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Soil carbon is the solid
carbon stored in global
soils. 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
carbon sink in regard to the global
carbon cycle, playing a role in
biogeochemistry,
climate change mitigation, and constructing global
climate models. Natural variation such as organisms and time has affected the management of carbon in the soils. The major influence has been that of human activities which has caused a massive loss of soil organic carbon. An example of human activity includes fire which destroys the top layer of the soil and the soil therefore get exposed to excessive oxidation.
Soil carbon is present in two forms: inorganic and organic. Soil inorganic carbon consists of mineral forms of carbon, either from
weathering of
parent material, or from reaction of soil minerals with atmospheric CO2.
Carbonate minerals are the dominant form of soil carbon in
desert climates. 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:
humus and
charcoal.[1]
Although exact quantities are difficult to measure, human activities have caused substantial losses of soil organic carbon.[2] 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.[3] The balance of soil carbon is held in
peat and wetlands (150 GtC), and in
plant litter at the soil surface (50 GtC). This compares to 780 GtC in the
atmosphere, and 600 GtC in
all living organisms. 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
contracted from the atmosphere by terrestrial plant
photosynthesis 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.[4][5]
Detritus resulting from
plant senescence is the major source of soil organic carbon. Plant materials, with
cell walls high in
cellulose and
lignin, are decomposed and the not-
respired carbon is retained as
humus. 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.
Topsoil ranges from 0.5% to 3.0% organic carbon for most
upland soils. Soils with less than 0.5% organic C are mostly limited to
desert areas. Soils containing greater than 12–18% organic carbon are generally classified as
organic soils. High levels of organic C
develop in soils supporting
wetland ecology,
flood deposition,
fire ecology, and
human activity.
Fire derived forms of carbon are present in most soils as unweathered
charcoal and weathered
black carbon.[8][9] Soil organic carbon is typically 5–50% derived from char,[10] with levels above 50% encountered in
mollisol,
chernozem, and
terra preta soils.[11]
Root exudates are another source of soil carbon.[12] 5–20% of the total plant carbon fixed during photosynthesis is supplied as root exudates in support of
rhizospheric mutualistic biota.[13][14] Microbial populations are typically higher in the rhizosphere than in adjacent
bulk soil.
SOC and other soil properties
Soil organic carbon (SOC) concentrations in sandy soils influence soil bulk density which decreases with an increase in SOC.[15] Bulk density is important for calculating SOC stocks [16] 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
soil fertility, in sandy soils. SOC was higher in sandy soils with higher pH. [17] 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
clay content accounts for 58%.[18] 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.[19] When organic matter decomposes, the organic matter binds with silt and clay forming aggregates.[20] 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.[21]
Organic carbon is vital to soil capacity to provide
edaphicecosystem services. The condition of this capacity is termed
soil health, 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.[22] Carbon, as it relates to the organic matter of soils, is a major component of soil and
catchment 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.[2] First was the use of
fire, which removes soil cover and leads to immediate and continuing losses of soil organic carbon.
Tillage and
drainage both expose soil organic matter to oxygen and oxidation. In the
Netherlands,
East Anglia,
Florida, and the
California Delta, subsidence of
peat lands from oxidation has been severe as a result of tillage and drainage.
Grazing management that exposes soil (through either excessive or insufficient recovery periods) can also cause losses of soil organic carbon.
Natural variations in soil carbon occur as a result of
climate,
organisms,
parent material, time, and relief.[23] The greatest contemporary influence has been that of humans; for example, carbon in
Australianagricultural soils may historically have been twice the present range that is typically 1.6–4.6%.[24]
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
burning crop stubbles or over-cultivation) are discouraged; on the other hand, incorporation of organic material (such as in
manuring) 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
nitrogenfertilizers.[23]
Data available on soil organic carbon
Europe
The most homogeneous and comprehensive data on the organic carbon/matter content of
European soils remain those that can be extracted and/or derived from the
European Soil Database in combination with associated databases on
land cover, climate, and
topography. 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,[25] 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[26] 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[27] in EU agricultural soils. This modelling framework has been updated by integrating the soil erosion component to estimate the lateral carbon fluxes.[28]
Much of the contemporary literature on soil carbon relates to its role, or potential, as an atmospheric
carbon sink to offset
climate change. Despite this emphasis, a much wider range of soil and
catchment health aspects are improved as soil carbon is increased. These benefits are difficult to quantify, due to the complexity of
natural resource systems and the interpretation of what constitutes soil health; nonetheless, several benefits are proposed in the following points:
Reduced
erosion,
sedimentation: 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
waterways, nutrients and
turbidity: nutrients and sediment tend to be retained by the soil rather than leach or wash off, and are so kept from waterways.
Water balance: greater soil water holding capacity reduces overland flow and recharge to
groundwater; 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
global warming.
Greater
biodiversity: soil organic matter contributes to the health of soil flora and, accordingly, the natural links with biodiversity in the greater
biosphere.
Forest soils
Forest soils constitute a large pool of carbon. Anthropogenic activities such as
deforestation cause releases of carbon from this pool, which may significantly increase the concentration of
greenhouse gas (GHG) in the
atmosphere.[29] 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.[30][31] Tropical deforestation represents nearly 25% of total anthropogenic GHG emissions worldwide.[32] 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.
West Africa has experienced significant loss of forest that contains high levels of soil organic carbon.[35][36] This is mostly due to expansion of small scale, non-mechanized agriculture using burning as a form of land clearance [37]
^Bird, M. (2015). "Test procedures for biochar in soil". In Lehmann, J.; Joseph, S. (eds.). Biochar for Environmental Management (2 ed.). p. 679.
ISBN978-0-415-70415-1.
^Mergel, A. (1998). "Role of plant root exudates in soil carbon and nitrogen transformation". In Box, J. Jr. (ed.). Root Demographics and Their Efficiencies in Sustainable Agriculture, Grasslands and Forest Ecosystems. Proceedings of the 5th Symposium of the International Society of Root Research. 82. Madren Conference Center, Clemson University, Clemson, South Carolina, US: Springer Netherlands. pp. 43–54.
doi:
10.1007/978-94-011-5270-9_3.
ISBN978-94-010-6218-3.
^Panagos, Panos; Hiederer, Roland; Liedekerke, Marc Van; Bampa, Francesca (2013). "Estimating soil organic carbon in Europe based on data collected through a European network". Ecological Indicators. 24: 439–450.
doi:
10.1016/j.ecolind.2012.07.020.
^Panagos, Panos; Ballabio, Cristiano; Yigini, Yusuf; Dunbar, Martha B. (2013). "Estimating the soil organic carbon content for European NUTS2 regions based on LUCAS data collection". Science of the Total Environment. 442: 235–246.
Bibcode:
2013ScTEn.442..235P.
doi:
10.1016/j.scitotenv.2012.10.017.
PMID23178783.
^FAO. 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