Soil carbon refers to the carbon component contained within soil organic matter, which is a complex mixture of decomposing plant and animal materials, microbes, and microbial byproducts. This carbon originates primarily from atmospheric carbon dioxide (CO2) absorbed by plants through photosynthesis. When plants and animals die, their remains are broken down by soil organisms, and a portion of this organic material becomes integrated into the soil matrix. Soil organic carbon is a naturally occurring solid form of carbon, distinct from inorganic carbon forms like carbonates.
The Role of Carbon in Soil Health
Carbon directly supports the physical and chemical integrity of soil. Carbon compounds function as a binding agent, helping aggregate individual soil particles into larger, stable clumps. This improves soil structure and creates more pore spaces, allowing for better air circulation and easier root penetration.
Organic matter also acts like a sponge, increasing the soil’s capacity to hold water. Soils rich in organic carbon retain more moisture, making them more resilient during drought and reducing runoff. This directly benefits plants by providing a consistent water supply.
The decomposition of organic matter releases nutrients for plants. As microorganisms break down carbon-rich residues, they cycle nutrients such as nitrogen, phosphorus, and sulfur. This natural nutrient cycling reduces the need for external chemical inputs, contributing to fertile and productive soil.
Soil’s Function in the Global Carbon Cycle
Beyond its benefits to soil health, soil plays a role in the planet’s climate system as a major carbon reservoir. Plants absorb atmospheric carbon dioxide through photosynthesis, converting it into organic compounds. When these plants die, the carbon they contain is transferred to the soil as organic matter.
This process, known as carbon sequestration, stores carbon in the soil. Globally, soils hold approximately 1,500 billion tonnes of carbon in their organic matter, making them the second largest active carbon store after the oceans. This amount is more than twice the carbon stored in all terrestrial vegetation and the atmosphere combined.
While carbon is constantly exchanged between soil and atmosphere, human activities have weakened the soil’s ability to retain carbon, leading to its release. Maintaining and increasing soil carbon levels through appropriate management practices can help reduce atmospheric CO2 concentrations.
Practices That Influence Soil Carbon Levels
Human activities impact the amount of carbon stored in soils, either increasing or depleting these levels. Agricultural practices can enhance soil carbon. No-till farming, for instance, minimizes physical disturbance to the soil, which reduces the oxidation of organic matter and prevents the release of stored carbon into the atmosphere.
Planting cover crops adds biomass to the soil both above and below ground. Their root systems contribute organic material and enhance microbial activity, improving soil structure. Crop rotation, growing different crops in sequence, diversifies organic residues returned to the soil, supporting a wider range of soil microbes and improving carbon sequestration.
Applying organic amendments such as compost and animal manure directly adds carbon-rich material to the soil. These amendments enhance soil organic carbon density and promote the stability of soil aggregates. Combining mineral fertilizers with manure can lead to increased soil organic carbon sequestration.
Conversely, some practices reduce soil carbon. Intensive tillage breaks up soil aggregates, exposing organic matter to oxygen and accelerating its decomposition by microbes. This process releases stored carbon as CO2 into the atmosphere. Leaving soil bare, without plant cover, increases its susceptibility to erosion and reduces organic matter input, leading to a decline in soil carbon levels over time.
Deforestation depletes soil carbon stocks. The removal of natural vegetation reduces the input of organic material from roots and plant litter. Cultivated lands following deforestation can lose significant soil organic carbon. Soil erosion, whether by wind or water, physically removes the carbon-rich topsoil layer, leading to a direct loss of stored carbon from the land.
Measuring and Monitoring Soil Carbon
Scientists use various methods to quantify and track soil carbon levels. Direct measurement involves collecting soil core samples from the field. These samples are then analyzed in a laboratory to determine the amount of organic carbon present.
While direct measurements offer high accuracy, they can be labor-intensive and costly for large areas. To overcome these limitations, estimation and modeling techniques are used. Remote sensing technologies, such as satellite imagery and drone-based sensors, capture data on vegetation health, soil characteristics, and topography.
Computer models use this remotely sensed data, combined with ground-truth measurements, to predict soil carbon content across extensive landscapes. These models can generate carbon distribution maps and estimate changes over time, providing a broader understanding of carbon dynamics without requiring widespread physical sampling.