Humic Matter in Soil and the Environment: Principles and Controversies - Chapter 2 pot

On the basis of color and solubility, Mulder 1862 classified them into 1 crenic and apocrenic acid, the yellowish to brown fractions soluble in water, 2 ulmic acid and humic acid, the br

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CHAPTER 2

OF HUMIC MATTER

2.1 CONCEPTS AND HISTORICAL BACKGROUND

2.1.1 Historical Concepts

Humic matter as defined earlier is composed of a variety of sub-

stances that can be obtained by fractionation on the basis of their

solubility in alkaline and acidic solutions The first humic substances

isolated were compounds soluble in bases, acid solution and water

They were extracted in Sweden by Berzelius (1839), who assigned

them the names of crenic and apocrenic acids, the fulvic acids of today

This investigation was continued by Mulder (1840), a former student

of Berzelius, who in the following years isolated additional humic

fractions On the basis of color and solubility, Mulder (1862) classified

them into (1) crenic and apocrenic acid, the yellowish to brown

fractions soluble in water, (2) ulmic acid and humic acid, the brown

and black fraction, respectively, soluble in alkali but insoluble in acid,

and (3) ulmin and humin, the fractions insoluble in alkali, acid and

water Mulder's accomplishment was followed twenty-seven years later

by the discovery of hymatomelanic acid, isolated by Hoppe-Syeler

(1889) as the ethanol-soluble fraction of humic acid Since then no

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The Nature and Distribution of Humic Matter 15

further achievements of major importance can be noticed in the study

and isolation of humic substances until Oden's (1914; 1919) concept

surfaces a t the start of the twentieth century Considered by many

scientists as based on a more solid scientific foundation, it is in essence

a revision of Mulder7s classification Oden recognizes (1) fulvic acid,

which replaces the use of the terms crenic and apocrenic acid, (2)

humic acid for the fraction soluble in alkali and insoluble in acid, (3)

hymatomelanic acid, a name used earlier by Hoppe-Syeler (1889) for

the humic acid fraction soluble in ethanol, and (4) humus coal, for

replacing humin and ulmin

2.1.2 Concepts in the Early Twentieth Century

Oden's theory, though challenged many times during the years,

has set the stage for the development of the concepts and types of

humic substances followed today It triggered in the beginning a flurry

of investigations and the ligno-protein theory was introduced during

this period by a number of people (Fuchs, 1930a, b, c; 1931; Hobson

and Page, 1932a, b, c; Waksman, 1938), a concept that has dominated

humic acid chemistry and formation till today This theory assumes

humic matter to be the product of reactions mainly between lignin and

protein, two important components of plant tissue Although it is a

very viable theory that is used by Flaig (1975) as the foundation of his

humic acid concept, other theories were presented when more became

known about humic acid chemistry toward the end of the century This

will be discussed in more detail in Chapter 4 on biochemistry and

formation of humic matter

The new burst of research activity has also resulted in the

development of new names, often amounting only to noise in the

nomenclature of humic substances Names, such as rotteprodukte

(rotten or decomposed products) and echte huminsauren (real humic

acid), have been proposed by Simon and Speichermann (1938)

Springer (1938) added humoligninsauren and lignohuminsauren

(lignin- or lignohumic acid) to the confusion Attempts, especially of

fractioning humic acid further into several subtypes, have come up

with more names By manipulation of the fractionation procedures, it

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16 Chapter 2

has been reported that humic acid can be subdivided into a- and

used for the initial or original humic acid fraction before ethanol

extraction for isolation of hymatomelanic acid, whereas the humic acid

residue, remaining after ethanol extraction, is called simply humic

acid The name P-humic acid has been reserved by Waksman (1936)

