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Organic Matter Decomposition |
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Organic Matter Decomposition
Through a series of complex reactions, microbes mediate the conversion of organic material such as leaves and twigs to the dark humus which colours and glues our soils. Without these processes, the soils would be loose, noncemented dusts and no life would be able to grow on them, and the world would be a very different place.
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Bacteria constitute the most abundant group of microbes, their numbers in soil directly proportional to the amount of organic matter in the soil. Most of the bacteria found in soil are heterotrophs (that which utilise readily available source of organic energy). Some bacteria like Ferrobacillus and Thiobacillus that make use of inorganic compounds (Fe and S respectively) are not directly involved in organic matter decomposition.
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Actinomycetes (mostly thermophiles) too take part in the decomposition process to a great extent by growing on complex substances like chitin and keratin.
Soil fungi are also heterotrophic (mostly) but their numbers vary and depend on whether a species has a dominant vegetative or reproductive
phase in the soil environment.
Soil algae vary in numbers and contribute a small amount of organic matter through their biomass but they do not play an active role in decomposition.
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The animal groups (earthworms, insects, snails) bring about mechanical reduction by biting and eating the organic matter (pulverising the material). Their excreta enzymes provide good nutrients for bacterial groups to multiply well and decompose the organic matter.
CO2, H2O, NO3, SO4,CH4, NH4 and H2S are the end products of decomposition. It depends on the availability of air. In more anaerobic environment (stagnant water) one can expect more of H2S and CH4.
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When we consider the order of decomposition, the first to get attacked are the sugars and starch (water soluble) which undergo rapid attack. Next are the crude proteins followed by hemicellulose, cellulose, oils, fats, wax, resins and lignin.
Organic matter has bound energy which is being taken up by the organisms forming various intermediary substances which are once again attacked by other organisms (which are capable of utilising intermediate products) to liberate CO2, SO4, NH3, etc.
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Thus the original organic matter is reduced in part to simpler substances which are utilised by various microbes for building up their cell structure. Some of the intermediary breakdown products form solution in soil water and are available for the plants. The undecomposed residues constitute the soil organic complex or humus. In the early stages of decomposition, heterotrophic bacteria are highly active followed by fungi, actinomycetes and other insect groups.
Once organic matter is added to the soil, there is an immediate increase in the heterotrophic microbial population (chiefly bacterial). Their life span is very short. After initial decomposition of the organic matter, they reach a stage of optimum growth and finally lyse. The lysed cells release simpler substances (partly taken up by the plants). The insoluble constituents form part of the humus. Thus organic matter added to the soil is converted by oxidative decomposition to simpler substances which are made available in stages for plant growth and the residue is transformed into humus. This process is called humification.
During organic matter decomposition, sugars, starch, hemicellulose and cellulose are rapidly broken down into CO2, H2O and some energy. Oils, fats,
waxes and resins are slowly decomposed to give CO2 and H2O. Lignin is resistant to decomposition in that some portions undergo slow decomposition and the end products are usually aromatic compounds (compounds containing benzene ring). The other portion chemically unite to become a part of soil humus.
Proteins are broken down slowly (since the clay minerals trap the protein molecules making it unavailable to the microbes). This problem is evaded by some microbes that excrete extracellular proteases, but since the enzymes themselves are proteinaceous, they lose their activity substantially. Apart from this difficulty, proteins also form an organic complex in soil with the lignin and become resistant to microbial attack, which converts it into amino acids, amides and ammonium which is oxidised to nitrites and nitrates which is again taken up by the plants.
Phosphorus in phytin, nucleic acids and cell membrane are broken down to orthophosphates. Similarly, sulphur containing amino acids are decomposed to liberate sulphates.
Three processes go on during decomposition:
1. Degradation of plant and animal remains (mineralisation) by cellulases and other enzymes.
2. Increase in biomass of microbes (polysaccharides and proteins) (immobilisation)
3. Accumulation or liberation of end products (index of microbial activity in soil).
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