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Biodegradation of lignocellulose in soil: basic understanding of degradation mechanism.
Abstract:
"In all things of nature there is something of the marvelous.”—Aristotle.
Soil is a natural reservoir for all kinds of living being which controls the biogeochemical cycles
through the regenerative and degradative process. Understanding the lignocellulosic breakdown in
the soil system could be a valuable source which can be of great benefit for the utilization of
lignocellulosic materials to extract the valuable chemicals for the welfare of human beings. This paper
discusses about the current knowledge on the soil biodegradation system particularly in the lignin
degradation process. Lignin degradation in soil system occurs in two stages, modification in its
chemical structure and aggregate formation. The degradation of the complex aromatic rings in the
lignin polymer occurs as a result of aggregate formation called humus, further degradation of these
colloid results in humic acid, fulvic acid and humin. These structures resemble with the lignin
aromatic structures. Therefore, the understanding of the formation of humus could help in elucidating
the lignin deconstruction mechanism in soil. The present challenge lies in understanding the detailed
structural change of the humic acid, fulvic acid, and humin with respect to lignin. The lignin
degradation pathway in soil provides us a new perspective for improvement of biological pretreatment
of the lignocellulosic biomass for making biofuel and chemicals.
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1. Introduction:
According to the latest survey made by Thomson Reuters (2009), the U.S. ethanol consumption is
forecast to increase from 5.6 billion gallons last year to 13.5 billion gallons in 2012, far more than the
7.5 billion gallons in 2012 originally estimated. In order to decrease the green house gas emissions
,an alternative energy source that is capable of producing reduced amount of CO2 and also has being
cost effective is needed(Groom et al, 2008). The CO2 emissions estimated increase from 5,890
million tons in 2006 to 7,373 million tons in 2030.12 % of the green house gases emissions are
decreased by the production and combustion of bioethanol (Jason et al, 2006). Biofuel can be
produced from sugarcane, corn, lignocellulosic feedstock such as wheat straw, wood chips, and other
oil yielding plants such as pongamia and jatropa. Among most of the sources for used for the
production of biofuel, the lignocellulosic based biofuels has shown a great capability of replacing the
fossil fuel as they help in green house gas emissions (gas that contributes to the greenhouse effect by
absorbing infrared radiation) and also reduced release of CO2. Apart from these they would not
intense effect the ecological diversity when compared to others (Powlson et al, 2005; Farrell et al,
2006; Groom et al, 2008). When there is such rapid increase in the demand for biofuel, the need for a
breakthrough is necessary to meet the demands and reduce the green house gas emissions.
In order to produce biofuel from lignocellulosic biomass, the greatest challenge lies in the
deconstruction of lignin polymer for the release of sugars, which are used for the biofuel production.
Basically, lignocellulosic materials are the plant cell wall composites which are composed of 40-55%
of cellulose, 24-40% of hemicelluloses and lignin of about 18-25% in hardwood (Howard et al,
2003). In wheat straw the amount of lignin is about 18-20% (Tuomela et al, 1999) and about 25-35%
in soft wood (Nausbaumer et al, 1996). In the current context, bioethanol production from
lignocellulosic biomass would be mainly from the cellulosic (source of C-6 sugars) and hemicellosic
(source for C-5 sugars) components, and other useful organic components can be derived from the
lignin component which can be used for making bioplastics, dispersants in cement industry, additives
in agricultural chemicals, textile dyes, and carbon fibers (Jerfrey at al, 1983; Howard et al, 2003;
Roberto et al, 2003; Parajo et al, 1998). Recovery of cellulose portion from the biomass needs the
disruption of lignin and hemicellulose matrix. However, this process is considered as a challenge
making it more efficient, cheapest and safest technology.
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Lignin is a structure polymer of the vascular plants which helps in protecting the plant cell wall.
Degradation of this lignin compound is a major task because covalent lignin carbohydrate linkage
prevents the enzymatic degradation of lignocellulose and this is due to the fact that lignin can
depolymerize to oxidative units or co –polymerize and form complex aromatic structure.