for a precipitate produced after adjusting the pH of a fulvic acid

solution to 4.8 with NaOH According to Stevenson (1994) this

p-fraction is an Al-humate with properties similar to an organic

substance obtained by Hobson and Page (1932a, b, c) named

confusing, but the identification of a fulvic acid fraction as a humate is

questionable It is also difficult to call it an AE-fulvate, since it is

insoluble in acid condition Hence the only alternative is the possibility

that the separation of humic acid from fulvic acid in the soil extract

has been conducted improperly or a proper fractionation procedure is

indeed unavailable Consequently, the analysis is fraught with many

errors or uncertainties, justifying claims for the production of artifacts

Although several people agree with a- humic acid a s defined above, it

makes more sense if this name is assigned to the insoluble part of

humic acid remaining after ethanol extraction This residue has

changed in composition and it seems more reasonable to assign it the

symbol a and retain the name humic acid for the original substrate

before ethanol treatment Hence, humic acid can be fractionated by

ethanol into an a-fraction and hymatomelanic acid In analogy to the

above, reference can be made to the division of humic acid into a brown

Germany By using neutral salt solutions, Springer (1938) succeeded

in separating humic acid into a (1) brown fraction (Braunhuminsaure),

soluble in NaC1, and (2) gray fraction (Grauhuminsaure), insoluble in

NaC1 The brown humic acid is said to be highly dispersible, contains

a lower carbon content, and according to Stevenson (1994) has

characteristics of humic matter in peat and in brown coal However,

Kononova (1966) is highly critical of humic matter originated from coal

or peat, since these two materials are formed in anaerobic conditions

completely opposite to the aerobic system present in soils responsible

for formation of soil humic matter Springer's gray humic acid has a

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The Nature and Distribution of Humic Matter 17

low degree of dispersion, and is easily coagulated In Stevenson's

opinion, it looks similar to the humic acid in mollisols

With research interest declining among soil scientists facing

resentment due to an apparent identity crisis on humic acids late in

the twentieth century as discussed in Chapter 1, scientists from a wide

variety of other disciplines have taken the lead in humic acid research

Chemists, geochemists, hydrologists and environmentalists have

become fascinated by the ubiquitous presence of humic matter in the

ecosphere In contrast to most soil scientists, they recognize its

profound role in environmental issues, and its effect on migration and

immobilization of industrial and nuclear waste and other pollutants

In industry, medicine and pharmacy, humic substances are recognized

as potential sources for production of valuable chemicals They are

considered commercially viable to be used as surfactants and as

drilling fluids in oil exploration, as well as in medicines for human

health Research in humic matter took a sharp turn away from soil

science to make rivers, lakes and oceans the centers for explorations

and investigations of humic substances The result is that new humic

compounds, assigned exotic names, have been discovered, enlarging

our concept of humic matter The name copropel is presented for a

humic substance, labeled as humus by Swain (1963) and Stevenson

(1994) It has been formed from the decomposition of microscopic

plants in eutrophic lakes and marshes A black mass of humified

material located a t deeper hypolimnetic areas of lakes and bays is

called sapropel, whereas a pondweed type of sapropel, believed to

originate from cellulose-rich plants, is called fdrna by the authors

above A deposit in dystrophic lakes consisting of an allochthonous

precipitate of humic acid and detritus is referred to as dy Marine slime

resulting from settled decomposed plankton detritus is calledpelogoea

and an amorphous, gummy accumulation of humic substances beneath

or within peat bogs is dopplerite Recently, less exotic names have been

used for humic matter present in the water medium It is known by

geochemists under the collective name of aquatic hurnic materials as

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18 Chapter 2

opposed to terrestrial humic materials for humic compounds in soils

(Christman and Gjessing, 1983)

