Mineralization and Immobilization Process

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Organic Matter Decomposition Degradation of Plant Residues Humus Genera of Microorganisms Capable of Utilising Different Components of Organic Matter Humic Acids Beneficial Role of Humic Acids

Mineralization and Immobilization Process

Mineralization and Immobilization Processes
When microorganisms grow and multiply on organic debris, carbon is utilised for building the cellular material of microbial cells with the release of carbon dioxide, methane and other volatile substances. In this process, microorganisms also assimilate nitrogen, phosphorus, potassium and sulphur which get bound in the cell protoplasm. Therefore, the C/N, C/P, C/K or C/S ratios in soil are governed by the extent of organic matter utilised by soil microorganisms depending on the oxygen content and the microbial biomass at a particular stage in decomposition.

Thus, three parallel processes go on during decomposition: (1) degradation of plant and animal remains by cellulases and other microbial enzymes, (2) the increase in the biomass of microorganisms which comprises polysaccharides and proteins and (3) the accumulation or liberation of end products. The term mineralization is used to designate the conversion of organic complexes of an element to its inorganic state which embodies the first of the three processes mentioned above.

Mineralization and Immobilization Process

Pathways in Organic Matter Decomposition

The second process which involves the microbial uptake of nutrients such as nitrogen, phosphorus and sulphur is opposite in magnitude to mineralization and is known as immobilization.

From the agronomic point of view, immobilization reduces the availability of nutrients to plant growth, the intensity of which is related to the total microbial biomass at a given time. The last of these processes provides an index of microbial activity in soil and is interlinked with nitrification and denitrification processes which are also mediated by microorganisms.

When fresh organic .matter is added to soil, owing to the resistance offered by fresh residues, the number of microorganisms developing on these substrates is small and therefore, immobilization is not so rapid whereas mineralization gradually increases resulting in the accumulation of ammonia and nitrates.

The rate of immobilization of nitrogen depends on the nature of the soil microflora, soil temperature, fertiliser nitrogen status and the C/N ratio of the organic matter added. Organic matter from diverse plant tissues vary widely in their C/N ratio. Optimum levels of C/N ratio in the range of 20-25 (1.4-1.7% N) seems to be ideal for maximum decomposition since there will be no immediate release of mineral nitrogen from residues over and above the amount required for microbial synthesis. In other words, a favourable soil environment is created to bring about an equilibrium between mineralization and immobilization processes.

This critical balance may be upset if the C/N ratio is less than 25 when mineralization is likely to exceed immobilization leading to accumulation of ammonium and nitrate forms of nitrogen. Therefore, one reliable way of measuring mineralization in soil is to find out the total available ammonium, nitrate and nitrite forms of nitrogen at a given time since the processes of ammonification, nitrification and denitrification take place almost concurrently in arable soils.

Plant debris containing higher amounts of lignin are resistant to microbial attack and hence mineralization proceeds slowly in organic ma erials like sawdust.

The lignin content plant residues may serve as an mdex of the vulnerability of organic residues to microbial attack. Studies with I5N-tracer techniques have shown that immobilized nitrogen in microbial cells is resistant to mineralization even after prolonged periods. These results have emerged from studies with pure cultures of microorganisms grown on synthetic substrate and hence cannot be equated to field conditions where chemically fixed nitrogen in clays and organic matter cannot easily be qistinguished from microbiologically immobilized nitrogen.

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Mineralization and Immobilization Process Effects of Residues of Crops on Plant Growth Soil Sickness Composting Vermicomposting Green Manure Anaerobic Decomposition of Organic Matter Biogas Degradation of Hydrocarbons

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