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Soil carbon is the solid stored in global . This includes both soil organic matter and inorganic carbon as carbonate minerals. It is vital to the soil capacity in our ecosystem. Soil carbon is a in regard to the global , playing a role in , climate change mitigation, and constructing global . play an important role in breaking down carbon in the soil. Changes in their activity due to rising temperatures could possibly influence and even contribute to climate change. Human activities have caused a massive loss of soil organic carbon; however, there is potential for human activity to intentionally divert carbon back to the soil.

Overview
Soil carbon is present in two forms: inorganic and organic. Soil inorganic carbon consists of mineral forms of carbon, either from of , or from reaction of soil minerals with atmospheric CO2. Carbonate minerals are the dominant form of soil carbon in . Soil organic carbon is present as soil organic matter. It includes relatively available carbon as fresh plant remains and relatively inert carbon in materials derived from plant remains: and . Soil carbon is critical for terrestrial organisms and is one of the most important carbon pools, with the majority of carbon stored in forests. Biotic factors include photosynthetic assimilation of fixed carbon, decomposition of biomass, and the activities of diverse communities of soil organisms. Climate, landscape dynamics, fires, and mineralogy are some of the important abiotic factors. Anthropogenic factors have increasingly changed soil carbon distributions. For example, anthropogenic fires destroy the top layer of the soil, exposing soil to excessive oxidation. Industrial nitrogen fixation, agricultural practices, and land use and other management practices are some anthropogenic activities that have altered soil carbon.

Global carbon cycle
Soil carbon distribution and accumulation arises from complex and dynamic processes influenced by biotic, abiotic, and anthropogenic factors. Although exact quantities are difficult to measure, soil carbon has been lost through land use changes, deforestation, and agricultural practices. While many environmental factors affect the total stored carbon in terrestrial ecosystems, in general, primary production and decomposition are the main drivers in balancing the total amount of stored carbon on land.
(2025). 9780128146088, Academic press, an imprint of Elsevier.
Atmospheric CO2 is taken up by photosynthetic organisms and stored as organic matter in terrestrial ecosystems.

Although exact quantities are difficult to measure, human activities have caused substantial losses of soil organic carbon.

(2025). 9780691146348, Princeton University Press. .
For example, the destruction of rainforests has resulted in a significant release of stored carbon from terrestrial ecosystems into the atmosphere as carbon dioxide (CO2). Of the 2,700 Gt of carbon stored in soils worldwide, 1550 GtC is organic and 950 GtC is inorganic carbon, which is approximately three times greater than the current atmospheric carbon and 240 times higher compared with the current annual fossil fuel emission. The balance of soil carbon is held in and wetlands (150 GtC), and in at the soil surface (50 GtC). This compares to 780 GtC in the , and 600 GtC in . The oceanic pool of carbon accounts for 38,200 GtC.

About 60 GtC/yr accumulates in the soil. This 60 GtC/yr is the balance of 120 GtC/yr by terrestrial plant reduced by 60 GtC/yr of plant respiration. An equivalent 60 GtC/yr is respired from soil, joining the 60 GtC/yr plant respiration to return to the atmosphere.

Impacts of climate change on soil
Climate change is a leading factor in as well as in its development of chemical and physical properties. Therefore, changes in climate will impact the soil in many ways that are still are not fully understood, but changes in fertility, , . , SOC, sequestration, aggregation etc. are predicted. In 1996, Least-Limiting Water Range (LLWR) was created to quantify the physical changes in soil. This indicator measures changes in available water capacity, , air filed porosity, soil strength, and oxygen diffusion rate. Changes in LLWR are known to alter ecosystems but it's to a different capacity in each region. For example, in polar regions where temperatures are more susceptible to drastic changes, melting permafrost can expose more land which leads to higher rates of and eventually, higher carbon absorption. In contrast, tropical environments experience worsening soil quality because soil aggregation levels decrease with higher temperatures.

