1

Supplementary Online Material

Humic substances extracted by alkali are invalid proxies for the dynamics and

functions of organic matter in terrestrial and aquatic ecosystems

Markus Kleber1 and Johannes Lehmann2

1Oregon State University, Corvallis, OR

2Cornell University, Ithaca, NY

2

Supplemental Table S1. Soil organic matter and humic substances over time.

Description of experiment, observation or statement of opinion. Blue color indicates definitions/descriptions that diverge from the classic “humic

substances paradigm”

Reference

22. Vers. Auf 2 Unzen gepulverten Torf goss ich 1 ½ Unze einer kaustischen Weinsteinsalzlauge, und erhitzte sie bis zum Aufkochen. Die Lauge wurde dunkelbraun. Den unaufgelösten Torf laugte ich mit kochendem Wasser aus, und trocknete ihn, wodurch er sehr hart wurde. Er wog 1 Unze 3 Qu., welche wie gewöhnlicher Torf brannte, und eine sehr weiβe Asche zurücklieβen.

23 Vers. Den Extract, vermittelst der alcalischen Lauge, nach dem vorigen Versuche, sättigte ich mit Vitriolsäure, wodurch er trübe wurde, und einen dunkelbraunen Niederschlag fallen lieβ. Dieser Niederschlag wog nach der Aussüβung und Trocknung ½ Unze 2 Scr.; er entzündete und verzehrte sich, wie anderer Torf. (p. 398)

Translation (by the authors)

22. Experiment. I added 45g strong potassium hydroxide to 60g pulverized peat, and heated it to boil. The alkali turned dark brown. I washed the undissolved peat with boiling water, and dried it, whereby it became very hard. It weighed 40g, which burnt similar to normal peat, and generated a very white ash.

23. Experiment. I saturated the alkaline solution from the previous experiment with sulfuric acid, whereby it turned opaque, and generated a dark brown precipitate. After washing and drying, this precipitate weighted 18g; it ignited and consumed itself as other peat.

(Achard, 1786)

J’ai ramassé de ces différentes humeurs sur les ormes du parc de Saint-Cloud, qui sont presque tous atteints de cette maladie, et je les ais soumises aux essais suivans. (p. 41)

Les alkalis le dissolvent avec une extreme facilité, et lui donent une couleur rouge foncée. Les acides s’opposent à sa dissolution, aussi, tant qu’il en conserve quelques traces, il se combine pas avec de l’eau. (p. 45)

Translation (by the authors)

I collected different liquids from elm trees in the park of Saint-Cloud, which all suffered from this disease, and I subjected them to the following experiments. (p. 41)

The alkalis dissolved them with extreme ease, and they resulted in a dark red color. Acids reverted this dissolution, and as long as it contained some traces, the

(Vauquelin, 1797)

3

substance was combinable with water. (p. 45)

l’humus, qu’il est trés-différent, pour entretenir la végétation, d’alimenter des plantes avec des solutions extractives, isolée et non renouvelées, ou de leur fournir du terreau. La partie insoluble de ce dernier, empêche les substances végétales non décomposées, de fournir aux plantes des sucs en fermentation toujour nuisibles à la végétation.

Résumé

Le carbone se trouve en plus grande proportion dans le terreau que dans les plantes d’où il provient. Cependant la proportion du carbone que contient le terreau ne paraît pas pouvoir être notablement augmentée, par l’action continuée des causes qui le produisent.

[…] Le gaz oxygène ne lui enlève que du carbone. L’humus, en perdent cet élément, abandonne en même temps sous forme d’eau son oxygène et son hydrogène, et un extrait soluble dans ce liquid. Le terreau parait être ainsi entièrement destructible à la température atmosphérique, par l’action réunie du gaz oxygène et des lavages.

Les sucs extractifs du terreaux contribuent dans certains proportion à sa fertilité: leurs cendres contiennent tous les principes des cendres des végétaux.

Le terreau pur est anti-septique. (p. 183-185)

Translation (by authors):

[…] property of humus makes it unique, to maintain vegetation, to feed plants with extractables, isolated and non-renewed, or to supply the soil. Its insoluble part prevents undecomposed plant material to provide plants with harmful compounds from fermenting vegetation.

Executive Summary

Carbon is found in greater proportion in the soil than in the plants from which it comes. However, the proportion of carbon contained in the soil does not seem to be significantly increased by the continued action of the causes that produce it.

