Soil carbon

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.

Impact of elevated CO₂ on soil carbon reserves

Overview

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]

Global carbon cycle

See also: Carbon cycle

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]

Organic carbon

 

Soil carbon cycle through the microbial loop

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).^[6][7]

Main article: Soil organic matter

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

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]

Soil health

Main article: Soil health

Organic carbon is vital to soil capacity to provide edaphic ecosystem 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.

See also: Soil carbon feedback

Managing soil 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 Australian agricultural 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 nitrogen fertilizers.^[23]

Data available on soil organic carbon

 

A portable soil respiration system measuring soil CO₂ flux

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]

Managing for catchment health

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.

The government of Tanzania—together with the Food and Agriculture Organization of the United Nations^[33] and the financial support of the government of Finland—have implemented a forest soil carbon monitoring program^[34] 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.^[35][36] This is mostly due to expansion of small scale, non-mechanized agriculture using burning as a form of land clearance ^[37]

See also

• Biochar
• Biosequestration
• Carbon cycle
• Carbon farming
• Carbon sequestration
• Coarse woody debris
• Mycorrhizal fungi and soil carbon storage
• Soil regeneration and climate change

References

1. Lal, R. (February 2007). "Carbon Management in Agricultural Soils". Mitigation and Adaptation Strategies for Global Change. 12 (2): 303–322. Bibcode:2007MASGC..12..303L. CiteSeerX 10.1.1.467.3854. doi:10.1007/s11027-006-9036-7. S2CID 59574069. Retrieved 16 January 2016.
2. Ruddiman, William (2007). Plows, Plagues, and Petroleum: How Humans Took Control of Climate. Princeton, NJ: Princeton University Press. ISBN 978-0-691-14634-8.
3. Yousaf, Balal; Liu, Guijian; Wang, Ruwei; Abbas, Qumber; Imtiaz, Muhammad; Liu, Ruijia (2016). "Investigating the biochar effects on C-mineralization and sequestration of carbon in soil compared with conventional amendments using stable isotope (δ13C) approach". GCB Bioenergy. 9 (6): 1085–1099. doi:10.1111/gcbb.12401.
4. Lal, Rattan (2008). "Sequestration of atmospheric CO₂ in global carbon pools". Energy and Environmental Science. 1 (1): 86–100. doi:10.1039/b809492f. Retrieved 16 January 2016.
5. "An Introduction to the Global Carbon Cycle" (PDF). University of New Hampshire. 2009. Retrieved 6 February 2016.
6. Bonkowski, Michael (2004). "Protozoa and plant growth: The microbial loop in soil revisited". New Phytologist. 162 (3): 617–631. doi:10.1111/j.1469-8137.2004.01066.x. PMID 33873756.
7. Naylor, Dan; Sadler, Natalie; Bhattacharjee, Arunima; Graham, Emily B.; Anderton, Christopher R.; McClure, Ryan; Lipton, Mary; Hofmockel, Kirsten S.; Jansson, Janet K. (2020). "Soil Microbiomes Under Climate Change and Implications for Carbon Cycling". Annual Review of Environment and Resources. 45: 29–59. doi:10.1146/annurev-environ-012320-082720.   Material was copied from this source, which is available under a Creative Commons Attribution 4.0 International License.
