Soil carbon is the solidcarbon stored in globalsoils. This includes bothsoil organic matter andinorganic carbon ascarbonate minerals. It is vital to the soil capacity in our ecosystem. Soil carbon is acarbon sink in regard to the globalcarbon cycle, playing a role inbiogeochemistry,climate change mitigation, and constructing globalclimate models.Microorganisms 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.[1] Human activities have caused a massive loss of soil organic carbon. For example, anthropogenic fires destroy the top layer of the soil, exposing soil to excessive oxidation.
Soil carbon is present in two forms: inorganic and organic.Soil inorganic carbon consists of mineral forms of carbon, either fromweathering ofparent material, or from reaction of soil minerals with atmospheric CO2.Carbonate minerals are the dominant form of soil carbon indesert climates.Soil organic carbon is present assoil organic matter. It includes relatively available carbon as fresh plant remains and relatively inert carbon in materials derived from plant remains:humus andcharcoal.[2] Soil carbon is critical for terrestrial organisms and is one of the most important carbon pools, with the majority of carbon stored in forests.[3] Biotic factors include photosynthetic assimilation of fixed carbon, decomposition of biomass, and the activities of diverse communities of soil organisms.[4] Climate, landscape dynamics, fires, and mineralogy are some of the important abiotic factors. Anthropogenic factors have increasingly changed soil carbon distributions. Industrial nitrogen fixation, agricultural practices, and land use and other management practices are some anthropogenic activities that have altered soil carbon.[5]
Soil carbon distribution and accumulation arises from complex and dynamic processes influenced by biotic, abiotic, and anthropogenic factors.[6] Although exact quantities are difficult to measure, soil carbon has been lost through land use changes, deforestation, and agricultural practices.[7] 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.[8] Atmospheric CO2 is taken up by photosynthetic organisms and stored as organic matter in terrestrial ecosystems.[9]
Although exact quantities are difficult to measure, human activities have caused substantial losses of soil organic carbon.[10] 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.[11] The balance of soil carbon is held inpeat and wetlands (150 GtC), and inplant litter at the soil surface (50 GtC). This compares to 780 GtC in theatmosphere, and 600 GtC inall 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/yrcontracted from the atmosphere by terrestrial plantphotosynthesis reduced by 60 GtC/yr of plantrespiration. An equivalent 60 GtC/yr is respired from soil, joining the 60 GtC/yr plant respiration to return to the atmosphere.[12][13]
Climate change is a leading factor insoil formation 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,salinity,moisture.temperature, SOC,sequestration, aggregation etc. are predicted.[14] In 1996, Least-Limiting Water Range (LLWR) was created to quantify the physical changes in soil. This indicator measures changes inavailable water capacity,soil structure, air filed porosity, soil strength, and oxygen diffusion rate.[14] 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 ofplant growth and eventually, higher carbon absorption.[14][15] In contrast, tropical environments experience worsening soil quality because soil aggregation levels decrease with higher temperatures.
Soil also has carbon sequestration abilities wherecarbon dioxide is fixed in the soil by plant uptakes.[16] This accounts for the majority of thesoil 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.
Carbon dioxide in the atmosphere is fixed by plants (or autotrophic microorganisms) and added to soil through processes such as (1) root exudation of low-molecular weight simple carbon compounds, or deposition of leaf and root litter leading to accumulation of complex plant polysaccharides. (2) Through these processes, carbon is made bioavailable to the microbial metabolic "factory" and subsequently is either (3) respired to the atmosphere or (4) enters the stable carbon pool as microbial necromass. The exact balance of carbon efflux versus persistence is a function of several factors, including aboveground plant community composition and root exudate profiles, environmental variables, and collective microbial phenotypes (i.e., the metaphenome).[17][18]
Detritus resulting fromplant senescence is the major source of soil organic carbon. Plant materials, withcell walls high incellulose andlignin, are decomposed and the not-respired carbon is retained ashumus. 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.
Fire derived forms of carbon are present in most soils as unweatheredcharcoal and weatheredblack carbon.[19][20] Soil organic carbon is typically 5–50% derived from char,[21] with levels above 50% encountered inmollisol,chernozem, andterra preta soils.[22]
Root exudates are another source of soil carbon.[23] 5–20% of the total plant carbon fixed during photosynthesis is supplied as root exudates in support ofrhizospheric mutualistic biota.[24][25] Microbial populations are typically higher in the rhizosphere than in adjacentbulk soil.
