What is the Carbon Cycle?

The carbon cycle refers to the phenomenon that carbon elements are exchanged in the earth's biosphere, lithosphere, hydrosphere, and atmosphere, and they continue to cycle with the movement of the earth. The carbon cycle in the biosphere is mainly manifested in the absorption of carbon dioxide from the atmosphere by green plants, the conversion of photosynthesis into glucose and release of oxygen with the participation of water, and the organism uses glucose to synthesize other organic compounds. Organic compounds pass through the food chain and become part of other organisms such as animals and bacteria. A part of the carbohydrates in the body is oxidized into carbon dioxide and water by respiration as the energy source metabolized by the organism, and releases the energy stored therein.

The two largest carbon pools on the planet are lithosphere and fossil fuels, which contain about 99.9% of the total carbon on the planet. The carbon activity in these two pools is slow and actually plays a role

The basic process of the carbon cycle in nature 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 living things and the atmosphere

Green plants obtain carbon dioxide from the air, convert them into glucose through photosynthesis, and then synthesize them into carbon compounds in the plant body. After passing through the food chain, they become carbon compounds in the animal body. Plants and animals

The role of forest ecosystems

The role of forest ecosystems in the carbon cycle When people realized that the rise of greenhouse gases, especially carbon dioxide, would cause global warming, which brought about a series of serious ecological and environmental problems, research on the carbon cycle was launched. As a large carbon sink that absorbs carbon dioxide and releases oxygen, forest ecosystems play a very important role in the carbon cycle. The global forest area is 4.161 billion hectares, of which tropical, temperate and cold regions account for 32.9%, 24.9% and 42.1%, respectively. Above-ground carbon in the global terrestrial ecosystem is 562 Gt, and the carbon content in the above-ground forest ecosystem is 483 Gt, accounting for 86%. The carbon content of the lower part of the global terrestrial ecosystem is 1,272 Gt, while the carbon content of the lower part of the forest is 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:

1.Biomass

A large amount of carbon is stored in the biomass of the forest ecosystem. For example, if the carbon content of the plant biomass is 45% to 50%, the biomass of the entire forest ecosystem is almost half of the carbon content. The biomass of a forest is most closely related to its growth stage. Generally, forests can be divided into young forest, middle-age forest, near-mature forest, mature forest / over-mature forest according to their age. It is the largest in mature forests and mature forests / overripe forests, in which the carbon accumulation rate is the largest in middle-aged forest ecosystems, while mature forests / overripe forests have basically stopped growing because of their biomass, and their carbon absorption and release are basically balanced. Estimating the potential of carbon uptake from the age structure of forests is a major aspect that determines the function of carbon sinks in forest ecosystems. The structure of forests in China is mainly young forests and middle-aged forests, so plants in China's forest ecosystems have great potential for fixing atmospheric carbon. According to Wang Xiaoke's estimates, the total potential carbon storage of plants in China's forest ecosystem is 8.41Pg, and the current actual total carbon storage is only 44.3% of the potential total carbon storage of plants. Therefore, if China's forest ecosystem is effectively protected, it will be an important carbon sink in China.

Forest products

The amount of carbon sequestered by forest products in forest ecosystems is a highly variable factor. General forest products can be divided into short-term products and long-term products according to their service life. Such as fuel wood, pulp wood and so on are short-term products, while plywood and construction wood are long-term products. To a large extent, the life of forest products also determines the carbon sink function of forest ecosystems. Long-lived forest products can delay carbon release and alleviate the increase in global atmospheric carbon concentration. Generally speaking, the service life of durable forest products can reach 100 to 200a. In such a long time, carbon can be completely achieved through reforestation Virtuous circle. Therefore, forest products that are durable and have a long service life should be processed as much as possible.

3. Plant litter and root debris

Although this part of the carbon content in the entire forest ecosystem is small, it is also a carbon pool that cannot be ignored. Slowing down its precipitation and decomposition also plays a certain role in the carbon sequestration of the forest ecosystem.

4. Forest soil

This is the largest carbon pool in the forest ecosystem. The soil carbon content of different forests is very different. In the northern forests, the forest soil accounts for 84% of the total carbon content; in temperate forest soils, carbon accounts for 62.9% of its total carbon content; in tropical forests, Carbon content accounts for half of the carbon storage of the entire tropical forest ecosystem. The carbon content of forest soils worldwide is 660 to 927 Gt, which is two to three times the above-ground level of forest ecosystems. Many scholars at home and abroad have recognized the important role of forest soil carbon pools and have carried out research on them. Research on soil carbon pools, their carbon cycle and global change has become a new development direction of soil science. ^[3]

5.Global carbon stocks

Carbon is one of the main elements in living matter and an important part of organic matter. In summary, there are four major carbon pools on the earth, namely atmospheric carbon pools, marine carbon pools, terrestrial ten-state system carbon pools, and lithosphere carbon pools. Carbon is continuously cyclically changed among major carbon pools such as the atmosphere, land and sea. Carbon in the atmosphere mainly exists in the form of gases such as carbon dioxide and methane, and mainly carbonate ions in water. In the lithosphere, it is the main component of carbonate rocks and sediments, and in terrestrial ecosystems it exists in vegetation and soil in the form of various organic or inorganic substances.

