Humic substance

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Humic acid isolated from peat Humic acid.jpg
Humic acid isolated from peat
Fulvic acid isolated from peat Fulvic acid.jpg
Fulvic acid isolated from peat

Humic substances (HS) are coloured recalcitrant organic compounds naturally formed during long-term decomposition and transformation of biomass residues. The colour of humic substances varies from yellow to brown to black. Humic substances represent the major part of organic matter in soil, peat, coal and sediments and are important components of dissolved natural organic matter (NOM) in lakes (especially, dystrophic lakes), rivers and sea water.

Contents

"Humic substances" is an umbrella term covering humic acid, fulvic acid, humin and hymatomelanic acid which differ in solubility. By definition, humic acid is soluble in water at neutral and alkaline pH, but insoluble at acidic pH < 2. Fulvic acid is soluble in water at any pH. Humin is not soluble in water at any pH. Hymatomelanic acid is part of humic acid that is soluble in ethanol.

This definition of humic substances is largely operational. It is rooted in the history of soil science and, more precisely, in the tradition of alkaline extraction, which dates back to 1786, when Franz Karl Achard treated peat with a solution of potassium hydroxide and, after subsequent addition of an acid, obtained an amorphous dark precipitate (i.e., humic acid). Aquatic humic substances were isolated for the first time later, in 1806, from spring water by Jöns Jakob Berzelius.

In terms of chemistry, fulvic acid, humic acid and humin share more similarities than differences and represent a continuum of humic molecules. All of them are constructed from similar aromatic, polyaromatic, aliphatic and carbohydrate units and contain the same functional groups (mainly, carboxylic, phenolic and ester) albeit in varying proportions.

Water solubility of humic substances is primarily governed by interplay of two factors: the amount of ionizable functional groups (mainly, carboxylic) and the molecular weight. In general, fulvic acid has higher amount of carboxylic groups and lower average molecular weight than humic acid. However, molecular weight distributions of humic and fulvic acids significantly overlap.

Age and origin of the source material determine the chemical structure of humic substances. In general, humic substances derived from soil and peat (which takes hundreds to thousands of years to form) have higher molecular weight, higher amount of functional groups, more carbohydrate units and less polyaromatic units than humic substances derived from leonardite (which takes millions of years to form).

Humic matter in isolation is the result of a chemical extraction from the soil organic matter or the dissolved organic matter and represent the humic molecules distributed in the soil or water. [1] [2] [3] A new understanding views humic substances not as high-molecular-weight macropolymers but as heterogeneous and relatively small molecular components of the soil organic matter auto-assembled in supramolecular associations and composed of a variety of compounds of biological origin and synthesized by abiotic and biotic reactions in soil. [4] It is the large molecular complexity of the soil humeome [5] that confers to humic matter its bioactivity in soil and its role as plant growth promoter. [6]

The academic definition of humic substances is under debate as "humification" becomes unsupported as a special case, leading to some radical definitions expanding HS to encompass all difficult-to-characterize soil organic matter, at the cost of clarity. There is also a call to forgo the traditional alkali extract method and directly analyze the soil, but its complexity prevents widespread adoption in agriculture. [7] In practice, this means some sources may apply a traditional acid-base analysis to compost, then state the results in term of "humic substances". [8]

Concepts of humic substances

The formation of humic substances in nature is one of the least understood aspects of humus chemistry and one of the most intriguing. There are three main theories to explain it: the lignin theory of Waksman (1932), the polyphenol theory, and the sugar-amine condensation theory of Maillard (1911). [9] [10] Those theories are insufficient to account for observations in soil research. [7] Humic substances are formed by the microbial degradation of dead plant matter, such as lignin, cellulose and charcoal. [11] [12] Humic substances in the lab are very resistant to further biodegradation. The structure, elemental composition and content of functional groups of a given sample depend on the water or soil source and the specific conditions of extraction. Nevertheless, the average properties of lab produced humic substances from different sources are remarkably similar.