Pretreatment is the process of releasing the cellulosic sugars by using various technologies such as
physical, physico-chemical, chemical, and biological processes. These different technologies that
have been used for pretreatment of lignocellulosic (Ye Sun et al, 2001) materials are acid treatment,
ammonia fiber/freeze explosion treatment, liquid hot water treatment, lime pretreatment etc. The acid,
lime and (ammonia fiber explosion) AFEX pretreatment technologies remove lignin significantly,
decrystallizing the cellulose which can be utilized for the bioethaol production. (Mosier et al,
2005).During the acid treatment dilute sulfuric acid is mostly used (Grohmann et al, 1985). For the
AFEX pretreatment, it is suitable for only hardwood rather than soft wood (Millan, 1994; Mosier et
al, 2005). As a part of pretreatment, the hemicelluloses are hydrolyzed during acid pretreatment and
alkali pretreatment .Thus decreasing the enzyme usage, making it cost efficient (Hahn-Ha et al,
2006).Though the present technologies are efficient enough to release cellulose sugars significantly,
for the technology to be cost effective the removed lignin components have to be made useful for
producing high value products which yet remains a challenge.
Biological pretreatment of the biomass is considered as one of the safest method to lignin
degradation, despite of the slow process (Akin et al, 1995; Gold et al, 1993). Biological pretreatment
is mainly focused on the use of enzymes (cellulases) produced by bacteria or fungi (e.g., Trichoderma
reesei) to hydrolyze cellulose (Hamelinck et al, 2005).As lignin is a complex organic polymer, in all
cases of pretreatment technologies, there is lack of enough information about the degradation
mechanism of the lignin which had become a potential barrier for an efficient deconstruction
mechanism. Due to the expensive pretreatment technologies, the bioethanol production is becoming
expensive process. For the production of bioethanol being cost effective, the scope lies in improving
the pretreatment technology in a way that the deconstructed lignin yields high value by-products
(Figure 1).
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1. Pretreatment Technologies
removal of lignin to release fermentable
sugars
Lignocellulosic Biomass
Thermo chemical Treatment
2. Hydrolysis of
fermentable sugars
(Release of C-6 and C-5
sugars)
Biological Treatment
3. Fermentation
and Biethanol
Production
Alkali Treatment
I.
Soil System
Lignin chemical modification
due to Microcosm, pH,
Temperature and other factors
((yet to be found).
Is this Possible?
Can this be cost effective???
II.
Aggregate formation and
addition of chelating agents
4. Lignin
based high
value
products
Figure 1: Block description of the application of soil system, comparison with other technologies .
Soil can be a potential system to depict the natural degradation of lignocellulosic biomass, as the
degradation of the plant and other organic material occurs in the soil which forms as a medium for the
growth of several microcosms. It plays a major role in the natural recycling mechanism of the complex
organic component of soil into their respective elemental forms (Skipper et al, 2005). During the
process of natural degradation all the complex materials in nature degrade into the elemental forms
through the process of composting (Miguel et al, 2002).
During the deconstruction mechanism of lignocellulosic biomass, the cellulose and hemicelluloses are
easily degraded where as lignin follows a specific pathway for its degradation (Stevenson, 1994) .The
chemical modification of lignin polymer allows the enzymes released by the microcosm to enter the
lignocellulosic matrix and digest the cellulose and hemicelluloses.
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The net desired compost is a result of micro environmental factors such as variations in temperature,
pH, pressure, and also due to the microbial interactions which have a direct or indirect effect on the
enzyme production system of the microorganism (Philippe et al, 2005).
Diversity in soil exceeds beyond that of eukaryotic organisms, they play a very important role during
nutrient cycle apart from their role in formation of soil aggregates, and they form the major
contributors’ in the complex interlinked food webs (Teuscher et al, 1960).These have the potential to
degrade any complex organic substance into simpler and more natural elemental forms in nature.
There potential for degradation can be observed in the cases where soil microcosm has shown to
degrade soil applied pesticides. The microbe could degrade the chemical before the chemical showed
its effect- enhanced degradation. Microorganisms living in the soil environment are responsible for
moderating the microenvironment by their enzymatic activity (Hatfield et al, 1994).