(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, but it is sufficient to connect the bonds and bridges of the ocean and terrestrial ecosystem carbon pools. The amount of carbon in the atmosphere directly affects the entire earth system. Material circulation and energy flow. The carbon-containing gases in the atmosphere mainly include carbon dioxide, methane and carbon monoxide. By measuring the content of these gases in the atmosphere, the size of the atmospheric carbon pool can be calculated. Therefore, compared to marine and terrestrial ecosystems, the amount of carbon in the atmosphere is The easiest to calculate and the most accurate. Since the carbon dioxide content in these gases is the largest and the most important, the carbon dioxide concentration 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 37400 Gt, which is more than 50 times the carbon content in the atmosphere. It plays a very important role in the global carbon cycle. From a millennial perspective, the ocean determines the concentration of carbon dioxide in the atmosphere. Carbon dioxide in the atmosphere is constantly exchanging with the surface of the ocean, so that the atmosphere and the surface of the ocean quickly reach equilibrium. About 30-50% of the carbon emissions caused by human speech will be absorbed by the ocean, but the ability of the ocean to buffer changes in the concentration of carbon dioxide in the atmosphere is not unlimited. The size of this ability depends on the number of cations that can be formed by rock erosion. The rate of carbon emissions caused by human activities is orders of magnitude greater than the rate provided by Yangliyu. Therefore, on 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 gradually decrease. .

(3) Terrestrial ecosystem carbon pool

It is estimated that the amount of carbon accumulated in terrestrial ecosystems is about 2,000 Gt. Among them, the amount of carbon accumulated in soil organic carbon pools is about twice that of vegetation carbon pools. From the perspective of carbon accumulation in different types of vegetation in the world, carbon accumulation in terrestrial ecosystems mainly occurs in forest areas, and forest ecosystems in the geosphere and biosphere The biogeochemical process plays an important "buffer" and "valve" function. About 80% of underground carbon accumulation and about 40% of underground carbon accumulation occur in forest ecosystems, and the remaining part is mainly stored in cultivated land and wetlands. , Tundra, alpine grasslands and desert semi-deserts; from the perspective of different climates, carbon accumulation mainly occurs in tropical areas. More than 50% of the global vegetation carbon and nearly 1/4 of the soil organic carbon are stored in tropical forests and savannas. Ecosystem, about 15% of vegetation carbon and nearly 18% of soil organic carbon are stored in temperate forests and grasslands, and the remaining land carbon accumulation mainly occurs in northern forests, tundra, wetlands, arable land, and desert and semi-desert regions .

In addition, vegetation carbon pools and silt organic carbon pools also contain different sub-carbon pools, and their turnaround times are long or short, which has formed a so-called "temporarysink". For example, increased carbon dioxide concentration accelerates the growth of trees and forms carbon sinks. These trees generally survive for decades to hundreds of years, then decay and decompose, and return to the atmosphere through heterotrophic breathing. Therefore, the carbon accumulation and carbon release of natural ecosystems are basically balanced on a longer time scale. Unless the intensity of the terrestrial ecosystem carbon pool increases, any carbon sink will sooner or later be balanced by the carbon source.

(4) Lithosphere carbon pool

Among the world's largest carbon monoxides, the lithosphere carbon pool is the largest. However, the turnover time of carbon in it is extremely long, more than about one million years, and it is the main component of carbonate rocks and sediments in the lithosphere. ^[4]

Geochemical cycle of carbon

The geochemical cycle of carbon controls the migration of carbon between surface or near-surface sediments and the atmosphere, biosphere, and ocean, and is the most important control of atmospheric carbon dioxide and marine carbon dioxide.

The sediment contains two forms of carbon: kerogen and carbonate. During the weathering process, kerogen reacts with oxygen to produce carbon dioxide, but the weathering of carbonates is complicated. Magnesium carbonate and calcium carbonate contained in the dolomite and calcite minerals are attacked by groundwater to produce calcium, magnesium, and bicarbonate ions that are soluble in water. They are eventually carried into the ocean by groundwater.

In the ocean, plankton and marine organisms such as corals ingest calcium and bicarbonate ions to form calcium carbonate skeletons and shells. After these creatures died, calcium carbonate was deposited on the ocean floor and eventually buried.

Carbon biological cycle

In the biological cycle of carbon, after the carbon dioxide in the atmosphere is absorbed by plants, it is converted into organic matter through photosynthesis, and then converted from organic matter to carbon dioxide and enters the atmosphere through biological respiration and bacterial decomposition. The biological cycle of carbon includes the migration of carbon between animals, plants and the environment.