[…] Oxygen takes away carbon. Humus, in losing this element, also loses oxygen and hydrogen as it forms water as well as a soluble extract in the water. The soil appears to be so completely destructible at atmospheric temperature, by the action of uniting oxygen and the solutes.

Extractables of soils contribute to some degree to its fertility: the ashes contain all the principles of the ashes of plants.

(De Saussure, 1804)

Das Holz, welches beim flammenden Verbrennen ebenfalls nur Kohlensäure und (Sprengel,

4

Wasser bildet, verwandelt sich durch ein langsames und dunkeles Verbrennen auch in eine besondere Säure, nehmlich in Humussäure oder Ulmin und Wasser. (p. 66)

Von den liquiden Alkalien wird sie sehr leicht und vollständig aufgelöst, und überhaupt besitzt sie eine große Verwandtschaft zu allen Alkalien. (p. 70)

Beym Verwesen der Pflanzenkörper, welches ebenfalls durch Sauerstoffgas bedingt ist, entstehen Kohlensäure, Wasser und Humussäure, und mehrere Flechtenarten liefern, wenn sie zugleich mit alkalischen Substanzen in Berührung sind, außer Kohlensäure und Wasser, farbige Säuren, welche jedoch nur als Repräsentanten der Humussäure zu betrachten find. Alle die besonderen Erzeugnisse des dunkeln Verbrennens organischer Materie sind aber nicht beständig, sondern verbrennen auch nach und nach ganz, wenn sie dem Einflusse der Luft und des Wassers ausgesetzt bleiben. Letztes beschleunigt das Verbrennen derselben dadurch, daß es aus der Luft Sauerstoffgas absorbiert, und dieses, gleichsam in tropfbar-flüssiger Form, der organischen Materie zuführt. Unendlich groß ist die Menge der organischen Substanz, welche von dem Wasser auf diese Art täglich vernichtet wird, und man kann annehmen, daß die Bäche, Flüsse und Meere der Luft eine vielleicht hundertmal größere Quantität Sauerstoffgas entziehen als durch alle die auf der Oberfläche des festen Theils der Erde statt findenden Processe des Athmens, des Verbrennens u.s.w. verbraucht wird. (p. 74-75)

1826)

Die pulverförmige braune Substanz, welche sich bei der Verwesung der Pflanzen bildet, nennt man bekanntlich Humus und theilt diesen nach seinen Eigenschaften in sauren, milden, halbverkohlten, oxydirten und harzhaltigen Humus. (p. 261)

Zur Darstellung der Säure wird der Torf mit verdünnter Salzäure ausgelaugt, mehrere Tage in einem verschlossenen Gefäβe mit Ammoniak digerirt und die abfiltrirte mit Wasser verdünnte Flüssigkeit wird mit Salzsäure versetzt, bis die sauer reagier, der entstandene Niederschlag wird nach einigen Tagen gesammelt und ausgewaschen. Diese Humussäure enthält stets etwas Eisenoxyd und Alaunerde, zu beiden Basen hat sie sehr grosse Anziehung; zur Abscheidung derselben wird sie in verdünnter kohlensaurer Natronauflösung aufgelöst und wieder durch Salzsäure präcipitirt. (p. 262)

(Sprengel, 1826)

[…] die Säuren, im verdünnten Zustand und mit Unterstützung von Wärme, den Zucker, das Gummi und die Stärke in eine schwarzbraune Substanz verwandeln, die die Huminsäure nennen, aus dem Grunde, weil dieselbe Substanz Bestandtheil des Humus (Dammerde) ausmacht […] In all diesen Fällen wird sie in zwei Modificationen erhalten, als Säure und als indifferenter Koerper, die wir mit

(Berzelius, 1839)

5

den Namen Huminsäure und Humin bezeichnet haben. (p. 386)

"Man sieht leicht, dass die Chemiker bis jetzt gewohnt waren, alle Zersetzungsproducte organischer Verbindungen von brauner oder braunschwarzer Farbe mit Humussaeure oder Humin zu bezeichnen, je nachdem sie in Alkali loeslich waren oder nicht, dass aber diese Producte in ihrer Zusammensetzung und Entstehungsweise nicht das geringste miteinander gemein haben. Man hat nun nicht den entferntesten Grund zu glauben dass das eine oder andere dieser Zersetzungsproducte, in der Form und mit den Eigenschaften begabt, die man den vegetabilischen Bestandtheilen der Dammerde zuschreibt, in der Natur vorkommt.