Soil organic carbon (SOC) concentrations in sandy soils influence soil bulk density which decreases with an increase in SOC.[26] Bulk density is important for calculating SOC stocks[27] and higher SOC concentrations increase SOC stocks but the effect will be somewhat reduced by the decrease in bulk density. Soil organic carbon increased thecation exchange capacity (CEC), a measure ofsoil fertility, in sandy soils. SOC was higher in sandy soils with higher pH.[28] 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 whereasclay content accounts for 58%.[29] 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.[30] When organic matter decomposes, the organic matter binds with silt and clay forming aggregates.[31] 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.[32]
Organic carbon is vital to soil capacity to provideedaphicecosystem services. The condition of this capacity is termedsoil health, a term that communicates the value of understanding soil as a living system as opposed to anabiotic component. Specific carbon related benchmarks used to evaluate soil health include CO2 release, humus levels, and microbial metabolic activity.
The exchange of carbon between soils and the atmosphere is a significant part of the world carbon cycle.[33] Carbon, as it relates to the organic matter of soils, is a major component of soil andcatchment 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.[10] First was the use offire, which removes soil cover and leads to immediate and continuing losses of soil organic carbon.Tillage anddrainage both expose soil organic matter to oxygen and oxidation. In theNetherlands,East Anglia,Florida, and theCalifornia Delta, subsidence ofpeat 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 ofclimate,organisms,parent material, time, and relief.[34] The greatest contemporary influence has been that of humans; for example, carbon inAustralianagricultural soils may historically have been twice the present range that is typically 1.6–4.6%.[35]
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 asburning crop stubbles or over-cultivation) are discouraged; on the other hand, incorporation of organic material (such as inmanuring) 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 ofnitrogenfertilizers.[34]
The most homogeneous and comprehensive data on the organic carbon/matter content ofEuropean soils remain those that can be extracted and/or derived from theEuropean Soil Database in combination with associated databases onland cover, climate, andtopography. The modelled data refer tocarbon content (%) in the surface horizon of soils in Europe. In an inventory on available national datasets, sevenmember 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,[36] 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[37] at regional level show important findings. Finally, a new proposed model for estimation of soil organic carbon in agricultural soils has estimated current topSOC stock of 17.63 Gt[38] in EU agricultural soils. This modelling framework has been updated by integrating the soil erosion component to estimate the lateral carbon fluxes.[39] Currently, the EU-ORCaSA[40] project is developing a multi-ecosystem framework for measuring, reporting and verification of soil organic carbon changes to support policy making.[41]
Much of the contemporary literature on soil carbon relates to its role, or potential, as an atmosphericcarbon sink to offsetclimate change. Despite this emphasis, a much wider range of soil andcatchment health aspects are improved as soil carbon is increased. These benefits are difficult to quantify, due to the complexity ofnatural resource systems and the interpretation of what constitutes soil health; nonetheless, several benefits are proposed in the following points:
Reducederosion,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.
Cleanerwaterways, nutrients andturbidity: 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 togroundwater; 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 toglobal warming.
Greaterbiodiversity: soil organic matter contributes to the health of soil flora and, accordingly, the natural links with biodiversity in the greaterbiosphere.
Forest soils constitute a large pool of carbon. Anthropogenic activities such asdeforestation cause releases of carbon from this pool, which may significantly increase the concentration ofgreenhouse gas (GHG) in theatmosphere.[42] Under theUnited 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 theIntergovernmental Panel on Climate Change's good practice guidance.[43][44] Tropical deforestation represents nearly 25% of total anthropogenic GHG emissions worldwide.[45] 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 forReducing 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.[48][49] This is mostly due to expansion of small scale, non-mechanized agriculture using burning as a form of land clearance[50]
^Schlesinger, William H.; Bernhardt, Emily S. (2020).Biogeochemistry: an analysis of global change (4th ed.). London: Academic press, an imprint of Elsevier.ISBN978-0-12-814608-8.
^Bird, M. (2015). "Test procedures for biochar in soil". In Lehmann, J.; Joseph, S. (eds.).Biochar for Environmental Management (2 ed.). Routledge. 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.Bibcode:2013EcInd..24..439P.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
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