Fractionation

Humic substances in soils and sediments can be divided into three main fractions: humic acids, fulvic acids, and humin. Their presence and relative abundance is inferred by lab extraction, a process which alters their original form beyond recognition.

Humic acid as traditionally produced in a laboratory is not a single acid; rather, it is a complex mixture of many different acids containing carboxyl and phenolate groups so that the mixture behaves functionally as a dibasic acid or, occasionally, as a tribasic acid. Humic acid used to amend soil is manufactured using these same well established procedures. Humic acids can form complexes with ions that are commonly found in the environment creating humic colloids. [15]

As a nutrition supplement, fulvic acid can be found in a liquid form as a component of mineral colloids. Fulvic acids are poly-electrolytes and are unique colloids that diffuse easily through membranes, whereas all other colloids do not. [16]

A sequential chemical fractionation called Humeomics can be used to isolate more homogeneous humic fractions and determine their molecular structures by advanced spectroscopic and chromatographic methods. [17] Substances identified in humic extracts and directly in soil include mono-, di-, and tri-hydroxycarboxylic acids, fatty acids, dicarboxylic acids, linear alcohols, phenolic acids, terpenoids, carbohydrates, and amino acids. [18]

Criticism

Decomposition products of dead plant materials form intimate associations with minerals, making it difficult to isolate and characterize soil organic constituents. 18th century soil chemists successfully used alkaline extraction to isolate a portion of the organic constituents in soil. This led to the theory that a 'humification' process created 'humic substances'; most commonly 'humic acid', 'fulvic acid', and 'humin'. [7] However, these humic substances have not been observed in soil. [19] Although 'humification' theory is unsupported by evidence, "the underlying theory persists in the contemporary literature, including current textbooks." [7] Attempts to redefine 'humic substances' in valid terms have resulted in a proliferation of incompatible definitions, "with far-reaching implications beyond our ability to communicate scientifically accurate soil processes and properties." [7]

Chemical characteristics

In nature

Since the dawn of modern chemistry, humic substances are among the most studied among the natural materials. Despite long study, their molecular structure remains elusive. The traditional view is that humic substances are heteropolycondensates, in varying associations with clay. [20] A more recent view is that relatively small molecules also play a role. [21] Humic substances account for 50 – 90% of cation exchange capacity. Similar to clay, char and colloidal humus hold cation nutrients. [22]

In traditional, base-soluble extracts

Model structure of a humic acid, having a variety of components including quinone, phenol, catechol, and sugar moieties Humic acid.svg
Model structure of a humic acid, having a variety of components including quinone, phenol, catechol, and sugar moieties
Model structure of a fulvic acid Fulvic acid.png
Model structure of a fulvic acid

A typical humic substance is a mixture of many molecules, some of which are based on a motif of aromatic nuclei with phenolic and carboxylic substituents, linked together; the illustration shows a typical structure. The functional groups that contribute most to surface charge and reactivity of humic substances are phenolic and carboxylic groups. [23] Humic acids behave as mixtures of dibasic acids, with a pK 1 value around 4 for protonation of carboxyl groups and around 8 for protonation of phenolate groups. There is considerable overall similarity among individual humic acids. [24] For this reason, measured pK values for a given sample are average values relating to the constituent species. The other important characteristic is charge density. The molecules may form a supramolecular structure held together by non-covalent forces, such as van der Waals force, π-π, and CH-π bonds. [21]

The presence of carboxylate and phenolate groups gives the humic acids the ability to form complexes with ions such as Mg2+, Ca2+, Fe2+, and Fe3+. Many humic acids have two or more of these groups arranged so as to enable the formation of chelate complexes. [25] The formation of (chelate) complexes is an important aspect of the biological role of humic acids in regulating bioavailability of metal ions. [24]

Determination of humic acids in water samples

The presence of humic acid in water intended for potable or industrial use can have a significant impact on the treatability of that water and the success of chemical disinfection processes. For instance, humic and fulvic acids can react with the chemicals used in the chlorination process to form disinfection byproducts such as dihaloacetonitriles, which are toxic to humans. [26] [27] Accurate methods of establishing humic acid concentrations are therefore essential in maintaining water supplies, especially from upland peaty catchments in temperate climates.