8. Bird, M. (2015). "Test procedures for biochar in soil". In Lehmann, J.; Joseph, S. (eds.). Biochar for Environmental Management (2 ed.). p. 679. ISBN 978-0-415-70415-1.
9. Skjemstad, Jan O. (2002). "Charcoal carbon in U.S. agricultural soils". Soil Science Society of America Journal. 66 (4): 1249–1255. Bibcode:2002SSASJ..66.1249S. doi:10.2136/sssaj2002.1249.
10. Schmidt, M.W.I.; Skjemstad, J.O.; Czimczik, C.I.; Glaser, B.; Prentice, K.M.; Gelinas, Y.; Kuhlbusch, T.A.J. (2001). "Comparative analysis of black C in soils" (PDF). Global Biogeochemical Cycles. 15 (1): 163–168. Bibcode:2001GBioC..15..163S. doi:10.1029/2000GB001284. S2CID 54976103.
11. Mao, J.-D.; Johnson, R. L.; Lehmann, J.; Olk, J.; Neeves, E. G.; Thompson, M. L.; Schmidt-Rohr, K. (2012). "Abundant and stable char residues in soils: implications for soil fertility and carbon sequestration". Environmental Science and Technology. 46 (17): 9571–9576. Bibcode:2012EnST...46.9571M. CiteSeerX 10.1.1.698.270. doi:10.1021/es301107c. PMID 22834642.
12. 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. ISBN 978-94-010-6218-3.
13. Pearson, JN; Jakobsen, I (1993). "The relative contribution of hyphae and roots to phosphorus uptake by arbuscular mycorrhizal plants, measured by dual labeling with 32P and 33P". New Phytologist. 124 (3): 489–494. doi:10.1111/j.1469-8137.1993.tb03840.x.
14. Hobbie, JE; Hobbie, EA (2006). "15N in symbiotic fungi and plants estimates nitrogen and carbon flux rates in arctic tundra". Ecology. 87 (4): 816–822. doi:10.1890/0012-9658(2006)87[816:nisfap]2.0.co;2. hdl:1912/911. PMID 16676524.
15. Périé, Catherine; Ouimet, Rock (2008-05-02). "Organic carbon, organic matter and bulk density relationships in boreal forest soils". Canadian Journal of Soil Science. 88 (3): 315–325. doi:10.4141/cjss06008. ISSN 0008-4271.
16. Périé, Catherine; Ouimet, Rock (2008-05-02). "Organic carbon, organic matter and bulk density relationships in boreal forest soils". Canadian Journal of Soil Science. 88 (3): 315–325. doi:10.4141/cjss06008. ISSN 0008-4271.
17. Oorts, K; Vanlauwe, B; Merckx, R (2003-12-01). "Cation exchange capacities of soil organic matter fractions in a Ferric Lixisol with different organic matter inputs". Agriculture, Ecosystems & Environment. Balanced Nutrient Management Systems for cropping systems in the tropics: from concept to practice. 100 (2): 161–171. doi:10.1016/S0167-8809(03)00190-7. ISSN 0167-8809.
18. Curtin, D.; Smillie, G. W. (May 1976). "Estimation of Components of Soil Cation Exchange Capacity from Measurements of Specific Surface and Organic Matter". Soil Science Society of America Journal. 40 (3): 461–462. Bibcode:1976SSASJ..40..461C. doi:10.2136/sssaj1976.03615995004000030041x. ISSN 0361-5995.
19. Oorts, K; Vanlauwe, B; Merckx, R (2003-12-01). "Cation exchange capacities of soil organic matter fractions in a Ferric Lixisol with different organic matter inputs". Agriculture, Ecosystems & Environment. Balanced Nutrient Management Systems for cropping systems in the tropics: from concept to practice. 100 (2): 161–171. doi:10.1016/S0167-8809(03)00190-7. ISSN 0167-8809.
20. Oorts, K; Vanlauwe, B; Merckx, R (2003-12-01). "Cation exchange capacities of soil organic matter fractions in a Ferric Lixisol with different organic matter inputs". Agriculture, Ecosystems & Environment. Balanced Nutrient Management Systems for cropping systems in the tropics: from concept to practice. 100 (2): 161–171. doi:10.1016/S0167-8809(03)00190-7. ISSN 0167-8809.
