Soil carbon is the solid form of carbon found in soils around the world. This includes soil organic matter, which comes from plants and animals, and inorganic carbon, such as carbonate minerals. Soil carbon helps soil perform important functions in ecosystems. It acts as a carbon sink, meaning it stores carbon from the atmosphere. This process is important for how carbon moves through the environment, reducing the effects of climate change, and helping scientists create models to study global climate patterns. Microorganisms in the soil help break down carbon. If their activity changes because of rising temperatures, it could affect climate change. Human activities have changed how carbon is distributed in soil. These activities have caused a large loss of soil organic carbon. However, people can take actions to return carbon to the soil intentionally.

Overview

Soil carbon exists in two types: inorganic and organic. Inorganic soil carbon comes from minerals, which form either by the breakdown of original rock or soil material or through reactions between soil minerals and carbon dioxide from the air. In desert areas, carbonate minerals are the main type of inorganic soil carbon. Organic soil carbon is found in soil organic matter. This includes carbon from fresh plant material that is easily used by organisms and carbon from older plant materials that are harder to break down, such as humus and charcoal. Soil carbon is essential for life on land and is one of the largest storage areas for carbon, with most of it found in forests. Living factors include the process of plants absorbing carbon through photosynthesis, the breakdown of plant and animal material, and the work of various soil organisms. Non-living factors include climate, changes in land shapes, fires, and the types of minerals in the soil. For example, human-caused fires can remove the top layer of soil, making it more likely to lose carbon through oxidation. Activities like industrial nitrogen production, farming, and changes in how land is used have affected soil carbon levels.

Global carbon cycle

Soil carbon distribution and accumulation result from complex and changing processes influenced by living things, non-living things, and human activities. Many environmental factors affect the total stored carbon in land ecosystems, but photosynthesis, respiration, and decomposition are the main processes that balance the amount of stored carbon on land. Atmospheric carbon dioxide is absorbed by photosynthetic organisms and stored as organic matter in terrestrial ecosystems. Microbes, fungi, plant roots, and other soil life release carbon dioxide back into the atmosphere through respiration. Additionally, microbes and saprophytic fungi break down organic matter in the soil—some carbon is released into the atmosphere, while some remains in the soil as humus from microbial waste. These natural processes are the foundation of the carbon cycle.

Of the 2,700 gigatons of carbon stored in soils worldwide, 1,550 gigatons are organic, and 950 gigatons are inorganic. This amount is about three times greater than the current atmospheric carbon and 240 times larger than the current annual fossil fuel emissions. The balance of soil carbon is found in peat and wetlands (150 gigatons) and in plant litter on the soil surface (50 gigatons). This compares to 780 gigatons of carbon in the atmosphere and 600 gigatons in all living organisms. The ocean holds 38,200 gigatons of carbon.

Approximately 60 gigatons of carbon per year accumulate in the soil. This amount is the result of 120 gigatons of carbon per year taken in from the atmosphere by plant photosynthesis, minus 60 gigatons of carbon per year released through plant respiration. An equal amount of 60 gigatons of carbon per year is released from the soil through respiration, joining the 60 gigatons of plant respiration to return to the atmosphere.

Forms of Carbon in Soil

In dry and semi-dry areas, most soil carbon is found as inorganic carbonates, such as calcite and dolomite. These carbonates can come from the bedrock or form in the soil itself. In wetter areas, most soil carbon is organic. Organic carbon is part of soil organic matter, which contains about 50% carbon by dry weight. The rest is mostly hydrogen, oxygen, nitrogen, and sulfur. Organic matter comes from plant roots, fallen leaves, crop residue, and animal and microbial remains. As these materials break down, they form smaller molecules that can be stored in the soil.

Soil organic carbon is split between living soil organisms and dead organic material from plants and animals. These together make up the soil food web, with living organisms relying on the dead material for energy. Soil biota includes earthworms, nematodes, protozoa, fungi, bacteria, and arthropods.

Decaying plant material is the main source of soil organic carbon. Plants with tough cell walls, like those high in cellulose and lignin, break down slowly, leaving carbon stored as humus. Cellulose and starches break down quickly, so they stay in the soil for shorter periods. More lasting forms of organic carbon include lignin, humus, and carbon trapped in soil aggregates or charcoal. These forms resist breakdown and remain in the soil for long periods.

Soil organic carbon is most common in the topsoil. Most upland soils have 0.5% to 3.0% organic carbon. Soils with less than 0.5% are usually found in deserts. Soils with more than 12–18% organic carbon are classified as organic soils. High organic carbon levels are found in wetlands, areas with flood deposits, fire-prone regions, and areas affected by human activity.

Charcoal from fires is present in most soils, either as unbroken pieces or as weathered black carbon. Soil organic carbon is often 5–50% from char, with some soils like mollisols, chernozems, and terra preta having more than 50% from char.

Root exudates are another source of soil carbon. Plants release 5–20% of their photosynthesis-produced carbon through roots to support helpful soil organisms. These exudates provide carbon to fungi in exchange for nutrients. Microbial life is more active near plant roots than in other parts of the soil.