Die Eigenschaften des Humus und der Humussaeure der Chemiker sind von den Pflanzenphysiologen unbegreiflicherweise uebertragen worden auf den Koerper der Dammerde*, den man mit dem naemlichen Namen belegt; an diese Eigenschaften knuepfen sich die Vorstellungen ueber die Rolle, die man ihm in der Vegetation zuschreibt"

*Unter Dammerde (terreau) versteht man ein Gemenge von verwitterten Mineralsubstanzen mit Ueberresten vegetabilischer und Thierstoffe; ihrer ganzen Beschaffenheit nach laesst sie sich als Erde betrachten, in welcher sich Humus im Zustande der Zersetzung befindet." p 278

(Liebig, 1840),

Darauf kochte ich den zurückbleibenden Torf mit einer Auflösung

von kohlensaurem Natron. Es blieb ein sehr grosser Theil, welcher unlöslich war, zurück. Dieser bestand aus Planzenrückständen, welche noch nicht in Humin und Ulmin verwandelt waren,und aus diesen beiden Körpern selbst, welche sich in dem Alkali nicht auflösen (p. 321)

…in angesäuertem Wasser […] unlösliche, aber in reinem Wasser lösliche, das ist die Huminsäure. (p. 322)

(Mulder, 1840)

In the first place it is evident that the organic matter of soils is made up of a large number of chemical compounds. This might very well be assumed on theoretical grounds and it has been pointed out in previous bulletins that plants, which constitute the greater portion of the material out of which the organic matter of soils is made, contain a vast number of chemical compounds that are added to the soil on the death and decay of the plants. Now, while there seems to be no disposition to question this fact, there has been the assumption on the part of many agriculturists that in some mysterious way this conglomerate of plant compounds added to the soil becomes transformed by decomposition into a single group of closely related bodies, humic acid, etc., and that no matter how varied may be the organic remains, or how diverse the condition of decay, soils vary in organic matter chiefly in amount. That this is not so is sufficiently proven by the facts here established. (p. 43)

(Schreiner and Shorey,

1840)

6

While it is true that the tendency in the decay of organic matter is toward simpler compounds and ultimately to a few simple compounds or the elements, the material known as soil organic matter is in the transition stage from the complex compounds of living organisms to the simple ultimate products. When the organic matter has decayed to the stage of such simple compounds as ammonia, nitric acid, free nitrogen or carbon dioxide, it is no longer organic matter and much of it may escape from the soil altogether. (p. 45)

In connection with this discussion of hydrocarbons in soils it should be noted that there are in nearly all soils dark-colored particles of organic matter that are very resistant to solvents and the action of either acids or alkalies. These sometimes are evidently particles of charcoal or coal that may have found their way into the soil, but a considerable portion is often made up of structureless bituminouslike material having the elementary composition so far as examined of lignites or brown coals. (p. 12)

Da sich aber durch Einwirkung von Luft und Wasser auf pflanzliche und tierische Gewebeteile fast stets gewisse stark dunkelgefärbte Stoffe neu bilden, wurde diesen sehr frühzeitig als für den Ackerboden charakteristisch und als Humusstoffen im engeren Sinne besondere Bedeutung beigemessen. (p. 100)

[…] scheint es vorläufig am zweckmäβigsten, die Farbe als das charakteristische Merkmal desselben [der Humusstoffe] anzusehen. Diese amorphen, dunkelbraunen Massen ohne erkennbare Zellstruktur bilden such stets, wenn die Organismen genügend lange Zeit den Atmosphärilien ausgesetzt bleiben. Je älter eine pflanzliche Ablagerung ist, desto mehr tritt diese dunkle Färbung hervor, und man spricht daher mit recht von verschiedenen Graden der Humifizierung. (p. 101-102)

Humusstoffe sind jene gelbbraun bis dunkelschwarzbraun gefärbten Substanzen unbekannter Konstitution, welche durch Zersetzung der organischen Substanz entweder in der Natur durch Einfluβ der Atmosphärilien oder im Laboratorium durch chemische Einwirkung (vornehmlich von Säuren oder Laugen) gebildet werden. (p. 102) Als Huminsäuren sollen daher diejenigen Humusstoffe bezeichnet werden, welche Wasserstoffionen abzuspalten vermögen und mit starken Basen unter Wasserbildung typische Salze geben. (p. 105)