The enzymes released by the microorganism not only affect the biomass directly but also indirectly act
as inhibitors and activators for other microorganisms (Tuomela et al, 1999).These groups of organisms
have the ability to modify the microenvironment as the enzymes and other byproducts’ released due to
the metabolic activity effects the temperature, pH and other elements which act as key enhancers for
degradative reactions, thus playing an important role in the soil degradation system. Though the
deconstruction of the lignin polymer in soil is a slow process, the chemical modification of the lignin
makes it feasible for the formation of a colloid resulting in its further degradation. As the nature of
formation of aggregate colloid and chemical modification are interrelated in the lignin pathway, thus
the detailed understanding of the lignin degradation mechanism in soil would provide a novel
pretreatment system for the bioethanol production from lignocellulosic biomass.
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2. Soil as a potential source for a novel pretreatment technology.
Soil consists of different kinds of elements which support all kinds of life forms. For plant growth, it has 14
elements which are considered to be essential because they are absorbed by plants in relatively large amounts
(Arora et al, 1991).Organic component of the soil is mainly distinguished into undecomposed,
decomposed and decomposing organic matter, where in the newly formed organic matter is the end
product. The dead organic matter in the soil system is subjected to different kinds of thermo chemical
and pressure induced mechanisms where in microbial activity also takes place (Teusher et al,
1960).The soil system provides a mixed system which involves various sets of activities and reactions
provided by the different sectors of soil. This includes biotic and abiotic factors such as temperature,
pH, enzymes released by the plants, water content, and external environment. Biotic factors include
soil flora and fauna, insects, termites, earthworms, microorganisms. The soil forms a support system
for the growth of all kinds of biotic organisms.
There are mainly four kinds of chemical reactions which occur during the process of degradation in
soil, ie oxidation, reduction, hydrolysis, and carbonation. Apart from the chemical reactions, the
microorganisms activity is also involved which is limited by the presence of different availability of the
energy, environmental conditions, and formation of certain detrimental substances which would create a
resistance for their growth. Microorganisms in soil are mainly found near the areas of humified plant
debris, cell wall remenants, fibrous materials, granular and amorphous materials (Foster, 1988).
The Microbial activity in soil is mainly dependent on the supply pH oxygen, amount of organic matter present
and the amount of inorganic compounds present with respect to the pH of the soil. The decomposition of
cellulose in soil occurs at pH 6.8 to 7.5, therefore the formation of spring turf in acidic soils takes place
(Teusher et al, 1960).
The properties of organic soil components manly depend on pH, as cations such as H+,Ca++,Mg
++
,K+,Na+ are
attached to colloids which changes the charge of that colloids and thus form aggregates. The organic
composition in soil is mainly fixed in the form of micro aggregates (< 250 #m diameter) bound into
macro aggregates (> 250 #m diameter), the bond strength in microaggregates are stronger when
compared to micro-aggregates (Tisdall, 1994).Macro-aggregates are stabilized by saprophytic fungi
where as micro-aggregates are stabilized by live or dead roots, fungi, invertebrates and microorganisms
(Lynch et al, 1985). These are degradation products as a result of series of reactions.
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Though soil provides a complex system, its interaction mechanism involving different sets of soil
system provides a method of discovering novel degradation pathways for lignocellulosic pretreatment of
biomass.
Figure 2: The following figure describes the soil system and various factors involved in the
lignocellulosic degradation process.
Wind
Sunlight
radiation
Cellulose
Hemicellulos
es
Lignin
Enzymes
Other complex
compounds
Organic acids
Degraded into smaller sub units.
Polyurinoids
Amino acids
Chemically modified/partially degraded
Unknown x ??
