Soil organic matter

Source: Wikipedia, the free encyclopedia.

Soil organic matter (SOM) is the

ecosystem services.[1] SOM is especially critical for soil functions and quality.[2]

The benefits of SOM result from a number of complex, interactive,

micronutrients, which the mineralization of SOM slowly releases. As such, the amount of SOM and soil fertility are significantly correlated.[3]

SOM also acts as a major sink and source of

global carbon cycle and therefore for climate change mitigation.[6] Therefore, SOM/SOC dynamics and the capacity of soils to provide the ecosystem service of carbon sequestration through SOM management have received considerable attention.[7]

The concentration of SOM in soils generally ranges from 1% to 6% of the total mass of topsoil for most upland soils. Soils whose upper horizons consist of less than 1% of organic matter are mostly limited to deserts, while the SOM content of soils in low lying, wet areas can be as great as 90%. Soils containing 12% to 18% SOC are generally classified as organic soils.[8]

SOM can be divided into three genera: the living

microbes, fresh and partially decomposed detritus, and humus. Surface plant litter, i. e., fresh vegetal detritus, is generally excluded from SOM.[9]

Sources

The primary source of SOM is vegetal detritus. In

decomposers are included in the SOM, and form a food web
of organisms that prey upon each other and subsequently become prey.

Above detritivores there are also herbivores that consume fresh vegetal matter, the residue of which then passes to the soil. The products of the metabolisms of these organisms are the secondary sources of SOM, which also includes their corpses. Some animals, like earthworms, termites, ants, and millipedes contribute to both vertical and horizontal translocation of organic matter.[1]

Additional sources of SOM include plant root exudates[10] and charcoal.[11]

Composition

The water content of most vegetal detritus is in the range of 60% to 90%. The dry matter consists of complex organic matter that is composed primarily of carbon, oxygen, and hydrogen. Although these three elements make up about 92% of the dry weight of the organic matter in soil, other elements are very important for the nutrition of plants, including nitrogen, phosphorus, potassium, sulfur, calcium, magnesium, and many

micronutrients.[1]

Organic compounds in vegetal detritus include:

  • Carbohydrates that are composed of carbon, hydrogen, and oxygen, and range in complexity from rather simple sugars to the large molecules of cellulose.
  • oleic
    . They also include carbon, oxygen, and hydrogen.
  • Lignins that are complex compounds, form the older parts of wood, and also are composed primarily of carbon, oxygen, and hydrogen. They are resistant to decomposition.
  • Proteins that include nitrogen in addition to carbon, hydrogen, and oxygen; and small amounts of sulfur, iron, and phosphorus.[1]
  • Charcoal, which is elemental carbon that is derived from incomplete combustion of organic matter. It is resistant to decomposition.

Decomposition

Main article: Decomposition

Vegetal detritus in general is not soluble in water and therefore is inaccessible to plants. It constitutes, nevertheless, the raw matter from which

Soil microbes decompose it through enzymatic biochemical processes, obtain the necessary energy from the same matter, and produce the mineral compounds that plant roots are apt to absorb.[12] The decomposition of organic compounds specifically into mineral, i. e., inorganic, compounds is denominated "mineralization". A portion of organic matter is not mineralized and instead decomposed into stable organic matter that is denominated "humus".[1]

The decomposition of organic compounds occurs at very different rates, depending on the nature of the compound. The ranking, from fast to slow rates, is:

  1. Sugars, starches, and simple proteins
  2. Proteins
  3. Hemicelluloses
  4. Cellulose
  5. Lignins and fats

The reactions that occur can be included in one of three genera:

  • Enzymatic oxidation
    that produces carbon dioxide, water, and heat. It affects the majority of the matter.
  • A series of specific reactions liberates and mineralizes the essential elements nitrogen, phosphorus, and sulfur.
  • Compounds that are resistant to microbial action are formed by modification of the original compounds or by microbial synthesis of new ones to produce humus.[1]

The mineral products are:

Element Mineral Products
Carbon CO2, CO32−, HCO3−, CH4, C
Nitrogen NH4+, NO2−, NO3−, N2 (gas), N2O (gas)
Sulfur S, H2S, SO32−, SO42−, CS2
Phosphorus H2PO4−, HPO42−
Others H2O, O2, H2, H+, OH−, K+, Ca2+, Mg2+, etc.

Humus

Main article: Humus

As vegetal detritus decomposes, some microbially resistant compounds are formed, including modified lignins, oils, fats, and waxes. Secondly, some new compounds are synthesized, like

polysaccharides and polyuronids. These compounds are the basis of humus. New reactions occur between these compounds and some proteins and other products that contain nitrogen, thus incorporating nitrogen and avoiding its mineralization
. Other nutrients are also protected in this way from mineralization.

Humic substances

Humic substances are classified into three genera based on their solubility in acids and alkalis, and also according to their stability:

  • Fulvic acid
    is the genus that contains the matter that has the lowest molecular weight, is soluble in acids and alkalis, and is susceptible to microbial action.
  • Humic acid
    is the genus that contains the intermediate matter that has medial molecular weight, is soluble in alkalis and insoluble in acids, and has some resistance to microbial action.
  • Humin is the genus that contains the matter that has the greatest molecular weight, is the darkest in color, is insoluble in acids and alkalis, and has the greatest resistance to microbial action.[1]

Function in carbon cycling

See also: Soil carbon

Soil has a crucial function in the global

organic carbon, which occurs primarily in the form of SOM, accounts for approximately 1,550 gigatons of the total global carbon pool,[13][14] with soil inorganic carbon (SIC) accounting for the remainder. The pool of organic carbon exists in dynamic equilibrium between gains and losses; soil may therefore serve as either a sink or source of carbon, through sequestration or greenhouse gas emissions, respectively, depending on exogenous factors.[15]

See also

References

  1. ^
    ISBN 978-0133254488
    . Retrieved 17 December 2023.
  2. doi:10.2136/sssaj1994.03615995005800030021x
    . Retrieved 17 December 2023.
  3. doi:10.1038/371783a0
    . Retrieved 17 December 2023.
  4. doi:10.4141/CJSS06008
    . Retrieved 24 December 2023.
  5. doi:10.2136/sssaj1997.03615995006100040026x
    . Retrieved 24 December 2023.
  6. ^ "Restoring soils could remove up to '5.5bn tonnes' of greenhouse gases every year". Carbon Brief. London, United Kingdom. 2020-03-16. Retrieved 24 December 2023.
  7. ^ Ontl, Todd A.; Schulte, Lisa A. (2012). "Soil carbon storage". The Nature Education Knowledge Project. Cambridge, Massachusetts. Retrieved 24 December 2023.
  8. ^ "Organic matter in soil: overview of composition, distribution, and content". Ocean Agro LLC. Nandesari Vadodara, India. 2018. Retrieved 25 December 2023.
  9. ^ Bot, Alexandra; Benites, José (2005). "The importance of soil organic matter: key to drought-resistant soil and sustained food production. Chapter 1. Introduction". Food and Agriculture Organization of the United Nations. Rome, Italy. Retrieved 25 December 2023.
  10. ISBN 978-94-010-6218-3
    . Retrieved 31 December 2023.
  11. doi:10.2136/sssaj2002.1249
    . Retrieved 31 December 2023.
  12. S2CID 92606851
    .
  13. doi:10.1111/j.1365-2389.1996.tb01386.x
    .
  14. doi:10.1016/j.geoderma.2016.01.034
    .
  15. ^ Lal, R. Soil Carbon Sequestration to Mitigate Climate Change. Geoderma, 123(1): 1–22 (2004).
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