The few soil scientists, who have continued with humic acid

research, have also yielded some results An alleged new humic acid

fraction, identified by the namegreen humic acid, has been isolated by

Kumada and Sato (1962) Chromatography using a cellulose column

separated a humic acid extract of a spodosol into a green and brown

fraction The green fraction, called green humic acid, is believed to be

derived from a fungal metabolite (Kumada and Hurst, 1967) and has

attracted considerable attention, especially in Japan and New Zealand

In Japan, much value has also been placed in the use of visible light

spectrophotometry in the identification of humic substances The

nature of absorption spectra and values of A log K (= K,,, - KO, in

which K = extinction) are applied to distinguish humic acids into four

major types, e.g., types A, B, R,, and P (Kumada, 1965; 1987; Kumada

and Miyara, 1973; Yoshida et al., 1978) The P-type of humic acid

produces, after separation by gel filtration and column chromatography

with cellulose powder or sephadex, a P, (brown) and P, (green) fraction

The P, humic fraction corresponds to the green humic acid discussed

above However, the existence of green humic acid is later rescinded by

Kumada (1987), who considers the name as incorrect since humic acid

is by definition brown to black in color In his opinion the green

fraction is an impurity commonly co-extracted with the brown (PJ

fraction

In the United States, with many of the prominent authorities

adhering to the concept of humic substances being operational

compounds, no new discoveries have been noticed in recent years

Though not really a major breakthrough, Stevenson's (1994) suggestion

for distinguishing generic humic substances should perhaps be

mentioned in all fairness, though this term had been recognized earlier

by Kononova (1966) and Kumada (1987) The name of generic fulvic

acid is used by Stevenson for fulvic acid purified by the XAD-resin

procedure Accordingly, he believes that fulvic acid can be distin-

guished into a (1) generic or true fulvic acid, obtained by purification

with amberlite-XAD resins, and (2) fulvic acid, obtained after

purification using conventional ion exchangers and dialysis procedures

However, the analogy presented by Stevenson in reference to the

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subdivision ofhumic acid by MacCarthy et al (1979) makes the generic

concept a very confusing issue Peat humic acid is in fact separated

into two subfractions by MacCarthy and coworkers using a pH

gradient elution technique, whereas Stevenson's generic fulvic acid is

just a XAD-resin purified version of fulvic acid, involving no separation

into subfractions a t all Consequently, the correctness ofusing the term

'generic' is still open for questions, whereas the subdivision of humic

acid as discussed above is also suspect More convincing research data

are needed differentiating unequivocally the generic from the

conventional type of fulvic acid No supporting data have been

presented confirming Stevenson's contention that generic fulvic acid is

lower in carbohydrate and peptide contents than conventional fulvic

acid

In summary it can be stated that several new concepts on humic

matter and a variety of new humic substances have been presented or

discovered toward the modern era This has no doubt broadened the

concept of hurnic matter Some of them have been used occasionally

today, though several scientists tend to consider them only of academic

importance However, Waksman's (1938) proposal t o delete all the

names and replace them by humus has not found wide acceptance On

the other hand, Oden's concept on humic acid, fulvic acid and

hymatomelanic acid and Mulder's idea of humin seem to have

weathered all criticisms The trend can be noticed that they are widely

used today by the majority of scientists, though reluctantly by some

Names such as green humic acid, gray and brown humic acid have

been used sometimes, depending on research purpose and interest

2.2 DISTRIBUTION OF HUMIC MATTER

The distribution of humic matter is not limited to the soils eco-

system and to climatic conditions Thought a t first to be present only

in soils, humic matter is currently assumed to be the most widely

distributed organic carbon containing material on the earth's surface

It is present in soils, in water of streams, lakes and the oceans, and in

their foam and sediments, from the tropics to the arctic regions Its

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20 Chapter 2

presence as a major constituent of the huge deposits of peat, lignite or

leonardite, coal and oilshale adds to the dimension of its wide

occurrence in the world Geochemists are even of the opinion that the

greatest storehouses of humic matter are the oilshales (Swain, 1975)

