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Decomposition
of
Organic
Matter
in
Soil |
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Decomposition
of
Organic
Matter
in
Soil - Fungi alongwith other microbes, chiefly bacteria (including actinomycetes) play vital role in the decomposition of organic matter in soil, thus releasing the nutrients locked up in the dead organic matter of plant, animal and microbial matter and bringing about the recycling of nutrients in nature.
In soil, microbes oxidise organic carbon to CO2 and liberate bound materials.
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The decomposition process actually begins while the plants are still intact in their environments i.e. at the senescent stage and this continues till plant remains are totally decomposed. The microbes appearing at different stages differ in their species - composition and arc infact nutritional or ecological groups of microbes.The sequence of the appearance of microbes on decaying plant remains on and in the soil has been worked out mostly for fungi. It has been shown that the primary colonisers on senescent plant parts are likely to be weak parasites and/or saprophytes of sugar fungi group (mucorales).
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On dead tissues, the ascomycetes and their imperfect forms (cellulose decomposers) are the secondary colonisers. At this stage, some mucorales may also appear as secondary saprophytic sugar fungi.
Finally, the basidiomycetes (lignin decomposers) appear in the succession.
Thus chemical make-up of the substrates determines the qualitative features of the fungal flora appearing at a particular stage of succession.
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Microbes, such as fungi and bacteria (including actinomycetes) playa vital role in the formation of brown or black organic complexes which remain in a dynamic stage, through the decomposition of organic materials and through the synthesis of substances that also become part of the humus. Humus is sparingly soluble in water but is rendered into solution by alkali, It contains amino acids, purines, pyrimidines, aromatics, uronic acids, amino sugars, pentose and hexose sugars, sugar alcohols, methyl sugars, aliphatic acids etc. Humus is the finally brokendown amorphous state of dead organic matter.
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In humus formation, microbes attack and degrade plant and animal remains. The plant residues comprise chiefly cellulose, hemicellulose, pectic substances and lignin.
Cellulose decomposing fungi include chiefly ascomycetes and deuteromycetes. These are chiefly species of the genera, Aspergillus, Penicillium, Chaetomium, Trichoderma, Fusarium, Stachybotrys, Memnoniella, Humicola, Phoma, Cladosporium, Alternaria, Acremonium, Myrothecium, Thielavia etc.
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They have a strong cellulose synthesising machinery, which breakdown cellulose to simple sugars. Three types of hemi-cellulases are involved in the degradation of hemicellulases. These are endo- enzymes, exo-enzymes and glycosidases.
The fungi involved in their breakdown belong to all major groups. Most important ones are species of Alternaria, Aspergillus, Chaetomium, Fusarium, Glomerella, Penicillium and Trichodenna.
Pectic substances, the primary constituents of the middle lamella and primary cell wall are brokendown also by microbial pectinases.
Pectinases belong to three major categories (i) hydrolytic that carry out hydrolytic cleavage of the pectin polymers (ii) transeliminases that bring about trans elimmative cleavage of glycosidic links by the action of lyases and (iii) pectinesterases which hydrolyse the methoxyl groups of the pectins, converting them to pectinic acids.
Pectinases are produced by various species of Rhizopus, Aspergillus, Penicillium, Monilia, Fusarium, Rhizoctonia, Geotrichum etc.
Lignin, chiefly in woody parts is degraded by basidiomycetes, such as aphyllophorales and agaricales. Lignolytic enzymes of these microbes include phenol oxidases, laccases and peroxidases. Besides fungi, bacteria also are involved in decomposition of organic matter.
These include mostly aerobic bacteria, such as pseudomonads, bacilli and actinomycetes. Some anaerobic bacteria, including the characteristic methanogenic bacteria, growing in primitive conditions are also involved in carbon cycle of nature.
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