Als Humus in weitesten Sinne werden in der Bodenkunde und Agrikulturchemie oft sämtliche organischen Stoffe des Bodens bezeichnet. […] Dieser weiteste Humusbegriff umfaβt daher sowohl frische und unzersetzte Pflanzenbestandteile als auch in Zersetzung begriffene oder völlig zersetzte Stoffe. (p. 100)

Besonders das Leben von Bakterien und anderen Kleinlebewesen des Bodens scheint an den Humus eng geknüpft zu sein. Inwieweit dieselben bei der Bildung der Humusstoffe eine wesentliche Rolle spiele, dürfte vorläufig unentschieden sein. Jedenfalls spielen sich jedoch die Lebensprozesse gröβerer Mengen niederer Organismen im Humus ab, und wir dürfen erwarten, die

(Oden, 1919)

7

Ausscheidungen und Spaltungsprodukte derselben im Humus vorzufinden. (p. 101)

[…] the most important part of the soil organic matter, the humic matter, is a high molecular organic colloid, […] it is a secondary product derived from the complex organic constituents of plant residues, themselves of unknown or little-known constitution, and since it is not known from which of these plant constituents it is derived. […] The organic matter of the soil may be classified into two parts, the humic matter and the non-humic matter. (p. 455)

In this series of papers it is proposed to avoid altogether the use of the term "humus", owing to the widely varying meanings attached to this word by different workers; the term "humic matter" will be used as a description of the dark-coloured high-molecular colloidal organic matter which is a characteristic constituent of the soil; nonhumic matter in the soil includes not only the colourless organic substances, largely soluble, which result from the chemical and biological decomposition of plant and animal residues and from the further decomposition of humic matter itself, but also the undecomposed parts of those residues containing cellulose, lignin, waxes, and so on, which are also colourless. (p. 456)

The term "humic acid" will be used in this series of papers for the part insoluble in alcohol. (p. 456)

Of that part of the humic matter not dissolved by cold, weak alkali, a portion is soluble in hot alkali. When it is convenient to use a name for this fraction, it will be designated as "humin". (p. 456)

(Page, 1930)

A resistant complex consisting of lignin and protein, two of the important constituents of plant, animal, and microbial residues, has been synthesized in the laboratory. This complex is similar in physical appearance and possesses the various chemical, physic-chemical and biological properties characteristic of the major portion of the soil organic matter, which is usually referred to as “humus” or “humic acid.” (p. 66)

Although chemists and agronomists have frequently used the terms “humus,” “humic acid,” “ulmic acid,” “humin,” “ulmin,” and numerous other designate complexes with seemingly different chemical properties, it has become now generally recognized that all these preparations were not definite compounds but depended largely upon the procedures used for their extraction. All these

(Waksman and Iyer,

1932)

8

dark colored organic complexes are left in soil or are formed there as a result of the decomposition of plant and animal residues. The nature of the soil, the nature of the residues, and the conditions of decomposition largely influence their chemical nature.

Numerous attempts have been made since the 1830 to synthesize by chemical procedures dark colored complexes which would have the same properties as the humus of the soil, of peat, and of decomposing plant residues (23). The dark color of the soil humus, its solubility in dilute alkalies, and its precipitation by acids, as well as its relatively high carbon content, served as a basis for these attempted syntheses; numerous complexes were this prepared having properties similar to those of humus in soils and composts. The treatments consisted in the action of acids at high temperatures on carbohydrates, the interaction of sugars with amino acids, and the oxidation of phenol and its derivatives. The dark colored preparations formed under these conditions cannot be compared with full justification with those formed under natural conditions. (p. 43)

However, these views that humic substances are complex products of synthetic nature and that micro-organisms participate in their synthesis were not developed prior to the 20th century. At the turn of the century many investigators believed that humus was a complex and indeterminate mixture of non-specific nature which are products of the decomposition of plant residues. (p.24)

From the data at present available it has been established that the humic-acid molecule consists of (a) an aromatic ring (b) nitrogen containing compounds in cyclic forms and in the form of peripheral chains and (c) possible reducing substances. (p. 65)