Microcosm
Humus
3. Soil biology and the biological micro-environment
The microenvironment in the soil is not consistent with the overall soil system. It varies with the layer
of the soil, penetration of sunlight, amount of moisture and presence of nutrient sources, CO2, O2, and
oxidation – reduction potential .These limit the boundaries of microbial activity on the available
substrate at specific micro-sites in the soil. Degradation of organic compounds which are insoluble in
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water takes place with the help of the presence of H+ concentration, where in various elements due to
the H+ concentration where in it helps in solubility of these elements (Arora et al, 1991).
The main contributors of H+ ions in soil are the compounds which have the hydrogen on reaction with
water release hydrogen into the soil resulting in an acidic environment. The microflora in soil includes
the algae where in it utilizes the sunlight, present in the upper zone of soil. Heterotrphs which includes
bacteria and fungi are responsible for the initial degradation of the organic compounds which are easily
degradable. In the soil ecosystem their exists two kinds of communities, primary microcosm which are
responsible for the initial degradation of simple carbonaceous nutrients and secondary community
which degrades the substrates produced by the ecosystem.
Functionally all the organisms present in the soil are interdependent through the process of degradation
of different forms of organic material .Animals such as nematodes and few fungi directly derive their
food from the living plants where as bacteria and other fungi have litter as their substrate. Foster (1988)
reviewed the location of the various types of soil-dwelling organisms and found that fungi, which
constitute about 80% of the biomass in many soils, tend to be restricted to the rhizosphere of roots, to
larger pores between aggregates and to the surface of aggregates. The soil microenvironment provides
the microcosm an atmosphere which makes it an interdependent system with an adaptability to adjust in
any extreme conditions and degrade the organic matter present in soil.
4. Biodegradation system of lignocellulosic components in soil
Carbohydrates, proteins, lipids, and lignin majorly form the organic matter of the soil system (Tuomela
et al, 1999).Among the lignocellulosic components of the plant cell wall, Cellulose is an unbranched
polymer made of glucose subunits, it forms the major constituent of the plant cell wall, thus its
degradation forming the major component of the carbon and energy flux in soil (Lynch, 1981). During
the process of hydrolysis the cellulose with the help of cellulase enzyme is broken down into glucose
units. In soil system, during the process of cellulolysis a group of enzymes synergistically act on the
different binding sites as a result of which the polymer degrades. (Jeewon et al, 1997). Lignocellulosic
crop residues, such as cereal straw, provide the principal input of cellulose to arable soils (Lynch,
1979).
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The spontaneous crystallization of cellulose is attributed to its uniform chemical structure where in the
glucose residue is tilted by 1800C (Schwarz, 2001) where as hemicellulose has an extremely
heterogeneous chemical composition. Lignin is a complex, variable, hydrophobic, cross-linked, threedimensional aromatic polymer of p-hydroxyphenylpropanoid units connected by C–C and C–O–C links
(Jeewon et al, 1997). Chemical modification of lignin structures takes place in the presence of oxygen
where the microorganisms produce enzymes of the peroxidase type .These enzymes in the presence of
hydrogen peroxidase chemically modify the structure by breaking the lignin side chains .Thus the
intermediates are unstable and form hydrophobic partially degraded structures in the presence of water
or oxygen. In the absence of oxygen and water lignin is not degraded and accumulation of these
complex polymer occur in soil (Kovalev et al, 2008)
After the chemical modification of lignin, the hydrophobic partially degraded compounds form
aggregates with other available organic compounds and polyurinoids form macro aggregates which are
called humus. They form a major part of humus composition which is required for the production of
humic acid. Colloid humus compounds consist of humic acids and water insoluble salts which have
very high affinity towards calcium and magnesium humate which act as polyelectrolyte which makes
the compound attach to more cations. During the process of organic decomposition cellulose produce
polyurinoides which are mucilaginous substances, aid in humus congregation. Organic acids, such as
humic acid and fulvic acid form an intermediary compound of organic matter decomposition also react
in the similar way. The process of degradation of humus substances is very slow despite of the
availability of organic nutrients in the soil environment. The slow degradation of the phenolic, amino
and sugar constituents of the humus takes plays where in the depolymerisation form the rate limiting
step as the concentration of the monomeric structures is relatively low when compared to organic
monomers which are readily available in the soil.