Soil also has carbon sequestration abilities where is fixed in the soil by plant uptakes. This accounts for the majority of the soil organic matter (SOM) in the ground, and creates a large storage pool (around 1500 Pg) for carbon in just the first few meters of soil and 20-40% of that organic carbon has a residence life exceeding 100 years.

Organic carbon
Soil organic carbon is divided between living and dead derived from biomass. Together these comprise the soil food web, with the living component sustained by the biotic material component. Soil biota includes , , , , and different .

resulting from is the major source of soil organic carbon. Plant materials, with high in and , are decomposed and the not-respired carbon is retained as . Cellulose and starches readily degrade, resulting in short residence times. More persistent forms of organic C include lignin, humus, organic matter encapsulated in soil aggregates, and charcoal. These resist alteration and have long residence times.

Soil organic carbon tends to be concentrated in the topsoil. ranges from 0.5% to 3.0% organic carbon for most soils. Soils with less than 0.5% organic C are mostly limited to areas. Soils containing greater than 12–18% organic carbon are generally classified as . High levels of organic C supporting , , , and .

Fire derived forms of carbon are present in most soils as unweathered and weathered .

(2025). 9780415704151, Routledge.
Soil organic carbon is typically 5–50% derived from char, with levels above 50% encountered in , , and soils.

are another source of soil carbon. 5–20% of the total plant carbon fixed during photosynthesis is supplied as root exudates in support of rhizospheric mutualistic biota. Microbial populations are typically higher in the rhizosphere than in adjacent .

SOC and other soil properties
Soil organic carbon (SOC) concentrations in sandy soils influence soil bulk density which decreases with an increase in SOC. Bulk density is important for calculating SOC stocks and higher SOC concentrations increase SOC stocks but the effect will be somewhat reduced by the decrease in bulk density. Soil organic carbon increased the cation exchange capacity (CEC), a measure of , in sandy soils. SOC was higher in sandy soils with higher pH. found that up to 76% of the variation in CEC was caused by SOC, and up to 95% of variation in CEC was attributed to SOC and pH. Soil organic matter and specific surface area has been shown to account for 97% of variation in CEC whereas content accounts for 58%. Soil organic carbon increased with an increase in silt and clay content. The silt and clay size fractions have the ability to protect SOC in soil aggregates. When organic matter decomposes, the organic matter binds with silt and clay forming aggregates. Soil organic carbon is higher in silt and clay sized fractions than in sand sized fractions, and is generally highest in the clay sized fractions.

Soil health
Organic carbon is vital to soil capacity to provide ecosystem services. The condition of this capacity is termed , a term that communicates the value of understanding soil as a living system as opposed to an abiotic component. Specific carbon related benchmarks used to evaluate soil health include CO2 release, humus levels, and microbial metabolic activity.

Losses
The exchange of carbon between soils and the atmosphere is a significant part of the world carbon cycle. Carbon, as it relates to the organic matter of soils, is a major component of soil and health. Several factors affect the variation that exists in soil organic matter and soil carbon; the most significant has, in contemporary times, been the influence of humans and agricultural systems.

Although exact quantities are difficult to measure, human activities have caused massive losses of soil organic carbon. First was the use of , which removes soil cover and leads to immediate and continuing losses of soil organic carbon. and both expose soil organic matter to oxygen and oxidation. In the , , , and the , subsidence of lands from oxidation has been severe as a result of tillage and drainage. management that exposes soil (through either excessive or insufficient recovery periods) can also cause losses of soil organic carbon.

Managing soil carbon
Natural variations in soil carbon occur as a result of , , , time, and relief.
(2025). 9780195515503, Oxford University Press. .
The greatest contemporary influence has been that of humans; for example, carbon in soils may historically have been twice the present range that is typically 1.6–4.6%.
(2025). 9780195517620, Oxford University Press. .

It has long been encouraged that farmers adjust practices to maintain or increase the organic component in the soil. On one hand, practices that hasten oxidation of carbon (such as or over-cultivation) are discouraged; on the other hand, incorporation of organic material (such as in ) has been encouraged. Increasing soil carbon is not a straightforward matter; it is made complex by the relative activity of soil biota, which can consume and release carbon and are made more active by the addition of .