At the present time there are no grounds for doubting that humic substances are high-molecular-weight compounds. (p.84)

(Kononova, 1961)

Humus represents a quasi-equilibrium stage in the decomposition of plant and microbial constituents, and as such its chemical composition is likely to fluctuate to some extent with variations in these materials. The major components, apart from polysaccharides, are thought to be polymers formed from the recombination of units such as quinones, amino-acids and aldehydes, compounds of varied complexity synthesised from simple molecules by the soil micro-organisms, particularly fungi, and possibly cell wall material from the micro-organisms. (p. 855)

(Whitehead and Tinsley,

1963)

9

The resistance of humic acid to microbial and to chemical decomposition could be explained if it were a large spherical molecule, consisting of many heterogeneous units, irregularly crosslinked by different covalent bonds, so that innumerable extra-cellular enzymes from many different micro-organisms would be needed to dissemble it piece by piece from the outer surface. This would be a lengthy process unless a greater variety of microorganisms were encouraged by the addition of readily decomposable debris (pp. 153ff)

(Swaby and Ladd, 1962)

Humic substances may be described as polymers (300 – 30000 molecular weight) containing phenolic OH and carboxylic groups with a lower number of aliphatic groups. (p. 514)

(Stumm and Morgan,

1996)

A series of relatively high-molecular weight, brown to black colored substances, formed by secondary synthesis reactions. (p. 36)

(Stevenson, 1982)

A general category of naturally occurring, biogenic, heterogeneous organic substances that can generally be characterised as being yellow to black in colour, of high molecular weight and refractory. (p. 4)

Humic substances are not biologically predestined to carry out a specific biochemical action and thus cannot be defined in functional terms. As a result, humic substances must be defined operationally (p.4)

(Aiken, et al., 1985)

The term humic substances refers to an operationally defined, heterogeneous mixture of naturally occurring materials. These substances are ubiquitous in nature and arise from the decay of plant and animal residues in the environment. (p. 1)

(MacCarthy, et al., 1990)

A series of relatively high-molecular-weight, brown- to black-colored substances formed by secondary synthesis reactions: the term is used as a generic name to describe the colored material or its fractions obtained on the basis of solubility characteristics; these materials are distinctive to the soil (or sediment) environment in that they are dissimilar to the biopolymers of microoorganisms and higher plants (including lignin). (p. 32-33)

(Stevenson, 1994)

Humic substances: Organic molecules with chemical structures which do not allow them to be placed into discrete categories of biopolymers including polysaccharides and sugars, proteins and amino acids, fats, waxes and other lipids, and lignin.

(Baldock and Nelson, 2000)

I. Humic substances comprise an extraordinarily complex, amorphous mixture of highly heterogeneous, chemically reactive yet refractory molecules, produced

(MacCarthy, 2001)

10

during early diagenesis in the decay of biomatter, and formed ubiquitously in the environment via processes involving chemical reaction of species randomly chosen from a pool of diverse molecules and through random chemical alteration of precursor molecules.

II. The molecular heterogeneity inherent in humic substances renders the humic material highly refractory, thereby serving a key role in the Earth’s ecological system. (p. 740-741)

.....on the degradative journey from a recognizable biochemical compound to CO2, organic carbon may become a component of a group of natural organic substances that are refractory (recalcitrant) in soil, sediment and natural waters, and do not fall into any discrete category of biochemistry. These compounds are collectively known as humic substances. (p. 129)

(Essington, 2004)

The transformations are known collectively as the humification process, and the final product, or humus, is a mixture of substances that has some resistance to further degradation. (p. 1)

Because empirical, and consequently molecular formulas cannot be deduced for humic molecules it is obvious that the concepts of configuration and accurate structural formula cannot be applied. Thus, the basic concepts inherent in the definitions of the empirical formula, structural isomerism, and stereoisomerism that are applicable to discrete compounds do not apply for HS, at least on the basis of what we know at this time. To do so would require that there be biological or genetic control of the synthesis process, and there is no evidence for such. Humic substances must be considered as gross mixtures. (p. 47)

(Clapp, et al., 2005)

In this textbook, we use the general term soil organic matter (SOM) to encompass all the organic components of a soil: (1) l9iving biomass (intact plant and animal tissues and microorganisms); (2) dead roots and other recognizable plant residues or litter (although in practice, residue particles that do not pass 2-mm sieve openings are often excluded from consideration); and (3) a largely amorphous and colloidal mixture of complex organic substances no longer identifiable as tissues. Only the third category of complex organic material is properly referred to as soil humus (p. 510)

Humic substances comprise about 60 to 80% of the soil organic matter. They are comprised of huge molecules with variable, rather than specific, structures and composition (p. 511)

Humic substances generally are dark-colored, amorphous substances with molecular weights varying from 20000 to 300,000 g/mol. Because of their complexity, they are the organic materials most resistant to microbial attack. (p.

