Process of Organic matter Decomposition
The organic materials (plant and animal residues) incorporated in the soil is attacked by a variety of microbes, worms, and insects in the soil if the soil is moist. Some of the constituents are decomposed very rapidly, some less readily and others very slowly.
The list of constituents in terms of ease of decomposition
- Sugars, starches and simple proteins → Rapid Decomposition
- Crude proteins.
- Fats, waxes, resins.
- Lignin → Very slow Decomposition.
The organic matter is also classified on the basis of their rate of decomposition
- Rapidly decomposed: Sugars, starches, proteins etc.
- Less rapidly decomposed: Hemicelluloses, celluloses etc.
- Very slowly decomposed: Fats, waxes, resins, lignins etc.
- Simple decomposition products, Aerobic – CO2, H2O, NO2, SO4.
When organic material is added to soil, three general reactions take place.
a. The bulk of the material undergoes enzymatic oxidation with CO2, water, energy and heat as the major products
b. The essential elements such as N, P and S are released and or immobilized by a series of reactions.
(A) Decomposition of Soluble Substances
Sugar and water-soluble nitrogenous compounds are the first to be decomposed as they offer a very readily available source of carbon, nitrogen, and energy for the microorganisms. Thus, when glucose is decomposed under aerobic conditions, the reaction is as under:
Sugar + Oxygen → CO2 + H2O
Under partially oxidized conditions,
Sugar + Oxygen → Aliphatic acid (Acetic, formic etc.)
Hydroxy acids (Citric, lactic etc.)
Alcohols (ethyl alcohol etc.)
Some of the reactions invoiced may be represented as under:
C6H12O6+ 2O2 → 2CH3COOH + 2CO2 + 2H2O
2 C6H12O6 + 3O2 → 2 C6H8O7+ 4 H2O
C6H12O6+ 2O2 → 2C2H5OH + 2CO2
Soluble nitrogenous compounds like amino acids, amides, ammonium compounds, nitrates etc., are also attacked by the microorganisms. The transformation of organic nitrogenous compounds into ammonia is called ammonification. During the course of action under aerobic conditions by heterotrophic organisms, oxygen is taken up and carbon dioxide is released. Ammonification process involves a gradual simplification of complex compounds.
Protein → polypeptides → amino acids → ammonia or ammonium salts.
Organic nitrogen → NH3
The ammonification occurs as a result of the action of enzymes produced by microorganisms. Their action is chiefly hydrolytic and oxidative (in the presence of air).
The process of conversion of ammonia to nitrite (NO2) and then to nitrate (NO3) is known as nitrification. The production of nitrate is more rapid than that of nitrite, while the formation of ammonia is the slowest process. That is why soil usually contains more nitrate nitrogen than nitrite at any time. Nitrification is an aerobic process involving the production of nitrates from ammonium salts.
The process, which involves the conversion of soil nitrate into gaseous nitrogen or nitrous oxide, is called Denitrification. Waterlogging and high pH will increase N loss by Denitrification.
Waterlogging (e.g., Rice field) and high pH will increase nitrogen loss by denitrification.
(B) Decomposition of Insoluble Substances
1. Breakdown of Protein: Proteins are complex organic substances containing nitrogen, sulfur, and sometimes phosphorus, in addition to carbon, hydrogen, and oxygen. During the course of decomposition of plant materials, the proteins are first hydrolyzed to a number of intermediate products, e.g., proteases, peptones, peptides, etc., as polypeptides. The changes may be represented as under:
The process of conversion of proteins to amino acids is known as aminization.
(2) Breakdown of Cellulose: Cellulose is the most abundant carbohydrate present in plant residues. The microorganisms break up cellulose into cellobiose and glucose. Glucose is further attacked by organisms and converted into organic acids:
The decomposition of cellulose in acid soils proceeds more slowly than in neutral and alkaline soils. It is quite rapid in well aerated soils and comparatively slow in those poorly aerated.
(3) Breakdown of Hemicellulose: to microbial decomposition, hemicelluloses are first hydrolyzed to their component sugars and uranic acids. The sugars are further attacked by microorganisms. They are converted to organic acids, alcohols, and water. The uranic acids are broken down to pentose and carbon dioxide. The newly synthesized Hemicelluloses thus form a part of the hummus. Hemicelluloses decompose faster than cellulose.
(4) Breakdown of Starch: Chemically it is a glucose polymer. It is first hydrolyzed to maltose by the action of enzymes (amylases). Maltose is next converted to glucose by another enzyme (maltase). Glucose is soluble in water is utilized for growth and other metabolic activities.
(C) Decomposition of Ether-soluble substances
Fats is first broken down by microorganisms through the agency of enzyme lipase into glycerol and fatty acids. Glycerol is next oxidized to organic acids which along with the other fatty acids are finally oxidized to carbon dioxide and water.
(D) Decomposition of Lignin
Lignin is deposited on the cell wall to impart strength to the framework of the plant. Lignin decomposes slowly, much slower than cellulose. Complete oxidation of lignin gives rise to carbon dioxide and water.
(E) Simple Decomposition Products
As the enzymic changes of the soil organic matter proceed, simple products begin to manifest themselves. Some of these especially carbon dioxide and water, appear immediately. Others such as nitrate-nitrogen, accumulate only after the peak of the vigorous decomposition is over. The more common simple products resulting from the activity of the soil microorganisms are as follows:
Carbon: CO2, CO32, HCO3, CH4, C
Nitrogen: NH4+, NO2, NO3, N2 (gas)
Sulfur: S, H2S, SO32, SO42, CS2
Phosphorus: H2PO4, HPO42
Others: H2O, O2, H2, H+, OH, K+, Ca2+, Mg2+ etc.
(F) Mineralization of Organic Sulphur
Many organic compounds especially those of nitrogenous nature, carry sulphur. Heterotrophic bacteria simplify the complex organic compounds, then autotrophic bacteria (sulfur bacteria) oxidize it into sulfate form.
(G) Mineralization of Organic Phosphorus
A large proportion of the soil phosphorus is carried in organic combinations. Upon attack by microorganisms, the organic phosphorus compounds are mineralized; that is, they are changed to inorganic combinations. It depends upon soil pH. As the pH goes up from 5.5 to 7.5 the available phosphorus changes from H2PO4 to HPO4. Both of these forms are available to higher plants.