21. Oorts, K; Vanlauwe, B; Merckx, R (2003-12-01). "Cation exchange capacities of soil organic matter fractions in a Ferric Lixisol with different organic matter inputs". Agriculture, Ecosystems & Environment. Balanced Nutrient Management Systems for cropping systems in the tropics: from concept to practice. 100 (2): 161–171. doi:10.1016/S0167-8809(03)00190-7. ISSN 0167-8809.
22. Eric Roston (October 6, 2017). "There's a Climate Bomb Under Your Feet; Soil locks away carbon just as the oceans do. But that lock is getting picked as the atmosphere warms and development accelerates". Bloomberg.com. Retrieved 6 October 2017.
23. Young, A.; Young, R. (2001). Soils in the Australian landscape. Melbourne: Oxford University Press. ISBN 978-0-19-551550-3.
24. Charman, P.E.V.; Murphy, B.W. (2000). Soils, their properties and management (2nd ed.). Melbourne: Oxford University Press. ISBN 978-0-19-551762-0.
25. 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.
26. 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. PMID 23178783.
27. Lugato, Emanuele; Panagos, Panos; Bampa, Francesca; Jones, Arwyn; Montanarella, Luca (2014-01-01). "A new baseline of organic carbon stock in European agricultural soils using a modelling approach". Global Change Biology. 20 (1): 313–326. Bibcode:2014GCBio..20..313L. doi:10.1111/gcb.12292. ISSN 1365-2486. PMID 23765562. S2CID 10826877.
28. Lugato, Emanuele; Panagos, Panos; Fernandez-Ugalde, Oihane; Orgiazzi, Alberto; Ballabio, Cristiano; Montanarella, Luca; Borrelli, Pasquale; Smith, Pete; Jones, Arwyn (2018-11-01). "Soil erosion is unlikely to drive a future carbon sink in Europe". Science Advances. 4 (11): eaau3523. Bibcode:2018SciA....4.3523L. doi:10.1126/sciadv.aau3523. ISSN 2375-2548. PMC 6235540. PMID 30443596.
29. IPCC. 2000. Land use, land-use change, and forestry. IPCC Special Report. United Kingdom, Cambridge University Press.
30. IPCC. 2003. Good practice guidance for land use, land-use change and forestry. Kanagawa, Japan, National Greenhouse Gas Inventories Programme.
31. IPCC. 2006. Guidelines for national greenhouse gas inventories. Kanagawa, Japan, National Greenhouse Gas Inventories Programme.
32. Pan Y., Birdsey R., Fang J., Houghton R., Kauppi P., Kurz W., Phillips O., Shvidenko A., et al. (2011). "A Large and Persistent Carbon Sink in the World's Forests". Science. 333 (6045): 988–93. Bibcode:2011Sci...333..988P. CiteSeerX 10.1.1.712.3796. doi:10.1126/science.1201609. PMID 21764754. S2CID 42458151.
33. "Forest monitoring and assessment".
34. 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
35. Ur Rehman, Hafeez; Poch, Rosa M.; Scarciglia, Fabio; Francis, Michele L. (2021). "A carbon-sink in a sacred forest: Biologically-driven calcite formation in highly weathered soils in Northern Togo (West Africa)". CATENA. 198: 105027. Bibcode:2021Caten.19805027U. doi:10.1016/j.catena.2020.105027. S2CID 228861150.
36. Anikwe, Martin AN (2010). "Carbon storage in soils of Southeastern Nigeria under different management practices". Carbon Balance and Management. 5 (1): 5. Bibcode:2010CarBM...5....5A. doi:10.1186/1750-0680-5-5. ISSN 1750-0680. PMC 2955576. PMID 20868522.
37. Feng, Yu; Zeng, Zhenzhong; Searchinger, Timothy D.; Ziegler, Alan D.; Wu, Jie; Wang, Dashan; He, Xinyue; Elsen, Paul R.; Ciais, Philippe; Xu, Rongrong; Guo, Zhilin (2022). "Doubling of annual forest carbon loss over the tropics during the early twenty-first century". Nature Sustainability. 5 (5): 444–451. Bibcode:2022NatSu...5..444F. doi:10.1038/s41893-022-00854-3. hdl:2346/92751. ISSN 2398-9629. S2CID 247160560.