(Brady and Weil, 2008)

11

512)

Studies using radioactive isotopes have shown that some organic carbon incorporated into humus thousands of years ago is still present in soils, evidence that humic materials can be extremely resistant to microbial attack. (p. 513)

SOIL ORGANIC MATTER

See Humic Substances (p.686)

In addition, soils also contain heterogeneous, physicochemically active, biodegradation-resistant macromolecular materials generically referred to as humic substances. […] Humic substances can be operationally classified into three separate fractions defined in terms of their solubility properties (p. 315)

A proportion of soil organic matter shows a definite chemical composition with structural features that are shared with the macromolecular constituents of the biomass e.g., celluloses, hemicelluloses, lignins, cutins, suberins, and proteins, etc., from plant and microbial origins. (p. 315)

Formation of humic substances

Assuming that humic substances are the most resistant organic fractions of the soil, the humification process should be considered to be of capital importance with regard to the understanding of soil quality and the mechanisms of C sequestration. Owing to the fact that the overall formation of humic substances requires hundreds of years and the contribution of a large variety of agents and starting materials, it is to be expected, perhaps, that information on the subject is only of a general nature, with several controversial points still requiring clarification. While classical concepts have postulated lignin and aromatic microbial metabolites as major starting materials, recent evidence indicates the importance of alternative humification mechanisms that are exclusively based on aliphatic precursors such as carbohydrates (either by catalytic dehydration reactions in soil reducing microcompartments or by the effect of fires, for example) and lipids (alteration of aliphatic biomacromolecules that include non-hydrolyzable esters or condensation of unsaturated fatty acids). (p. 319)

Clearly, different processes are involved in soil C sequestration (Figure C29): such as selective preservation of biomass, diagenetic alteration of biomacromolecules and humification by neoformation sensu stricto (microbial, enzymatic or abiotic). (p. 98)

(Chesworth, 2008)

I. Humic substances are organic compounds in humus not synthesized directly to sustain the life cycles of the soil biomass. More specifically, they are dark-colored, biologically refractory, heterogeneous organic compounds produced as the by product of microbial metabolism. They differ from the biomolecules

(Sposito, 2008)

12

present in humus because of their long-term persistence and their molecular architecture.

II. Humic substances emerge from a slow process of biological decomposition, oxidation, and condensation as characteristic organic mixtures having two fundamental properties: 1. Supramolecular association: self-organized assemblies of diverse low-molecular mass organic compounds that have either a predominantly hydrophilic (fulvic acid) or hydrophilic-hydrophobic (humic acid) nature, with the latter being mediated in aqueous solution by hydrogen bonds and hydrophobic interactions.

2. Biomolecular provenance: identifiable biopolymeric fragments that form an integral part of a labile molecular architecture and that govern both conformational behavior and chemical reactivity. (p. 70)

Soil humus in itself is not biologically refractory. […] humus in aqueous extracts can be degraded readily under aerobic conditions over periods of days to weeks. Evidently, anaerobic conditions and, more significantly, interactions with soil particles are essential in protecting humus from microbial attack and conversion to CO2. (p. 82)

With the two concepts remaining till nowadays, the meaning of the word ‘‘humus’’ is still unprecise. One can advice the use of it more in the sense of a horizon concept and keep ‘‘SOM’’ for humus-as-a-constituent. Nevertheless, the qualifier ‘‘humic’’ and its derivatives are always used in the sense of a ‘‘constituent,’’ e.g., ‘‘humic substances.’’

(Feller, 2009).