Data available on soil organic carbon
Europe
The most homogeneous and comprehensive data on the organic carbon/matter content of soils remain those that can be extracted and/or derived from the European Soil Database in combination with associated databases on , climate, and . The modelled data refer to carbon content (%) in the surface horizon of soils in Europe. In an inventory on available national datasets, seven member states of the European Union have available datasets on organic carbon. In the article "Estimating soil organic carbon in Europe based on data collected through a European network" ( Ecological Indicators 24, pp. 439–450), a comparison of national data with modelled data is performed. The LUCAS soil organic carbon data are measured surveyed points and the aggregated results at regional level show important findings. Finally, a new proposed model for estimation of soil organic carbon in agricultural soils has estimated current top SOC stock of 17.63 Gt in EU agricultural soils. This modelling framework has been updated by integrating the soil erosion component to estimate the lateral carbon fluxes. Currently, the EU-ORCaSA project is developing a multi-ecosystem framework for measuring, reporting and verification of soil organic carbon changes to support policy making.

Managing for catchment health
Much of the contemporary literature on soil carbon relates to its role, or potential, as an atmospheric to offset . Despite this emphasis, a much wider range of soil and health aspects are improved as soil carbon is increased. These benefits are difficult to quantify, due to the complexity of systems and the interpretation of what constitutes soil health; nonetheless, several benefits are proposed in the following points:

  • Reduced , : increased soil aggregate stability means greater resistance to erosion; mass movement is less likely when soils are able to retain structural strength under greater moisture levels.
  • Greater productivity: healthier and more productive soils can contribute to positive socio-economic circumstances.
  • Cleaner , nutrients and : nutrients and sediment tend to be retained by the soil rather than leach or wash off, and are so kept from waterways.
  • : greater soil water holding capacity reduces overland flow and recharge to ; the water saved and held by the soil remains available for use by plants.
  • Climate change: Soils have the ability to retain carbon that may otherwise exist as atmospheric CO2 and contribute to .
  • Greater : soil organic matter contributes to the health of soil flora and, accordingly, the natural links with biodiversity in the greater .

Forest soils
soils constitute a large pool of carbon. Anthropogenic activities such as cause releases of carbon from this pool, which may significantly increase the concentration of (GHG) in the atmosphere.IPCC. 2000. Land use, land-use change, and forestry. IPCC Special Report. United Kingdom, Cambridge University Press. Under the United Nations Framework Convention on Climate Change (UNFCCC), countries must estimate and report GHG emissions and removals, including changes in carbon stocks in all five pools (above- and below-ground biomass, dead wood, litter, and soil carbon) and associated emissions and removals from land use, land-use change and forestry activities, according to the Intergovernmental Panel on Climate Change's good practice guidance.IPCC. 2003. Good practice guidance for land use, land-use change and forestry. Kanagawa, Japan, National Greenhouse Gas Inventories Programme.IPCC. 2006. Guidelines for national greenhouse gas inventories. Kanagawa, Japan, National Greenhouse Gas Inventories Programme. Tropical deforestation represents nearly 25% of total anthropogenic GHG emissions worldwide. Deforestation, forest degradation, and changes in land management practices can cause releases of carbon from soil to the atmosphere. For these reasons, reliable estimates of soil organic carbon stock and stock changes are needed for Reducing emissions from deforestation and forest degradation and GHG reporting under the UNFCCC.

The government of —together with the Food and Agriculture Organization of the United Nations and the financial support of the government of —have implemented a forest soil carbon monitoring programFAO. 2012. "Soil carbon monitoring using surveys and modelling: General description and application in the United Republic of Tanzania". FAO Forestry Paper 168 Rome. Available at: http://www.fao.org/docrep/015/i2793e/i2793e00.htm to estimate soil carbon stock, using both survey and modelling-based methods.

West Africa has experienced significant loss of forest that contains high levels of soil organic carbon. This is mostly due to expansion of small scale, non-mechanized agriculture using burning as a form of land clearance

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