Though most of the humic matter is a natural product synthesized in

the environment, some are now assumed to be anthropogenic in origin,

such as in polluted waterways, drainage ditches, and sewage ponds or

sewage lagoons Especially in the 'Old World,' such as in Europe,

anthropogenic humic matter, identified as humus of harbor and city

agglomeration sediments has started to become recognized (Cieslewicz

et al., 1996) No doubt, such types of humic matter are also abound in

other parts of the world, where stable civilizations have had the

opportunity to accumulate a lot of organic waste during the centuries

2.2.1 Humic Matter in Soils

Humic matter occurs in all kinds of soils since it is the major

fraction of soil humus Though variations as to its content can be

noticed due to differences in climate and drainage, humic matter can

be found in soils from the lowlands to high in the mountains of warm

tropical to frigid arctic climates It is also present in humid region to

arid region soils Famous for their high organic matter contents are

mollisols, soils under grass vegetation of the semihumid regions

Contents have been reported as high as 5 to 6% in terms of organic

carbon (Stevenson, 1994) This is equivalent to approximately 9- 10%

organic matter, and half of this is estimated to be humic matter The

latter has a composition, characterized by a humic acid content slightly

dominating that of fulvic acid, as noticed by its fulvic acidlhumic acid

ratio ranging from 0.9 to 0.6 (Tan, 1978) Another soil with a similar

high humic matter content is the andosol, a soil occurring in the humid

tropics to the arctic regions (Arnalds et al., 1995; Tan, 1984; Theng,

1980) However, the composition and type of its humic matter differ

markedly from those of the mollisols The fulvic acid content makes up

more than half of the humic matter in andosols, which is in sharp

contrast with that in mollisols The fulvic acid Jhumic acid ratio is often

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noticed to range between 4.0 -1.0 in andosols of the humid tropics

