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Degradation of Plant Residues |
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Degradation of Plant Residues
Soil can be defined as a natural medium for plant growth composed of minerals, organic materials and living organisms. While physical weathering of rocks caused by changes in temperature and the consequent chemical decomposition contributes largely to the formation of the soil, biological activities such as root growth and microbial metabolism in the soil contribute to its texture and fertility. Undoubtedly, the amount of organic matter present in any soil determines its natural suitability for plant cultivation.
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Soil organic matter comprises residues of plant and animals at all stages of decomposition mediated by soil microorganisms. Various organic compounds which reach the soil by way of animal and plant residues are made up of complex carbohydrates, simple sugars, starch, cellulose, hemicelluloses, pectins, gums, mucilage, proteins, fats, oils, waxes, resins, alcohols, aldehydes, ketones, organic acids, lignin, phenols, tannins, hydrocarbons, alkaloids, pigments and other products. The size of particles in the organic matter, the nature and abundance of microorganisms involved, the extent of availability of C, N, P and K, the moisture content of soil, its temperature, pH and aeration, presence of inhibitory substances (such as tannins) etc. are some of the major factors which influence the rate of organic matter decomposition.
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Plant residues contain 15-60 percent cellulose, 10-30 per cent hemicellulose, 5-30 per cent lignin, 2-15 per cent protein and 10 per cent sugars, amino acids and organic acids. Cellulose occurs in a semicrystalline form with a molecular weight of 106 and has glucose units with B(1-4) linkages. The individual chains of glucose are held together by hydrogen bonds. Cellulase enzyme complex decomposes cellulose into disaccharide cellobiose which is hydrolyzed by the enzyme cellobiase to glucose. Hemicelluloses are various polymers of hexoses, pentoses and sometimes uronic acids with commonly occurring monomers such as xylose and mannose. Pectin is an example of hemicelluloses and is an important constituent of the middle lamella of cell walls. Pectin is degraded by the enzyme pectinase which is a complex of several enzymes.
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Lignin is much more complex than celluloses and is formed by chemical reaction involving phenols and free radicals without any specific order. Lignin gets encrusted on the cellulose and hemicellulose matrix. Compounds like caffeic acid and ferulic acid have structures similar to lignin and they have been used 'in studies on degradation of lignin.
The lignin molecule has only three elements carbon, hydrogen and oxygen. The molecule is a polymer of aromatic nuclei with either a single repeating unit or several similar units as building blocks. The repeating units range from about 200 to 1000 depending upon the origin of lignin and the methods used to determine the molecular weight.
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Degradation of lignin is brought about by fungi mainly belonging to Basidiomycetes. The genera of fungi which degrade lignin as well as cellulose are Clitocybe, Collybia, Mycena, Marasmius, Polystictus, Armillaria, Polyporus, Stereum, Ganoderma, Pleurotus, Trametes, Fomes and Ustulina.
Bacteria constitute the most abundant group of microorganisms. In normal fertile soils, 10-100 million bacteria are present per g of soil. This figure may increase depending on the organic matter content of any particular soil. The bulk of soil bacteria are heterotrophic and utilize readily available source of organic energy from sugars, starch, cellulose and protein. On the other hand, autotrophic bacteria which occupy a smaller portion of the biomass in soil and use inorganic sources such as iron (Ferrobacillus) and sulphur (Thiobacillus) are not directly involved in organic matter decomposition.
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The number of actinomycetes may be as high as 200 million per g of soil and may increase in manured soils. Thermophilic (tolerating 50 to 65°C) forms are not uncommon in compost piles. Ac¬tinomycetes grow on complex substances such as keratin, chitin and other complex polysaccharides and thus play-an active role in humus formation. Soil fungi are mostly heterotrophs and use organic residues easily but their numbers vary in soil depending on whether a species has a dominant vegetative or reproductive phase in the soil environment. Sporulating fungi such as Mucor, Penicillium and Aspergillus appear on agar plates rather profusely than non-sporulating ones. Soil algae in cultivated soils vary greatly in numbers and may contribute a small amount of organic matter through their biomass but they do not have any active role in organic matter decomposition. The microorganisms involved in the decomposItion of organic matter are listed in. The end products of decomposition are CO2, H2O, NO3, SO4, CH4, NH4, and H2S depending on the availability of air.
In spite of the paucity of data on biomass in relation to litter decom¬position, especially on the microbial part of the complex, studies on the estimations of biomass in a deciduous woodland soil at meathop, Lan¬cashire (brown earth with a mull humus) indicate that annual litter production on dry weight basis was of the order of 7640.0 kg/ha and the total biomass was 497.5 kg/ha (including animals and microbes). The split up figures for individual groups in the biomass were as follows in kg/ha: bacteria, 7.3; actinomycetes, 0.2; fungi, 454.0 (contrary to the general Diagrammatic representation of breakdown of cellulose in lignocellulosic materials (based on recent findings). (From Chahal and Overend, 1982).
Degradation of Plant Residues
Part of plant tissue showing different types of cells and the action of an unnamed enzyme X on middle lamella. Individual cells released by the action of X. Some of the cells show broken estimates based on dilution-plating), protozoa, 1.0; nematodes, 2.0; earthworms, 12.0; Enchytraeidae, 4.0-; molluscs, 5.0; acari, 1.0; Collembola, 2.0; Diptera, 3.0; other arthropods, 6.0; total microflora, 461.5 and total microfauna, 36.0.
The relationship between organic matter and plant growth may be direct or indirect. Organic matter is a natural substrate for saprophytic microorganisms and provides nutrition to plants indirectly through the activity of soil microorganisms. It is essential for the formation of soil aggregates and hence soil structure which ultimately determines the extent of soil aeration and rooting habit of plants. Organic matter helps in the conservation of soil nutrients by preventing erosion and surface run-off of nutrients.
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