The terrestrial carbon cycle describes the movement of carbon atoms between the land, including living organisms and soil, and the atmosphere. This continuous exchange helps regulate Earth’s climate and supports all life. Carbon is a building block for living things, making its circulation foundational for ecosystem health and global climate stability.
Carbon’s Earthly Homes: Terrestrial Reservoirs
Carbon is stored in terrestrial environments within living biomass, soil organic matter, peatlands, and permafrost. Living biomass, particularly plants and trees, absorbs atmospheric carbon dioxide through photosynthesis, converting it into organic compounds for growth. Forests are notable carbon sinks, holding a substantial portion of the planet’s above-ground carbon. Globally, forests store approximately 861 gigatonnes of carbon in their various components, including branches, leaves, roots, and soils.
A vast amount of carbon is stored in soil organic matter (SOM), derived from the decomposition of plant and animal residues and microbial tissues. SOM can constitute between 1% and 6% of the total mass of topsoil in most upland areas, and it is estimated to contain about 58% carbon. This carbon in the soil is a dynamic pool, constantly undergoing processes of gain and loss, and it plays a role in soil health by improving structure, water retention, and nutrient availability.
Peatlands represent another substantial, long-term terrestrial carbon store. These wetlands are characterized by waterlogged conditions that slow decomposition, allowing organic matter to accumulate as peat. Peat soils contain over 600 gigatonnes of carbon globally, which is approximately 44% of all soil carbon. Similarly, permafrost, perpetually frozen soil, holds a significant amount of organic carbon due to inhibited decomposition. The northern permafrost region stores between 1,460 and 1,600 petagrams of organic carbon, a quantity nearly double the carbon currently in the atmosphere.
The Breath of the Planet: Natural Carbon Exchange Processes
The movement of carbon within the terrestrial environment is driven by several natural processes. Photosynthesis is a primary mechanism for carbon uptake. Plants, algae, and some bacteria absorb atmospheric carbon dioxide, using sunlight to convert it into organic compounds. This process is fundamental to carbon’s entry into terrestrial food webs and reduces atmospheric carbon dioxide, influencing Earth’s climate.
Respiration, from both plants and soil organisms, releases carbon dioxide back into the atmosphere. Plants respire to break down organic matter for energy, releasing carbon dioxide as a byproduct. Soil respiration involves the release of carbon dioxide from various soil organisms as they decompose organic matter. This process contributes significantly to atmospheric carbon, with estimates suggesting soil respiration releases around 77 petagrams of carbon annually, which exceeds current anthropogenic emissions from fossil fuels.
Decomposition occurs when bacteria, fungi, and other decomposers break down dead organic material. This breakdown releases carbon dioxide back into the atmosphere and incorporates some carbon into soil organic matter. In waterlogged conditions, decomposition can also produce methane. This continuous cycle ensures nutrients are recycled within ecosystems, making them available for new plant growth.
Natural disturbances, such as wildfires, also play a role in carbon exchange. Wildfires release large amounts of stored carbon from vegetation and soil into the atmosphere as carbon dioxide and other greenhouse gases. However, in many ecosystems, regrowing plants in burned areas subsequently absorb carbon, often leading to a net neutral effect over time. Volcanic activity is another natural process that can release stored carbon from geological reservoirs into the atmosphere.
Human Footprint: Altering the Terrestrial Carbon Balance
Human activities have significantly altered the natural balance of the terrestrial carbon cycle, primarily through deforestation, land-use change, and agricultural practices. Deforestation involves clearing forests for purposes such as agriculture or logging. When forests are cleared, substantial amounts of carbon stored in trees and soil are released into the atmosphere as carbon dioxide. This immediate release, coupled with reduced carbon absorption capacity, disrupts the natural balance. Tropical deforestation alone contributes approximately 1.7 petagrams of carbon per year to the atmosphere.
Agricultural practices also impact soil carbon levels. Intensive tillage can lead to soil erosion and the loss of soil organic carbon, releasing it into the atmosphere. Conversely, practices like no-till farming, cover cropping, and adding organic soil amendments can help retain or increase carbon in the soil by minimizing disturbance and promoting organic matter accumulation.
The degradation of peatlands is another human-induced alteration. Draining peatlands for agriculture or development exposes long-stored organic matter to oxygen, leading to its decomposition and the release of vast quantities of carbon dioxide and methane. Burning peatlands further exacerbates these emissions. Their degradation accounts for a disproportionately large share of global greenhouse gas emissions, estimated at nearly 3 gigatonnes of carbon dioxide equivalent per year.
These human-driven changes impact the carbon sequestration capacity of terrestrial ecosystems. The reduction of carbon sinks and increased carbon release into the atmosphere contribute to an imbalance in the global carbon cycle. This shift affects natural systems’ ability to regulate atmospheric carbon dioxide levels, influencing broader environmental conditions.