Organic carbon is important for soil’s ability to support ecosystems. Soil health refers to how well soil functions as a living system. Measures like carbon dioxide release, humus levels, and microbial activity are used to assess soil health.

In sandy soils, organic carbon affects soil density, which decreases as organic carbon increases. Soil density is important for calculating how much carbon is stored. Organic carbon also increases soil fertility by boosting cation exchange capacity (CEC). Sandy soils with higher pH have more organic carbon. Studies show that up to 76% of CEC differences are linked to organic carbon, and up to 95% are linked to organic carbon and pH. Soil organic matter and surface area explain 97% of CEC variation, while clay content explains 58%. Organic carbon increases with more silt and clay in the soil. Silt and clay particles help protect organic carbon by forming soil aggregates. When organic matter breaks down, it binds with silt and clay, creating stable soil structures. Organic carbon is higher in silt and clay particles than in sand, and it is highest in clay-sized particles.

Soil carbon and climate change

Climate change affects how soil forms because changes in temperature and moisture levels alter the soil's chemical and physical properties. These changes can impact soil fertility, salinity, moisture, temperature, soil organic carbon (SOC), sequestration, and aggregation. Forest soils hold a large amount of carbon. Human activities, such as deforestation, release carbon from these soils into the atmosphere, increasing greenhouse gas (GHG) levels. Scientists are studying how climate and soil particles interact.

Soil can store carbon by absorbing carbon dioxide through plants. This process creates most of the soil organic matter (SOM) in the ground, storing about 1,500 petagrams of carbon in the top few meters of soil. Around 20-40% of this carbon stays in the soil for more than 100 years. Researchers are looking for ways to restore carbon to soils through farming and other practices.

The exchange of carbon between soil and the atmosphere is a key part of the global carbon cycle. Soil carbon is important for soil and catchment health, as well as carbon storage. Many factors influence soil organic matter and carbon, but human activities and agriculture have been the most significant in recent times.

Measuring soil carbon levels is difficult, but human activities have caused large losses of soil organic carbon through land use changes, such as deforestation and farming. For example, cutting down rainforests releases stored carbon into the atmosphere as carbon dioxide (CO₂). Fires remove soil cover and cause immediate and ongoing carbon loss. Tillage and drainage expose soil to oxygen, speeding up carbon loss. In places like the Netherlands, East Anglia, Florida, and the California Delta, peatland subsidence has been severe due to these practices. Poor grazing management, which exposes soil for too long or too short a time, also causes carbon loss. Farmers are encouraged to adjust practices to keep or increase soil organic matter. Practices that speed up carbon oxidation, like burning crop stubble or over-cultivation, are discouraged. Adding organic material, such as manure, is encouraged. Increasing soil carbon is complex because soil organisms can consume or release carbon, and their activity increases with nitrogen fertilizers.

Many studies focus on soil carbon's role in reducing atmospheric carbon to fight climate change. However, increasing soil carbon also improves soil and catchment health in other ways. These benefits are hard to measure due to the complexity of natural systems, but some include:

• Less erosion and sedimentation: Stronger soil structure reduces erosion and landslides.
• Greater productivity: Healthier soils support better crop growth and economic benefits.
• Cleaner water: Soils hold nutrients and sediment, preventing them from polluting waterways.
• Better water balance: Soils that hold more water reduce runoff and help groundwater recharge.
• Climate change: Soils store carbon that would otherwise be atmospheric CO₂.
• Greater biodiversity: Healthy soil supports plant and animal life.

Under the United Nations Framework Convention on Climate Change (UNFCCC), countries must report GHG emissions and changes in carbon stocks in five pools (above- and below-ground biomass, dead wood, litter, and soil) and related emissions from land use changes. Tropical deforestation contributes about 25% of global human-caused GHG emissions. Deforestation, forest damage, and poor land management release carbon from soils into the atmosphere. Accurate estimates of soil carbon are needed to track emissions and support climate policies.

In 1996, the Least-Limiting Water Range (LLWR) was developed to measure physical soil changes, such as water capacity, structure, and oxygen levels. LLWR changes affect ecosystems differently in each region. For example, in polar areas, melting permafrost can increase plant growth and carbon absorption. In contrast, tropical regions often see worse soil quality due to reduced aggregation from high temperatures.

Tanzania, with help from the United Nations Food and Agriculture Organization and Finland, has started a program to monitor soil carbon using surveys and models. West Africa has lost much forest with high soil carbon due to farming that uses burning to clear land. Australian agricultural soils may have had twice as much carbon as they do now, which is typically 1.6–4.6%.

The most detailed data on European soil organic carbon comes from the European Soil Database combined with land cover, climate, and topography data. Modelled data show carbon content in the top soil layer of European soils. Seven European Union countries have national organic carbon datasets. A study in Ecological Indicators compared these datasets with modelled data. The LUCAS soil survey results show important findings at the regional level. A new model estimates 17.63 gigatons of soil organic carbon in EU agricultural soils, including erosion effects. The EU-ORCaSA project is creating a system to measure, report, and verify soil carbon changes to support policies.