As a lot of different bio-organic molecules in very diverse physical associations are mixed together in natural environments, it is cumbersome to measure their exact concentrations in the humic superstructure. For this reason, concentrations of humic acid are traditionally estimated out of concentrations of organic matter, typically from concentrations of total organic carbon (TOC) or dissolved organic carbon (DOC).

Extraction procedures are bound to alter some of the chemical linkages present in the soil humic substances (mainly ester bonds in biopolyesters such as cutins and suberins). The humic extracts are composed of large numbers of different bio-organic molecules that have not yet been totally separated and identified. However, single classes of residual biomolecules have been identified by selective extractions and chemical fractionation, and are represented by alkanoic and hydroxy alkanoic acids, resins, waxes, lignin residues, sugars, and peptides.

Ecological effects

Organic matter soil amendments have been known by farmers to be beneficial to plant growth for longer than recorded history. [28] However, the chemistry and function of the organic matter have been a subject of controversy since humans began postulating about it in the 18th century. Until the time of Liebig, it was supposed that humus was used directly by plants, but, after Liebig showed that plant growth depends upon inorganic compounds, many soil scientists held the view that organic matter was useful for fertility only as it was broken down with the release of its constituent nutrient elements into inorganic forms. At the present time, soil scientists hold a more holistic view and at least recognize that humus influences soil fertility through its effect on the water-holding capacity of the soil. Also, since plants have been shown to absorb and translocate the complex organic molecules of systemic insecticides, they can no longer discredit the idea that plants may be able to absorb the soluble forms of humus; [29] this may in fact be an essential process for the uptake of otherwise insoluble iron oxides.

A study on the effects of humic acid on plant growth was conducted at Ohio State University which said in part "humic acids increased plant growth" and that there were "relatively large responses at low application rates". [30]

A 1998 study by scientists at the North Carolina State University College of Agriculture and Life Sciences showed that addition of humate to soil significantly increased root mass in creeping bentgrass turf. [31] [32]

A 2018 study by scientists at the University of Alberta showed that humic acids can reduce prion infectivity in laboratory experiments, but that this effect may be uncertain in the environment due to minerals in the soil that buffer the effect. [33]

Anthropogenic production

Humans can affect the production of humic substances via a variety of ways: by making use of natural processes by composting lignin or adding biochar (see soil rehabilitation), or by industrial synthesis of artificial humic substances from organic feedstocks directly. These artificial substances may be similarly divided into artificial humic acid (A-HA) and artificial fulvic acid (A-FA). [34]

Lignosulfonates, a by-product from the sulfite pulping of wood, are valorized in the industrial fabrication of concrete where they serve as water reducer, or concrete superplasticizer, to decrease the water-cement ratio (w/c) of fresh concrete while preserving its workability. The w/c ratio of concrete is one of the main parameter controlling the mechanical strength of hardened concrete and its durability. The same wood pulping process can also be applied to obtain humus-like substances by hydrolysis and oxidation. A kind of artificial "lignohumate" can be directly produced from wood in this way. [35]

Agricultural litter can be turned into an artificial humic substance by a hydrothermal reaction. The resulting mixture can increase the content of dissolved organic matter (DOM) and total organic carbon (TOC) in soil. [34]

Lignite (brown coal) may also be oxidized to produce humic substances, reversing the natural process of coal formation under anoxic and reducing conditions. This form of "mineral-derived fulvic acid" is widely used in China. [36] This process also occurs in nature, producing leonardite. [37]