5. Different sets of microorganisms which are present in soil
Microorganisms play an important role during the process of degradation in soil, as in their absence total
nitrogen, potassium and phosphorous, sulfur, and carbon would be locked up unavailable in the form of
rock or gas and thus degradation of the organic matter would not take place. Due to the presence of
microbes the elements from the organic matte are released, which adds them back into the circulations
that they can be used again by the plant and animal life. The class of fungi which is mainly responsible
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for cellulose degradation in soil are Hyphochytridiomycete and Oomycete classes of Eucomycota and
Myxomycetes.Though fungi initiates the cellulolysis in soil, it’s a mixed microbial activity where in
bacteria also plays a major role (Arora et al, 1991).The action of cellulosytic enzymes is initiated by the
presence of the Ca+2 ions and thiol donating molecules (Arora et al, 1991).
The microenvironment in soil is mainly controlled by the metabolites metabolites such as acids, bases
and ligands produce by the microcosm. These components interact directly and indirectly with soil
system thus changing the properties of the soil with respect to the chemical environment. Thus the
microcosm in soil system is an interdependent, organized system where the enzymes released by the
microcosm help in altering the microenvironment which suits the degradation process. Though
understanding the intricate microbial system is a challenge, it provides us a basic understanding about
how the soil system works with the lignocellulosic degradation mechanism.
Among the different classes of fungi, Basidiomycetes are the class responsible for modifying lignin
(Eriksson et al, 1990 ; Ten Have et al, 2001; Bennett et al, 2002 ; Rabinovich et al, 2004 ; Carmen
,2009).These have the capability of surviving in a nitrogen deficient environment, presence of toxic and
antibiotic compounds, they are two types ,white rot and brown rot fungi (Schwarze et al , 2000; Zabel et
al ,1992).These mostly produce extracellular oxidative enzymes. Apart from degrading lignin, white rot
has the potential to degrade cellulose, hemicelluloses simultaneously or selectively. The change in the
mechanical properties macroscopically is due to the bacterial activity. Ascomycetous class of fungi
degrade on the different kinds of extractives in the lignocellulosic biomass .Different kinds of other
fungi, insects produce laccases have the potential to modify lignin (Mayer et al, 2002). Saccharomyces,
Zymomonas mobilis, Pichia striptis, Candida shehatae, Escherichia coli, Trichoderma reesei,
Clostridium thermocellum, Clostridium papyrosolvens,Neospora crassa,Fusarium oxysporium have the
ability to degrade all kinds of hexose sugars where as pentoses are degraded by Zymomonas
mobilis,Trichoderma reesei,Clostridium Papyrosolvens,Fusarium oxysporium (Lee ,1996).
Lignin peroxidase (LiP) and manganese peroxidase (MnP) produced by P. chrysosporium are described
as true ligninases because of their high redox potential (Martinex, 2002; Gold et al, 2000).Lip acts on
the non phenolic components where as Mnp acts on both phenolic and non phenolic lignin components
using Mn3+ as a catalytic oxidizer thorugh a series of lipid peroxidation reactions(Jensen et al ,1996)
(Angel et al ,2005). Hemicellulose is a polysaccharide which is composed of D-xylose,D-mannose,D10
galactose ,D-glucose,L-arabinose,4-O-methyl-glucoronic,D-galacturonic and D-glucoronic acids linked
by β-1,4- and sometimes by β-1,3-glycosidic bonds(Carmen ,2009).Small chemical oxidizers such as
activated oxygen species and enzyme mediators, are probably involved in the initial steps of
lignocellulosic degradation in soil (Angel et al ,2005).
Table 1.Anatomical, chemical features of different types of wood decaying fungi.
White rot
Decay aspect
and consistency
Brown rot
Bleached appearance,
lighter in color than
Soft rot
Soft consistency in wet
environments. Brown and
sound wood, moist, soft,
spongy, strength
crumbly in dry
environments.
loss after advanced decay.
Generally uniform
Brown, dry, crumbly,
powdery,
ontogeny of wood decay.
brittle consistency,
breaks up like cubes,
drastic
loss of strength at initial
stage of decay. Very
uniform
ontogeny of wood
decay.
