Global Carbon Cycle and Photosynthetic Considerations
The global carbon cycle involves both biological and physical processes. Only the biological components are discussed here. Plants and other photosynthetic organisms assimilate CO2 and eventually convert the CO2 to simple sugars. From simple sugars, plants synthesize a variety of compounds and store them in the form of plant tissue. When herbivores consume plant material, simple sugars are synthesized and transformed to other carbon-based compounds. Predators feeding on the herbivores redigest and again transform the carbon. At each step, some of the carbon returns to the atmosphere in the form of CO2 as a by-product of respiration. The remainder is incorporated and temporarily stored in living biomass. Carbon contained in living organisms is eventually released through decomposition. The rate of release is highly variable and depends on both environmental and biological conditions. In tropical forests, most of the carbon in detritus is rapidly recycled. In drier, cooler regions considerable quantities of carbon are stored in the soil for long periods of time.
Similar global carbon cycle process takes place in aquatic environments. Phytoplankton assimilate carbon dioxide that has diffused into the upper layers of water and dissolved carbonate and bicarbonate ions and convert it to organic sugars. The carbohydrate produced eventually passes through the aquatic food web. Significant portions of carbon, bound as carbonate in shells and exoskeletons of organisms, settle to the lake or ocean floor. This carbon is largely isolated from biotic activity and becomes incorporated into bottom sediments. It may then enter a long-term pool and may not recycle to the atmosphere again until it resurfaces as limestone through geological time.
If such condition where respiration and photosynthesis are not match, carbon either accumulates on land or is freed to the atmosphere. Unfortunately, the global rates of respiration and photosynthesis are neither known nor measured well enough to determine yearly alterations in carbon storage. On the other hand, land usage, for example the clearing of forests for croplands, is relatively well documented, both historically and with satellites, and thus can be used to determine changes in the storage of carbon.
Research has focused on the current and historic releases of carbon due to land usage. The approach we use is based on the fact that much of the carbon stored in trees and soils is released to the atmosphere when forests are cleared and cultivated.
Carbon release occurs speedily with burning; some of it occurs slowly as dead plant material decomposes. When forests regrow on cleared land, by nature they withdraw carbon from the atmosphere and store it again in trees and soils. The difference between the total amount of carbon released to the atmosphere and the total amount withdrawn from the atmosphere sets whether the land is a net source or sink for atmospheric carbon. Eventually, our approach is thus based on two types of data: annual rates of deforestation and the changes in carbon that follow changes in land use.
The total global carbon pool involved in global carbon cycle is estimated to be approximately 55,000Gt (1Gt= 1 billion metric tons). Fossil fuels, the result of ancient photosynthesis, account for approximately 10,000Gt. The ocean contains approximately 35,000Gt, primarily as bicarbonate (HCO3 ) and carbonate ions (CO3 ). Dead organic matter in the ocean contains 1650Gt of carbon. Living biomass in the ocean, primarily phytoplankton, is 3Gt. Terrestrial ecosystems contain approximately 1200Gt as dead organic matter and 550Gt as living matter. The atmosphere holds approximately 740Gt of carbon.
Although global carbon cycle is a combination of both physical and biological processes, photo- synthesis is obviously a central controlling factor. In order to understand the global carbon cycle, we must understand how photosynthetic reactions occur over broad temporal and spatial scales. Human activities have severely altered the global carbon cycle during the past century, with unknown future consequences.