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The formation of humic substances
- The formation of humic substances is one of the least understood aspects of
- chemistry and one of the most intriguing. Studies on this subject are of
long-standing and continued research can be justified on theoretical and
- Several pathways exist for the formation of humic substances during the
decay of plant
- and animal remains in soil, the main ones being shown in the picture:The
classical theory, popularized by Waksman, is that humic substances represent
modified lignins but the majority
of present-day investigators favor a mechanism involving quinones . In practice all four pathways must
be considered as likely mechanisms for the synthesis of humic and fulvic acids
in nature, including sugar-amine condensation .This
four pathways may operate in all soils, but not to the same extent or in the
same order of importance. A lignin pathway may predominate in poorly drained
soils and wet sediments (swamps, etc.) whereas synthesis from polyphenols may
be of considerable importance in certain forest soils. The frequent and sharp
fluctuations in temperature, moisture and irradiation in terrestrial surface
soils under a harsh continental climate may favor humus synthesis by
Pathway 1 - The lignin theory
- For many years it was thought that humic substances were derived from
- 1). According to this theory,
lignin is incompletely utilized by microorganism and the residuum becomes part
of the soil humus. Modification in lignin include loss of methoxyl (OCH3)
groups with the generation of o-hydroxyphenols and oxidation of aliphatic side
chains to form COOH groups. The modified material is subject to further unknown
changes to yield first humic acids and then fulvic acids.This pathway,
illustrated on the picture , is exemplified by Waksman's lignin-protein theory.
- The following evidence was cited by Waksman in support of the lignin theory
- acid formation:
- Both lignin and humic acid are decomposed with considerable difficulty by
the great majority of fungi and bacteria.
- Both lignin and humic acid are partly soluble in alcohol and pyridine.
- Both lignin and humic acid are soluble in alkali and precipitated by
- Both lignin and humic acid contain OCH3 groups.
- Both lignin and humic acid are acidic in nature.
- When lignins are warmed with aqueous alkali, they are transformed into
methoxyl-containing humic acids.
- Humic acids have properties similar to oxidized lignins.
- Although lignin is less easily attacked by microorganisms than other plant
- mechanisms exist in nature for its complete aerobic decomposition.
Otherwise undecomposed plant remains would accumulate on the soil surface and
the organic matter content of the soil would gradually increase until CO2
was depleted from the atmosphere. The ability of soil organisms to degrade
lignin has been understimated in some quarters and its contribution to humus
has been exaggerated.
- In normally aerobic soils lignin may be broken down into
- prior to humus synthesis.On the other hand, the fungi that degrade lignin
are not normally found in excessively wet sediments.Accordingly, it seems
logical to assume that modified lignins may make a major contribution to the
humus of peat, lake sediments, and poorly drained soils.
Pathway 2 and 3 - The polyphenol theory
- In pathway 3 lignin still plays an important role in humus synthesis, but
in a different way.
- In this case phenolic aldehydes and acids released from lignin during
microbiological attack undergo enzymatic conversion to quinones, which
polymerize in the presence or absence of amino compounds to form humiclike
- Pathway 2 is somewhat similar to pathway 3 except that the polyphenols are
- by microorganisms from nonlignin C sources (e.g., celulose). The
polyphenols are then enzymatically oxidized to quinones and converted to humic
substances.As noted earlier, the classical theory of Waksman is now considered
obsolete by many investigators. According to current concepts quinones of
lignin origin, together with those synthesized by microorganisms, are the major
building blocks from which humic substances are formed.
- The formation of brown-colored substances by reactions involving quinones
is not rare
- event, but is a well-known phenomenon that takes place in melanine
formation, such as in the flesh of ripe fruits and vegetables following
mechanical injury and during seed coat formation.
- Possible sources of phenols for humus synthesis include lignin,
- uncombined phenols in plants and tannins.Of these, only the first two have
received serious attention.
- Flaig's concept of
- humus formation is:
- Lignin, freed of its linkage with cellulose during decomposition of plant
residues, is subjected to oxidative splitting with the formation of primary
structural units (derivatives of phenylpropane).
- The side-chains of the lignin-building units are oxidized, demethylation
occurs, and the resulting polyphenols are converted to quinones by
- Quinones arising from the lignin (and from other sources) react with
N-containing compounds to form dark-colored polymers.
- The role of microorganisms as sources of polyphenols has been emphasized by
- Kononova.She concluded that humic substances were being formed by
cellulose-decomposing myxobacteria prior to lignin decomposition.
- The stages leading to the formation of humic substances were postulated to
- Fungi attack simple carbohydrates and parts of the protein and cellulose
in the medullary rays, cambrium, and cortex of plants residues.
- Cellulose of the xylem is decomposed by aerobic myxobacteria. Polyphenols
synthesized by the myxobacteria are oxidized to quinones by polyphenoloxidase
enzymes, and the quinones subsequently react with N compounds to form brown
- Lignin is decomposed. Phenols released during decay also serve as source
materials for humus synthesis.
Pathway 4 - Sugar-amine condensation
- The notion that humus is formed from sugars (pathway 4) dates back to the
early days of
- humus chemistry. According to this concept reducing sugars and amino acids,
formed as by-products of microbial metabolism, undergo nonenzymatic
polymerization to form brown nitrogenous polymers of the type produced during
dehydratation of certain food products at moderate temperatures.
- A major objection to this theory is that the reaction proceeds rather
slowly at the
- temperatures found under normal soil conditions. However, drastic and
frequent changes in the soil environment (freezing and thawing, wetting and
drying), together with the intermixing of reactants with mineral material
having catalytic properties, may facilitate condensation. An attractive feature
of the theory is that the reactants (sugars, amino acids etc.) are produced in
abundance through the activities of microorganisms.
- The initial reaction in
- sugar-amine condensation involves addition of the amine to the aldehyde
group of the sugar to form the n-substituted glycosylamine. The glycosylamine
subsequently undergoes to form the N-substituted-1-amino-deoxy-2-ketose. This
is subject to: fragmentation and formation of 3-carbon chain aldehydes and
ketones, such as acetol, diacetyl etc.; dehydration and formation reductones
and hydroxymethyl furfurals.
- All of these compounds are
- highly reactive and readily polymerize in the presence of amino compounds
to form brown-colored products.
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