Decomposition of Organic Matter in Soil

(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 → Carbon dioxide + Water

When the nutrient or oxygen supplies are restricted, different partially oxidised compounds are formed:

(i) Ammonification:

Soluble nitrogenous compounds, viz., 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

Or

Organic nitrogen → NH₃

The ammonification occurs as a result of action of enzymes produced by microorganisms. Their action is chiefly hydrolytic and oxidative (in the presence of air).

(ii) Nitrification:

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.

It is the work of autotrophic bacteria:

The net reactions are as follows:

The process which involves conversion of soil nitrate into gaseous nitrogen or nitrous oxide is called de-nitrification.

Water-logging (e.g., Rice field) and high pH will increase nitrogen loss by de-nitrification.

(B) Decomposition of Insoluble Substances:

1. Breakdown of Protein:

Proteins are complex organic substances containing nitrogen, sulphur, and sometimes phosphorus, in addition to carbon, hydrogen and oxygen. During the course of decomposition of plant materials, the proteins are first hydrolysed to a number of intermediate products, e.g., proteoses, peptones, peptides, etc., collectively known 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:

When subjected to microbial decomposition, hemicelluloses are first hydrolysed to their component sugars and uronic acids. The sugars are further attacked by microorganisms. They are converted to organic acids, alcohols, carbon dioxide and water. The uronic acids are broken down to pentose and carbon dioxide. The newly synthesised Hemicelluloses thus form a part of the humus. Hemicelluloses decompose faster than cellulose.

(4) Breakdown of Starch:

Chemically it is glucose polymer. It is first hydrolysed to maltose by the action of enzymes (amylases). Maltose is next converted to glucose by another enzymes (maltase). Glucose being soluble in water is utilized for growth and other metabolic activities.

(C) Decomposition of Ether-soluble substances:

Fats are first broken down by microorganisms through the agency of enzyme lipase into glycerol and fatty acids. Glycerol is next oxidised to organic acids which along with the other fatty acids are finally oxidised to carbon dioxide and water.

(D) Decomposition of Lignin:

Ligning is deposited on the cell wall to impart strength to the framework of 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:

Mineralization of Organic Sulphur. Many organic compounds especially those of nitrogenous nature, carry sulphur. Heterotrophic bacteria simplify the complex organic compounds, then autotrophic bacteria (sulphur bacteria) oxidize it into sulphate form.

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.

Modes of Organic Matter Decomposition:

There are two modes in the degradation of organic matters into simpler substances i.e. aerobic and anaerobic decomposition. Micro-organisms such as fungi, bacteria, actinomycetes and mould play a dominant role depending on the availability of oxygen. Both bacteria and fungi are facultatively aerobic or anaerobic.

Option conditions for good decomposition of organic matter in soil:

1. Adequate water

2. Adequate supply of nitrogen

3. Optimum pH

4. Good aeration

5. Warm climate

6. A fine state of mechanical disintegration

Aerobic decomposition of organic matter:

A lot of heat is generated during aerobic decomposition. About two-third of carbon released during aerobic decomposition. Nitrogen is liberated as free ammonia; this ammonia is oxidized as nitrate.

Anaerobic decomposition of organic matter:

Anaerobic decomposition is mainly a reduction process. Due to incomplete degradation of organic matter, methane (CH₄) and hydrogen (H₂) are evolved. In waterlogged situation (anaerobic), in addition to above organic acids some important organic acids such as acetic acid, formic acid and butyric acid are also evolved.

In this condition, organic matter is decomposed by primary as well as secondary microorganisms. The primary microorganism, initially convert the complex carbohydrates and proteins into simple carbon sources.

Difference between aerobic and anaerobic decomposition of soil organic matter:

1. During aerobic decomposition about two-third (2/3) of carbon are respired as CO₂ while remaining one-third (1/3) combines with the cell protoplasm of microorganisms.

In case of anaerobic decomposition, carbon is respired as CO₂ to a small extent, but the major part is got rid off-as methane.

2. Aerobic decomposition does not bring bad odours but during anaerobic decomposition, methane, hydrogen sulphate and others bring bad odours.

3. During aerobic decomposition, heat generated is very high in comparison to anaerobic break down.