Economic geology

In economic geology, the term humate refers to geological materials, such as weathered coal beds (leonardite), mudrock, or pore material in sandstones, that are rich in humic acids. Humate has been mined from the Fruitland Formation of New Mexico for use as a soil amendment since the 1970s, with nearly 60,000 metric tons produced by 2016. [38] Humate deposits may also play an important role in the genesis of uranium ore bodies. [39]

Technological applications

The heavy-metal binding abilities of humic acids have been exploited to develop remediation technologies for removing lead from waste water. To this end, Yurishcheva et al. coated magnetic nanoparticles with humic acids. After capturing lead ions, the nanoparticles can then be captured using a magnet. [40]

Ancient masonry

Archeology finds that ancient Egypt used mudbricks reinforced with straw and humic acids. [41]

See also

Related Research Articles

<span class="mw-page-title-main">Humus</span> Organic matter in soils resulting from decay of plant and animal materials

In classical soil science, humus is the dark organic matter in soil that is formed by the decomposition of plant and animal matter. It is a kind of soil organic matter. It is rich in nutrients and retains moisture in the soil. Humus is the Latin word for "earth" or "ground".

<span class="mw-page-title-main">Soil</span> Mixture of organic matter, minerals, gases, liquids, and organisms that together support life

Soil, also commonly referred to as earth or dirt, is a mixture of organic matter, minerals, gases, liquids, and organisms that together support the life of plants and soil organisms. Some scientific definitions distinguish dirt from soil by restricting the former term specifically to displaced soil.

<span class="mw-page-title-main">Lignite</span> Soft, brown, combustible, sedimentary rock

Lignite, often referred to as brown coal, is a soft, brown, combustible, sedimentary rock formed from naturally compressed peat. It has a carbon content around 25–35%, and is considered the lowest rank of coal due to its relatively low heat content. When removed from the ground, it contains a very high amount of moisture which partially explains its low carbon content. Lignite is mined all around the world and is used almost exclusively as a fuel for steam-electric power generation.

<span class="mw-page-title-main">Solubility</span> Capacity of a substance to dissolve in a solvent in a homogeneous way

In chemistry, solubility is the ability of a substance, the solute, to form a solution with another substance, the solvent. Insolubility is the opposite property, the inability of the solute to form such a solution.

The pedosphere is the outermost layer of the Earth that is composed of soil and subject to soil formation processes. It exists at the interface of the lithosphere, atmosphere, hydrosphere and biosphere. The pedosphere is the skin of the Earth and only develops when there is a dynamic interaction between the atmosphere, biosphere, lithosphere and the hydrosphere. The pedosphere is the foundation of terrestrial life on Earth.

Organic matter, organic material, or natural organic matter refers to the large source of carbon-based compounds found within natural and engineered, terrestrial, and aquatic environments. It is matter composed of organic compounds that have come from the feces and remains of organisms such as plants and animals. Organic molecules can also be made by chemical reactions that do not involve life. Basic structures are created from cellulose, tannin, cutin, and lignin, along with other various proteins, lipids, and carbohydrates. Organic matter is very important in the movement of nutrients in the environment and plays a role in water retention on the surface of the planet.

Humins are carbon-based macromolecular substances, that can be found in soil chemistry or as a by-product from saccharide-based biorefinery processes.

Glomalin is a hypothetical glycoprotein produced abundantly on hyphae and spores of arbuscular mycorrhizal (AM) fungi in soil and in roots. Glomalin was proposed in 1996 by Sara F. Wright, a scientist at the USDA Agricultural Research Service, but it was not isolated and described yet. The name comes from Glomerales, an order of fungi. Most AM fungi are of the division Glomeromycota. An elusive substance, it is mostly assumed to have a glue-like effect on soil, but it has not been isolated yet.

Potassium humate is the potassium salt of humic acid. It is manufactured commercially by alkaline extraction of brown coal (lignite) leonardite and is used mainly as a soil conditioner.

