The organic carbon cycle traces the path of carbon atoms through Earth’s living systems. It is the biological component of the larger global carbon cycle, focusing on how carbon is incorporated into organisms, transferred between them, and returned to the non-living environment. This cycle is driven by the processes of life.
Carbon Fixation in Ecosystems
The organic carbon cycle begins with carbon fixation, the conversion of inorganic carbon dioxide (CO2) into organic compounds through photosynthesis. Organisms that perform this process, like plants and algae, are called autotrophs. They capture atmospheric carbon and transform it into energy-rich molecules like sugars, forming the base of most ecosystems.
In terrestrial ecosystems, plants are the primary agents of carbon fixation. Forests, grasslands, and other vegetated areas pull large amounts of CO2 from the air, incorporating it into their structure. Terrestrial photosynthesis converts roughly 250 billion tons of carbon dioxide annually, making this carbon accessible to the rest of the ecosystem.
A similar process occurs in aquatic environments, where microscopic organisms called phytoplankton are the primary carbon fixers. While making up only 1-2% of the planet’s producer biomass, these algae perform about 40% of global carbon fixation. They absorb dissolved CO2 from the water, turning it into organic matter that sustains marine food webs.
Transfer Through Food Webs
Once carbon is fixed into the biomass of producers like plants and phytoplankton, it enters the food web through consumption. An organism’s position in this web is its trophic level, with producers forming the first level.
When a primary consumer (herbivore) eats a plant, it ingests the plant’s carbon compounds. The herbivore uses this carbon to build its own tissues and for energy. This moves the carbon to the second trophic level, and the process continues as carnivores or omnivores consume the herbivore.
At each step up the trophic levels, much of the carbon is lost. Only about 10% of the biomass from one level is incorporated into the next, as the rest is used for life processes. This inefficiency limits most food chains to four or five trophic levels.
Return to the Environment
Carbon continuously returns to the non-living environment, primarily the atmosphere and soil. Two main biological processes drive this return: respiration and decomposition. These pathways allow the carbon to be used again by producers.
All living organisms perform cellular respiration, breaking down organic compounds like sugars to release energy. A byproduct of this process is carbon dioxide, which is released back into the atmosphere. This represents a rapid return of carbon to the atmospheric pool.
The second pathway for carbon’s return is decomposition. When organisms die, their organic matter is broken down by decomposers like bacteria and fungi. These microorganisms consume the material, releasing CO2 through respiration and returning organic compounds to the soil, making them available for new plant growth.
Long-Term Storage of Organic Carbon
Not all organic carbon immediately returns to the atmosphere. A portion can be stored for extended periods, from decades to millions of years, in natural reservoirs. This long-term removal of carbon from the active cycle is known as sequestration.
Soil is a major long-term storage site for carbon. As organic matter decomposes, some of it becomes stable compounds that persist for centuries, enhancing soil structure and fertility. Globally, soils hold about three times more carbon than all living plants and animals combined.
Over geological timescales, some organic matter escapes decomposition. When buried under sediment without oxygen, heat and pressure can transform it into fossil fuels like coal, oil, and natural gas. This process traps carbon, removing it from the active cycle for millions of years.
Human Alterations to the Cycle
Human activities since the Industrial Revolution have altered the organic carbon cycle’s natural balance. The primary disruptions are the burning of fossil fuels and changes in land use. Both actions release massive amounts of carbon into the atmosphere far faster than natural processes can remove it.
The combustion of fossil fuels is the largest source of new CO2 in the atmosphere. This process releases carbon that was stored for millions of years in a geological instant. This rapid influx overwhelms the ability of natural sinks, like forests and oceans, to absorb the excess.
Land-use changes, like deforestation and certain agricultural practices, are another factor. Clearing forests reduces the planet’s capacity for carbon fixation. Practices like intensive tillage can also accelerate the decomposition of soil organic matter, releasing stored carbon into the atmosphere.