Host (woodtype)
Simultaneous rot Selective
delignification
Softwoods; seldom
hardwoods.
Hardwood, rarely
Forest ecosystems
softwood
and wood in
service.
Hardwod and
softwood
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Cell-wall
constituents
degraded
Cellulose, lignin
and hemicellulose.
Brittle fracture.
Initial attack selective
for hemicelluloses
and lignin,
later cellulose also.
Fibrous feature.
Anatomical
features Cell
wall attacked
progressively
from lumen.
Erosion furrows
associated
Cell wall attacked
progressively from
lumen. Erosion
furrows associated
with hyphae.
with hyphae.
Lignin degradation
in middle lamella
and secondary wall.
Cellulose,
hemicelluloses.
Cellulose and
hemicelluloses,
Lignin slightly
modified.
lignin slightly altered.
In some cases,
extended
degradation of
hardwood
(including middle
lamella).
Degradation at a
great distance
Cell wall attack in the
proximity
from hyphae
(diffusion
of hyphae starts
mechanism).
Entire
cell wall attacked
rapidly
with cracks and
clefts.
from cell lumen.
Logitudinal biconical
cylindrical cavities in
secondary wall
Secondary wall erosions
from cell lumen
Middle lamella dissolved
Facultative soft-rot decay
by diffusion
by some basidiomicetes.
mechanism (not in
contact with
hyphae), radial cavities
in cell wal
Causal agents
Basidiomycetes
(e.g. T. versicolor,
Irpex lacteus,
Basidiomycetes
exclusively
Ascomycetes
(Chaetomium
(e.g. C. puteana,
globosum, Ustulina
deusta)
Gloeophyllum
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P. chrysosporium
trabeum,
and Deuteromycetes
and Heterobasidium
Laetiporus
sulphureus,
(Alternaria alternata,
annosum)
and some Ascomycetes
(e.g. Xylaria
hypoxylon).
Basidiomycetes (e.g.
Piptoporus
betulinus,
Thielavia terrestris,
Paecilomyces spp.), and
Postia placenta and some bacteria. Some
white
Serpula
(Inonotus hispidus) and
lacrimans).
Ganoderma australe,
Phlebia tremellosa,
C. subvermispora,
Pleurotus spp. and
brown-rot (Rigidoporus
crocatus) basidiomycetes
cause facultative soft-rot
decay.
Phellinus pini).
(Based on Eriksson et al; Schwarze et al; Zabel and Morrell; Angel et al, 2005)
Table 2: Some important fungal strains which are responsible for lignocellulosic degradation.