<span class="mw-page-title-main">Dissolved organic carbon</span> Organic carbon classification

Dissolved organic carbon (DOC) is the fraction of organic carbon operationally defined as that which can pass through a filter with a pore size typically between 0.22 and 0.7 micrometers. The fraction remaining on the filter is called particulate organic carbon (POC).

<span class="mw-page-title-main">Shilajit</span> Thick, sticky tar-like substance

Shilajit, salajeet, mumijo or mumie is an organic-mineral product of predominantly biological origin, formed in the mountains.

Soil chemistry is the study of the chemical characteristics of soil. Soil chemistry is affected by mineral composition, organic matter and environmental factors. In the early 1870s a consulting chemist to the Royal Agricultural Society in England, named J. Thomas Way, performed many experiments on how soils exchange ions, and is considered the father of soil chemistry. Other scientists who contributed to this branch of ecology include Edmund Ruffin, and Linus Pauling.

Acid–base extraction is a subclass of liquid–liquid extractions and involves the separation of chemical species from other acidic or basic compounds. It is typically performed during the work-up step following a chemical synthesis to purify crude compounds and results in the product being largely free of acidic or basic impurities. A separatory funnel is commonly used to perform an acid-base extraction.

Soil organic matter (SOM) is the organic matter component of soil, consisting of plant and animal detritus at various stages of decomposition, cells and tissues of soil microbes, and substances that soil microbes synthesize. SOM provides numerous benefits to the physical and chemical properties of soil and its capacity to provide regulatory ecosystem services. SOM is especially critical for soil functions and quality.

The International Humic Substances Society is a scientific society with a focus on research into natural organic matter (NOM) in soil and water.

<span class="mw-page-title-main">Leonardite</span>

Leonardite is a soft waxy, black or brown, shiny, vitreous mineraloid that is easily soluble in alkaline solutions. It is an oxidation product of lignite, associated with near-surface mining. It is a rich source of humic acid and is used as a soil conditioner, as a stabilizer for ion-exchange resins in water treatment, in the remediation of polluted environments and as a drilling additive. It was named after A. G. Leonard, first director of the North Dakota Geological Survey, in recognition of his work on these deposits.

<span class="mw-page-title-main">Phlobaphene</span>

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<span class="mw-page-title-main">Naturally occurring phenols</span> Group of chemical compounds

In biochemistry, naturally occurring phenols are natural products containing at least one phenol functional group. Phenolic compounds are produced by plants and microorganisms. Organisms sometimes synthesize phenolic compounds in response to ecological pressures such as pathogen and insect attack, UV radiation and wounding. As they are present in food consumed in human diets and in plants used in traditional medicine of several cultures, their role in human health and disease is a subject of research. Some phenols are germicidal and are used in formulating disinfectants.

Cheluviation is the process in which the metal ions in the upper layer of the soil are combined with organic ligands to form coordination complexes or chelates, moving downwards through eluviation and then depositing.

<span class="mw-page-title-main">Irina Perminova</span> Russian chemist

Irina Vasilievna Perminova is Russian scientist, Professor, Dr. Habil. in Analytical Chemistry, Chief Scientist, Head of the Laboratory of Natural Humic Systems at the Division of Medicinal Chemistry and Fine Organic Synthesis of the Department of Chemistry of the Moscow University, Moscow, Russia

