total carbon storage of forest ecosystems in China is 8.41Pg, and the actual total carbon storage is only 44.3% of the total plant carbon storage.

Therefore, if China’s forest ecosystem is effectively protected, it will be an important carbon sink for China.

Forest Products

The carbon sequestration of forest ecosystem forest products is a very variable factor. General forest products can be classified into short-term products and long-term products according to their service life. 

For example, wood for fuel and wood for pulp are short-term products, while plywood and construction wood are long-term products. 

The longevity of forest products also largely determines the carbon sink function of forest ecosystems . 

Long-life forest products can delay carbon release and alleviate the increase of global atmospheric carbon concentration. Generally speaking, the service life of durable forest products can reach 100-200a.

After such a long time, carbon can be realized through reforestation. A virtuous circle. Therefore, we should try to process durable and long-life forest products.

Plant Litter and Root Debris

This part of carbon content accounts for a small proportion of the entire forest ecosystem, but it is also a carbon pool that cannot be ignored. Slowing down its sedimentation and decomposition also plays a certain role in the carbon sequestration of forest ecosystems.

Forest Soil

This is the largest carbon pool in the forest ecosystem. The soil carbon content varies greatly in different forests. In the northern forests, forest soils account for 84% of total carbon; in temperate forest soils, carbon accounts for 62.9% of total carbon; in tropical forests, in soils.

The carbon content accounts for half of the carbon stock of the entire tropical forest ecosystem. The carbon content of the global forest soil is 660-927 Gt, which is 2 to 3 times higher than that of the forest ecosystem.

Many scholars at home and abroad have recognized the important role of forest soil carbon pools and have studied them. Studying soil carbon pools and their carbon cycle and global changes has become a new development direction in soil science.

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Global Carbon Pool

Carbon is one of the main elements in living matter and an important part of organic matter. To sum up, there are four major carbon pools on the planet, namely the atmospheric carbon pool, the ocean carbon pool, the terrestrial system carbon pool and the lithospheric carbon pool. 

Carbon is constantly changing between major carbon pools such as the atmosphere, land and ocean. 

The carbon in the atmosphere mainly exists in the form of gases such as carbon dioxide and methane, and is mainly carbonate ions in water. 

It is the main component of carbonate rocks and sediments in the lithosphere, and is present in vegetation and soil in the form of various organic or inorganic substances in terrestrial ecosystems.

(1) Atmospheric Carbon Pool

The size of the atmospheric carbon pool is about 720 Gt C, which is the smallest among several large carbon pools. However, it is sufficient to link the carbon pools of the ocean and terrestrial ecosystems.

The carbon content in the atmosphere directly affects the entire Earth system. Material circulation and energy flow. The carbonaceous gases in the atmosphere are mainly carbon dioxide, methane and carbon monoxide.

By measuring the content of these gases in the atmosphere, the size of the atmospheric carbon pool is estimated. Therefore, the amount of carbon in the atmosphere is relative to marine and terrestrial ecosystems.

The easiest to calculate, and the most accurate. Since the carbon dioxide content of these gases is the largest and most important, the concentration of carbon dioxide in the atmosphere can often be regarded as an important indicator of the carbon content in the atmosphere.

(2) Marine Carbon Pool

The ocean has the ability to store and absorb carbon dioxide in the atmosphere. Its soluble inorganic carbon (DIc) content is about 37,400 Gt, which is more than 50 times the carbon content in the atmosphere.

It plays an important role in the global carbon cycle. From the perspective of the millennium, the ocean determines the concentration of carbon dioxide in the atmosphere. 

The carbon dioxide in the atmosphere is constantly exchanged with the surface of the ocean, thus quickly balancing the atmosphere with the surface of the ocean. 

Since about 30-50% of the carbon emissions caused by human movements will be absorbed by the ocean, the ability of the ocean to buffer changes in the concentration of carbon dioxide in the atmosphere is not unlimited.

The magnitude of this capacity depends on the amount of cations that can be formed by rock erosion.

Since the rate of carbon emissions caused by human activities is orders of magnitude larger than the rate at which yang is provided, at the millennium scale, as the concentration of carbon dioxide in the atmosphere continues to rise, the ability of the ocean to absorb carbon dioxide will inevitably decrease.

(3) Terrestrial Ecosystem Carbon Pool

It is estimated that the amount of carbon accumulated in terrestrial ecosystems is around 2 000 Gt. Among them, the amount of carbon accumulated in the soil organic carbon pool is about twice that of the vegetation carbon pool.

From the carbon accumulation of different vegetation types in the world, the carbon accumulation of terrestrial ecosystems mainly occurs in forest areas, and the forest ecosystems are in the earth circle and biosphere.

The bio geochemical process plays an important role of “buffer” and “valve”. About 80% of underground carbon accumulation and about 40% of underground carbon accumulation occur in forest ecosystems, and the rest are mainly stored in cultivated land and wetlands. , tundra, alpine grassland and desert semi-desert; from different climates, carbon accumulation mainly occurs in the tropics, more than 50% of the world’s vegetation carbon and nearly 1/4 of the soil organic carbon storage in tropical forests and savanna.

In the ecosystem, about 15% of vegetation carbon and nearly 18% of soil organic carbon are stored in temperate forests and grasslands, and the remaining part of terrestrial carbon accumulation occurs mainly in northern forests, tundra, wetlands, cultivated land, and desert and semi-desert areas.

In addition, the vegetation carbon pool and the soil organic carbon pool also contain different sub-carbon pools, and their turnaround time is longer or shorter, which forms the “temporarysink”.

For example, elevated carbon dioxide levels accelerate tree growth and form carbon sinks. These trees typically survive for decades to hundreds of years, then decay and decompose, returning to the atmosphere through heterotrophic breathing.

Therefore, the carbon storage and carbon release of natural ecosystems are basically balanced over a longer time scale. Unless the intensity of the terrestrial ecosystem carbon pool is increased, any carbon sink will be balanced by carbon sources sooner or later.

(4) Lithosphere Carbon Pool

Among the world’s largest carbon borers, the lithospheric carbon pool is the largest. However, carbon has a very long turnaround time, about one million years, and is a major component of carbonate rocks and sediments in the lithosphere.

Carbon Geochemical Cycle

The geochemical cycle of carbon controls the migration of carbon between surface and near-surface sediments and the atmosphere, the biosphere, and the oceans, and is the primary control of atmospheric carbon dioxide and ocean carbon dioxide.

The sediment contains two forms of carbon: kerogen and carbonate. 

During weathering, kerogen reacts with oxygen to produce carbon dioxide, which is complicated by weathering. 

Magnesium carbonate and calcium carbonate contained in dolomite and calcite minerals are attacked by groundwater to produce calcium ions, magnesium ions and bicarbonate ions which are soluble in water. They are eventually brought into the ocean by groundwater.

In the ocean, marine organisms such as plankton and coral ingest calcium and bicarbonate ions to form the bones and shells of calcium carbonate. After these creatures died, calcium carbonate was deposited on the bottom of the sea and eventually buried.

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Carbon cycle Conclusion

In the biological cycle of carbon, carbon dioxide in the atmosphere is absorbed by plants, converted into organic matter by photosynthesis, and then converted into carbon dioxide by organic respiration and bacterial decomposition into the atmosphere. 

The biological cycle of carbon includes the migration of carbon between animals, plants and the environment.