(Tan, 1965; 1964) Another marked difference is that the humic matter

in andosols is present in close association with Al and allophane,

whereas the humic matter in mollisols is more likely present as Ca-

humate and Ca-fulvate Large amounts of humic matter are also

noticed in spodosols, where they are concentrated in the B, or spodic

horizons This is in contrast to mollisols and andosols where the A

horizons contain most of the humic matter The B, horizon deposit of

humic matter is often so thick that it becomes a valuable source for

commercial humate production as is the case in Florida (Lobartini et

al., 1992; Burdick, 1965) Spodosol humic matter has been a favored

material for investigations in Canada, where the results are taken to

apply also for humic matter from other soils (Schnitzer and Khan,

1972; Schnitzer, 1972; 1976) Judging from its formation due to

leaching from the A and E horizons, the general opinion is that

spodosol humic matter is composed of large amounts of fulvic acids

However, recent studies with a spodosol, on the border of Georgia and

Florida, reveal its humic matter to have a composition characterized

by a FA/HA ratio = 0.13, suggesting a humic acid concentration 10

times higher than that of fulvic acid (Lobartini et al., 1991) This

finding supports an earlier report for the Unicamp Company in Florida

showing the Florida source of its humate products to contain 91.3%

humic acid and 8.7% fulvic acid (Tan et al., 1988) Since spodosols are

very acidic soils, conditions favoring dissolution of large amounts of Fe

and Al, the humic substances are mostly in the form of Fe- and

Al-humates and hlvates

Other groups of soils containing hurnic matter are the ultisols

and oxisols, soils generally low in organic matter content due to a rapid

rate of decomposition The oxisols of the humid tropics are notorious for

their low organic matter contents, with contents often reported to be

as low as 1% organic carbon The humic matter in these soils is often

noticed to contain more fulvic acid than humic acid and these

substances are assumed to be present as Fe- and Al-fulvates or

humates (Tan, 1978) However, the lowest organic matter contents are

detected in aridisols, soils of the arid regions and the sandy soils in the

deserts Because of deficiency of water, biomass formation and

decomposition reactions in these dry regions are very limited

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22 C h a p t e r 2

2.2.2 Humic Matter in Soils of the Wetlands

The soils discussed above are generally well-drained, and the

aerobic conditions encourage the occurrence of rapid decomposition

processes However, under the influence of poor drainage, such as

occurring in marshes and swamps or in general in soils of the

wetlands, anaerobic decomposition prevails (Tan, 2000) Due to lack of

oxygen, the decomposition process is very slow, if not inhibited, and

incomplete, and hence contributes to accumulation of huge amounts of

organic matter Many of the wetlands and some of the lake areas are

eutrophic, encouraging an excessive growth of aquatic weeds and other

plants The latter provide an overabundance of dead organic residue

filling gradually the inundated or wet areas The partly decomposed

organic matter will eventually develop into bogs, peat, and muck,

which are often believed to be precursors for formation of coal and

ultimately fossil fuel (Hatcher et al., 1985) When conditions are

favorable, sapropel, copropel and the like may also develop into peat

and bogs The only conditions required are anaerobic environments for

the accumulation and development into peat and its eventual

conversion into coal

Peat deposits are also not limited to climatic conditions and can

be found all over the world where large amounts of biomass are

available and where decomposition of organic residue is inhibited They

are distributed from the tropical Amazon basin of Brazil and the

coastal regions of Sumatra, Indonesia, to the Baltic coast in Europe

and tundras in Alaska and other arctic regions In addition to excess

water, the frigid temperature in the tundras is another reason for

inhibiting decomposition of organic residue In his opening address a t

the 1972 International Meeting of Humic Substances a t Nieuwersluis,

The Netherlands, Golterman (1975) underscored the importance of

peat as the producer of humic substances affecting the living

environment of the Dutch people Called sometimes peatlands or mires

in Europe and Canada and known as histosols in the United States,

these organic deposits are believed to cover an area of 500 million

hectares worldwide, representing an organic carbon reserve of 1012

metric tons (Mathur and Farnham, 1985), and only the organic carbon

reserve in oilshale is believed to exceed this amount Swain (1975)

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T h e Nature and Distribution of Humic Matter 23

presented data showing the earth's crust to possess a total organic

carbon reserve of 19 x 1015 metric tons, and most of it (18 x 1015 metric

tons) is stored in oilshales

In contrast to a mineral soil system, where the organic matter

content makes up only a small fraction compared to the mineral

fraction, the organic matter content is an integral and substantial part

of peats or histosols By definition organic soils contain >80% organic

matter and ~ 2 0 % mineral matter (Brady and Weil, 1996) A more

complex definition, used in Soil Taxonomy (Soil Survey Staff, 1990),

indicates that organic soils must have an organic carbon content >18%

in the presence of > 60% clay, or > 12% in the absence of clay Most of

the information indicates that peats contain large amounts of humic

acids (Zelazny and Carlisle, 1974; Kononova, 1966), though occasion-

ally it is reported that humic matter in peat is composed mostly of

fulvic acids (Schnitzer, 1967) The humic acid content appears to

increase from peat to muck, with the more humified muck noticed to

contain its humic matter mostly in the form of humic acid (Preston et

al., 1981) The elemental composition, spectral characteristics and

other chemical properties of peat humic acids are believed to be similar

to those of humic acids in mineral soils (Mathur and Farnham, 1985)

However, as indicated before, Kononova (1966) was highly critical of

humic matter originated from peat, since the material has been formed

in anaerobic conditions, completely opposite to the aerobic system

present in soils

2.2.3 Humic Matter in Aquatic Environments

It is now an established fact that the distribution of humic mat-

ter is not limited to soils, but it has also been detected in streams,

lakes, and oceans and in their sediments These humic substances may

influence ground water properties and are considered to play an

important role in the geochemical cycle of organic carbon in aquatic

systems They are distributed in what they call dissolved organic

matter (DOM) or dissolved organic carbon (DOC), which according to

Aiken (1985) can be distinguished into two big groups, hydrophobic

and hydrophilic groups Each of the two groups can be subdivided

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