References

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  24. 1 2 Ghabbour, E.A.; Davies, G. (Editors) (2001). Humic Substances: Structures, Models and Functions. Cambridge, U.K.: RSC publishing. ISBN   978-0-85404-811-3.{{cite book}}: |author2= has generic name (help)
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  29. ↑ Pan American Union. Dept. of Cultural Affairs. División de Fomento Científico, Pan American Union. Dept. of Scientific Affairs, Organization of American States. Dept. of Scientific Affairs (1984). Ciencia interamericana: Volumes 24–27. And since plants have shown their ability to absorb and translocate the complex molecules of systemic insecticides, they can no longer discredit the idea that plants are able to absorb the soluble humic nutrients, containing by far ...{{cite book}}: CS1 maint: multiple names: authors list (link)
  30. ↑ Arancon, Norman Q.; Edwards, Clive. A.; Lee, Stephen; Byrne, Robert (2006). "Effects of humic acids from vermicomposts on plant growth". European Journal of Soil Biology. 42: S65–S69. CiteSeerX   10.1.1.486.2522 . doi:10.1016/j.ejsobi.2006.06.004.
  31. ↑ Cooper, R. J.; Liu, Chunhua; Fisher, D. S. (1998). "Influence of Humic Substances on Rooting and Nutrient Content of Creeping Bentgrass". Crop Science. 38 (6): 1639. doi:10.2135/cropsci1998.0011183X003800060037x.
  32. ↑ Liu, Chunhua; Cooper, R. J. (August 1999). "Humic Substances Their Influence on Creeping Bentgrass Growth and Stress Tolerance" (PDF). TurfGrass Trends: 6.
  33. ↑ Kuznetsova, Alsu; Cullingham, Catherine; McKenzie, Debbie; Aiken, Judd M. (November 2018). "Soil humic acids degrade CWD prions and reduce infectivity". PLOS Pathogens. 14 (11): e1007414. doi: 10.1371/journal.ppat.1007414 . PMC   6264147 . PMID   30496301.
  34. 1 2 Tang, Chunyu; Li, Yuelei; Song, Jingpeng; Antonietti, Markus; Yang, Fan (2021-06-25). "Artificial humic substances improve microbial activity for binding CO2". iScience. 24 (6): 102647. Bibcode:2021iSci...24j2647T. doi:10.1016/j.isci.2021.102647. ISSN   2589-0042. PMC   8387571 . PMID   34466779.
  35. ↑ Yakimenko, Olga; Stepanov, Andrey; Patsaeva, Svetlana; Khundzhua, Daria; Osipova, Olesya; Gladkov, Oleg (3 July 2021). "Formation of Humic-Like Substances during the Technological Process of Lignohumate Synthesis as a Function of Time". Separations. 8 (7): 96. doi: 10.3390/separations8070096 .
  36. ↑ Gong, Guanqun; Xu, Liangwei; Zhang, Yingjie; Liu, Weixin; Wang, Ming; Zhao, Yufeng; Yuan, Xin; Li, Yajun (3 November 2020). "Extraction of Fulvic Acid from Lignite and Characterization of Its Functional Groups". ACS Omega. 5 (43): 27953–27961. doi:10.1021/acsomega.0c03388. PMC   7643152 . PMID   33163778.
  37. ↑ "Youngs, R.W. & Frost, C.M. 1963. Humic acids from leonardite – a soil conditioner and organic fertilizer. Ind. Eng. Chem., 55, 95–99" (PDF). Archived from the original (PDF) on 2010-06-01. Retrieved 2022-03-18.
  38. ↑ Newcomer, Robert W.; Nybo, John P.; Newcomer, Jacob K. (2020). "Humate in the upper Cretaceous Fruitland Formation in northwestern New Mexico" (PDF). New Mexico Geological Society Special Publication. 14: 41–46. Retrieved 26 October 2020.
  39. ↑ McLemore, Virginia T. (2020). "Uranium deposits in the Poison Canyon trend, Ambrosia Lake Subdistrict, Grants Uranium District, McKinley and Cibola Counties, New Mexico" (PDF). New Mexico Geological Society Special Publication. 14: 53–63. Retrieved 26 October 2020.
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  41. ↑ Lucas, A.; Harris, J.R. (1998). Ancient Egyptian Materials and Industries. New York: Dover Publications. p. 62. ISBN   978-0-486-40446-2.
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