Fungal strain
Comment
References
Basidiomycetes
Phanerochaete chrysosporium
Production of lignin
peroxidase and Mn-dependent
peroxidase
Venkatadri and
Irvine, 1993
Bonnarme et
al., 1993
Schoemaker
and Leisola, 1990
Linko, 1988
Zhong et al., 1988
Pellinen et al., 1989
Phanerochaete chrysosporium
Overproduction of lignin
peroxidase and Mn-dependent
Tien and Myer, 1990
Polystictus sanguineus
** Best laccase producer**
Arora and Garg, 1992
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Daldinia concentrical
Postia plancenta
Streptomyces 6iridosporus
Degradation of wheat
lignocellulose to release
soluble lignin-rich fragments
Seelenfreund et al , 1990
P. chrysosporium
Pine and straw alkali lignins - Deepak et al , 2008
Lignin peroxidases
Pulp and paper mill waste
water -Manganese peroxidases
Veratryl alcohol and its
methyl ether -Glyoxal oxidase
Decolorizes the textile waste
water -Peroxidases
Trichlorophenol,
petachlorophenol,
Toluene-Xylanase
Congo red, Amaranth,
Atrazine, -Cellulases
Azo dyes Succinimides
DDT, Benzene, Tolune,
Xylene Aryl-alcoholdehydrogenase
Humic acid- Cellobiose
dehydrogenase
Coal solubilizing agent
5. Lignin degradation mechanism -soil as its potential source.
Soil consists of different kinds of materials which undergo degradation continuously. During the
biodegradation process, among the lignocellulosic biomass which mainly consist of cellulose,
hemicelluloses, pectin, and lignin the cellulose and hemicelluloses are comparatively easily degraded
into monomer sugar units. Cellulose during the process of degradation forms polyurinoid components
which act as mucilaginous units for humus formation. Lignin is a complex aromatic polymer which is
composed of hydroxyl, methoxy and carboxyl groups. The random synthesis of lignin formation makes
it more strenuous task for its degradation as the bond formed between the two lignin monomers is not
similar to others.
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During the biodegradation process of lignin, the lignin structure is modified with the help of enzymes
such as laccase, peroxidases and esterases which are released by lignin degrading fungi initially as the
breakage of bonds is not a feasible. Apart from the lignin molecule there are many kinds of other
complex structures present in soil .The part of the soil components which are chemically modified along
with modified or partially degraded lignin undergo a dehydrative condensation to form humus
aggregate.
Figure 3: The following figure represents the soil system and its humification process.
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These aggregates are colloids which are composed of mainly polyuriods, partially degraded lignin, and
other complex aromatic structures. After the humus formation, it undergoes dehydration and
demethylation reactions which lead to decrease hydrogen to carbon ration .During the dehydration and
demethylation process, the humic acids and the protein get separated. The complex of humus aggregate
consists of humic acids, fulvic acids and humin. The lignin compound mostly is a part of humic acids.
During the process of dehydrative condensation of the humus formation the chemically modified lignin
further undergoes a chemical modification which results in a compound made of the other aromatic
structures. (Stevenson,1994) proposed a theory of humus formation where in the lignin after it is
chemically modified forms quinones and these undergo polymerization along with different kinds of
amino acids to give rise to humus macromolecules. (Sanchez-cortes et al, 2001) showed that the
polyphenolic compounds for the intermediate during the humus formation. The humus aggregates
basically have the capability to provide the necessary factors for further lignin degradation process.
Monolignol polymerization is another important step during lignification which remains poorly
understood and needs to be explored further in the context of lignin engineering. The dehydrogenative
polymerization of monolignols is thought to be catalyzed by peroxidases and laccases
The further understanding of different compounds which are involved in the humic acids ,their activity
towards the further degradation of lignin ,whether it is forming a more complex or simpler structure
would give us a chance of understanding the chemical structure, its nature of reactivity with other
aromatic compounds and in turn its degradation . The figure provides a clear idea about the chemical
changes that occur in soil and the humus formation and its further degradation process.
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Figure 4: Probable chemical changes that occur in the organic components present in soil,
resulting in the humus formation.
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6. Conclusion
During the process of biodegradation of lignocellulosic component in soil, the cellulose,
hemicelluloses and lignin undergo different kinds of chemical, microbial activities where in they
get modified or degraded. The complex organic polymers in order to be further degraded forms
the humus aggregate. The formation of humic substance stimulates the activities of micro flora
and micro fauna’ .This is mainly because of the colloidal nature of humus which has many
different kinds of chelating agents attached to it. Further analysis of humus in an compost
environment revealed that as the humic acids is formed the aerobic heterotrophs population
increases where as the oxygen content, ATP, and dehydrogenase activity decreases. This shows
that oxidation reaction are mainly responsible for the humic acid production .The degradation
system of lignin component to release the sugars during the pretreatment process requires a
method where in the total amount of sugars as well as the organic polymer is utilized to produce
ethanol and high value bi-products respectively, Soil provides a media of complex organic
components along with a pool of microorganisms where in the degradation process is not a
sequential chemical change in the organic component but a complex interaction between the
different components where in the complex formation from the simpler molecule is formed for
further
degradation into its elemental forms. Though there is equal role played by
microorganisms as well as other biotic and abiotic components the main role is played by the soil
in forming a compound which attracts the all the biotic components for the degradation process.
This provides us a new perspective for designing a lignin pretreatment pathway by utilizing other
complex organic components.
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