It would be pointless to draw up a classification system that takes account of several fractions based on charge density differences, or even differences in solubilities in organic solvent systems. (p. 8)

(Hayes, 2009)

A series of relatively high molecular weight, yellow to black colored substances formed by secondary synthesis reactions. The term is used as a generic name to describe the colored material or its fractions obtained on the basis of solubility characteristics. These materials are distinctive to the soil (or sediment) environment in that they are dissimilar to the biopolymers of microorganisms and higher plants (including lignin), (p. 43)

Humus: Total of the diverse organic components in the environment exclusive of undecayed plant and animal tissues, their “partial decomposition” products, and the soil biomass.

Soil organic matter: Same as humus.

(Huang and Hardie, 2009)

13

Humic substances: A series of relatively high-molecular-weight, yellow to black colored substances formed by secondary synthesis reactions. The term is used as a generic name to describe the colored material or its fractions obtained on the basis of solubility characteristics. (Table 2.1, p. 43)

There is a large volume of work documenting the polycondensation and polymerization of simple biomolecules (e.g., polyphenols, amino acids, and sugars), as catalyzed by enzymes and soil minerals, leading to the formation of humified substances (Bollag et al., 1998; Huang, 2000, 2004). Since these catalysts are ubiquitous in soil environments, and the substrate biomolecules are readily available from the continuous decomposition of organic residues as discussed in Section 2.3, it is certain that these reactions occur in the natural environment. Furthermore, these reactions are responsible for browning reaction of biomolecules observed during oxidative decomposition, and they provide an explanation for the dark color of humic substances in soils. (p. 61)

[…] the complex mixture of plant and microbial products, degraded to small organic molecules which have been described above, is consistent with the first stage of HS [humic substance] formation.

The second stage of HS formation is through the selective chemical and microbial synthesis. (p. 180)

Thus far, the question on how the energy, kinetic, and thermodynamics relationships operate in soil systems, especially those relations associated with the biontic humification of simple organic compounds into PKs [polyketides], has not been answered. (p. 202)

(Schnitzer and Monreal,

2011)

Organic molecules with chemical structures, which do not allow them to be placed into the category of nonhumic biomolecules. (p. 11-3)

(Baldock and Broos, 2011)

Humic acids (HA) are natural organic compounds that are important components of organic matter.

(Kulikova, et al., 2014)

New compounds are also being formed – humic substances - and added to the mixture called humus. (p. 3)

[…] soil organic matter is a term frequently used to indicate the dead organic fraction only, and the live fraction, although of equal importance, is usually ignored. This dead organic fraction was divided in the previous chapter into two groups: a group of organic matter at various degrees of decomposition, related to litter, and another group composed of completely decomposed materials,

(Tan, 2014)

14

which was identified as humus. (p. 37)

[…] humus is distinguishable into a nonhumified and a humified fraction (p. 39)

After their release. These substances in question are usually subject to further degradation and decomposition reactions, and are the main sources for the synthesis and formation of the humic fraction by a process called humification. (p. 39)

The humified compounds are the most active substances in soils, possessing electrical charges and exchange capacities exceeding those of the clay minerals. They are essentially new products in soils synthesized from the non-humified materials released during decomposition of plant and animal residue with or without the assistance of microorganisms. (p. 69)

The nonhumic matter is now defined to include all substances released by decomposition of residues of plants and other organisms. (p. 39)

Humic substances are the major organic constituents of soil (humus), peat, coal, many upland streams, dystrophic lakes and ocean water

http://en.wikipedia.org/wiki/Humic_aci

d

Humic substances (HS) are complex and heterogeneous mixtures of polydispersed materials formed in soils, sediments, and natural waters by biochemical and chemical reactions during the decay and transformation of plant and microbial remains (a process called humification). Humic substances are highly chemically reactive yet recalcitrant with respect to biodegradation.

http://www.humicsubstances.org/whatarehs.html

Following decomposition, a small fraction of the C in NPP and NSP is preserved either because it becomes metabolized to a recalcitrant state, such as humic substance, or is protected physically through association with secondary minerals or trapped within soil aggregates. Soil OM consists of recognizable, partially decayed plant residues, soil microorganisms, soil fauna, by-products of decomposition, and humic substances. The formation of humic substances results from many events of oxidation and reduction that create materials with increased C and H and lower O content compared to the original animal, microbial, and plant tissue. (p. 360)

OM in soils and sediments represents residual compounds and the organic structures remaining following the decomposition of plant, fauna, and microbial inputs. (p. 360)

(Horwath, 2015)

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