The carbon cycle is the natural process that moves carbon atoms continuously between the atmosphere, oceans, land, and living organisms. This constant exchange maintains the balance of carbon on Earth. Plants are central to this global system, acting as the primary biological bridge that transfers carbon from the atmosphere into the terrestrial environment. They regulate atmospheric carbon dioxide ($\text{CO}_2$) levels through processes that involve both capturing and releasing carbon.
Carbon Capture: The Photosynthesis Engine
Plants use photosynthesis to remove atmospheric carbon dioxide, acting as the world’s most effective natural carbon capture technology. This process occurs in the leaves within chloroplasts, which contain chlorophyll to absorb sunlight. The captured solar energy powers a chemical reaction combining carbon dioxide absorbed from the air and water taken through the roots to produce sugars. Atmospheric $\text{CO}_2$ enters the leaf through microscopic pores called stomata.
Inside the chloroplast, the enzyme RuBisCO initiates the Calvin cycle, converting the gas into three-carbon sugars. These sugars become the plant’s food source and the building blocks for organic compounds like cellulose or starch. The carbon atom from the atmospheric $\text{CO}_2$ is thus “fixed” into a solid, organic form, becoming part of the plant’s physical structure. Plants absorb roughly one-third of the $\text{CO}_2$ that human activities release into the atmosphere each year.
Carbon Release: Plant Respiration
Although plants are net absorbers of carbon, they release a portion of captured $\text{CO}_2$ back into the atmosphere through plant respiration. Like all living organisms, plants require energy to fuel metabolic functions, such as growth and nutrient transport. Respiration converts the stored sugars created during photosynthesis back into usable energy (adenosine triphosphate or ATP). This process breaks down organic carbon compounds, releasing carbon dioxide as a byproduct.
Plant respiration occurs continuously, day and night, though it is often more noticeable when photosynthesis stops. Annually, the carbon released by terrestrial plant respiration is substantial, estimated to be 40–60% of the carbon fixed during photosynthesis. This counter-process illustrates that plants are both a sink and a source of atmospheric carbon.
Long-Term Storage: Biomass and Soil Sequestration
The carbon not immediately used or respired enters two major pools for long-term storage: the plant’s biomass and the soil. Carbon stored in living plant structures, particularly in the woody tissues of trees, can be sequestered for decades or even centuries. This includes the trunk, branches, and deep root systems, which collectively make forests the most significant terrestrial carbon sinks due to their massive biomass.
The soil is the second, and largest, terrestrial carbon reservoir, holding approximately three times more carbon than is found in all living plants and animals. Plants transfer carbon to the soil through two primary mechanisms.
As plants shed leaves, die, or turn over their roots, this dead organic matter is deposited on or in the soil. Microorganisms then break down this litter, creating stable, long-lasting carbon compounds known as soil organic matter.
Additionally, living roots actively release carbon compounds, called exudates, into the soil to feed symbiotic soil microbes. These microbes convert the root exudates into stable soil carbon.
The Global Balance: Plants as Climate Regulators
The net effect of plant activities—the capture, release, and storage of carbon—positions them as powerful regulators of the global climate. Terrestrial ecosystems are considered a “net carbon sink,” meaning they absorb more carbon from the atmosphere than they release through respiration and decomposition combined. This net uptake helps to offset a significant portion of the $\text{CO}_2$ emissions generated by human activity. Forests, especially tropical and boreal forests, are the largest terrestrial sinks. Deforestation severely diminishes this regulatory capacity, releasing stored carbon back into the atmosphere and reducing the planet’s ability to mitigate rising $\text{CO}_2$ levels.