The most important function of soil microorganisms is the decomposition of various kinds of organic matter present in the soil. Virtually all types of organic matters eventually find their way to the soil or to the sea. The soil organic matter chiefly consists of residues of dead plant and animals, and the excretory products of the living beings.
These organic constituents need to be converted into simple inorganic forms (minerals) to make them available to the autotrophic organisms. This conversion of organic matter into simple inorganic forms is called mineralization.
The mineralization is rendered mainly through decomposition of organic matter by soil microorganisms, mainly fungi and bacteria. It is estimated that 90% of the mineralization of organic matter is the result of the metabolic activities of these two groups of microorganisms.
The remaining 10% results from the metabolism of all other organisms, as well as the combustion of fuel and other materials. The overwhelming contribution of microorganisms to the process of mineralization reflects their ubiquity, their significant contribution to the bulk of living material (their biomass), their high rates of growth and metabolism, and their collective ability to degrade a vast variety of naturally occurring organic materials.
In general, the organic compounds of organism-residues are attacked by organotrophic microorganisms capable of digesting and oxidizing them.
As the oxygen is consumed, anaerobic microorganisms capable of reducing nitrates and sulphates or carbonates commence their activities ultimately creating an aerobic condition. The various simple inorganic substances such as ammonia, sulphide, hydrogen, etc. are avidly oxidized by lithotrophs.
Thus, the organic compounds of organism-residues are completely decomposed to simple organic forms and subsequently oxidized. This cycle of events occur where environment is suitable. Extreme cold and heat are unsuitable for normal turnover of the matter. In cooler parts, decomposition rate is accelerated in spring, whereas in warmer parts rainy conditions favour the activity.
The organic residues which are added to the soil can be categorized into three groups; the easily degradable, moderately degradable and difficultly degradable. These three types of organic residues are attached by distinctly different types of microbes.
The result of microbial mineralization is the release of energy, water, gases, etc. on the one hand and formation of a complex amorphous substance (humus, process called humification) on the other hand. The whole process of mineralization terminating into humification is outlined in most simplified way in Fig. 30.1.
The rate of oxidation of biologically important elements such as nitrogen, carbon, sulphur, phosphorus, etc. during mineralization of organic matter is of paramount importance to plants. This is so because these elements, though physically present in large quantities, may not be available to plants as they are not found in suitable chemical form.
These are the microorganisms which bring about these elements to the plants for their use through their activities and also make them free for reuse.
This process of ‘use and reuse’ through regular recycling of essential elements by the activity of microorganisms during the process of decomposition or mineralization of organic matter is called ‘biogeochemical process’ which is essential for the maintenance of life on our planet. Different biogeochemical processes, often called biogeochemical cycles, are discussed ahead.
Thus, the organic matter added to the soil is converted by oxidative decomposition to simpler inorganic substances as a result of mineralization that are made available in stages for plant growth, and the residue transformed into humus as a result of the process called ‘humification’; the processes of mineralization and humification running hand-in-hand in nature.