The organic matter in the soil has a complex transformation process with the direct participation of soil organisms and is influenced by soil environmental factors. Some of them are completely mineralized to form simple minerals, part are used by soil organisms to synthesize proteins, lipids, sugars and other compounds that build their bodies, some undergo complex process of transformation and re-synthesis into macromolecular compounds known as humic substances.
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In other words, when organic matter enters the soil, it will be affected by two processes occurring simultaneously: mineralization and humusification. Depending on the soil conditions and the activity of soil organisms, either of these two processes may prevail in the soil.
The humus compounds after being formed are also subjected to slow decomposition to form minerals (Figure).
The process of mineralization and synthesis of soil humus
Mineralization of organic matter in the soil (inorganic)
Mineralization is the process of breaking down organic compounds into simple minerals such as CO2, H2O, NO3-, NH4+, Ca2+, Mg2+, K+…
This is a complex transformation process that goes through many different stages. Complex organics are first broken down into simpler organics called intermediates. For example, from protein molecules are broken down to form peptide chains, then amino acids; The hydrocarbons are broken down to form sugar compounds, which are then further broken down to form mineral end products.
Depending on the environmental conditions and the activity of soil microorganisms, the mineralization of organic matter can take place in two different ways: rotting and rotting.
Rotting is an aerobic process that takes place in the presence of sufficient oxygen. The end products of this process are mainly substances in the oxidized form such as CO2, H2O, NO3-, PO43-, SO42-. This is an exothermic process and as a result increases the temperature of the soil.
Rotting is an anaerobic process that occurs in the absence of oxygen due to flooding or by rapidly growing aerobic microorganisms that have used up oxygen in the soil. The end product of the decay process besides substances in the oxidized form such as CO2, H2O also has a large amount of substances in the reduced form such as CH4, H2S, PH3, NH3 …
The rate of mineralization of organic matter in the soil depends on the nature of the organic matter, environmental conditions and the activity of soil organisms. In general, sugar and starch compounds are most susceptible to mineralization; followed by proteins, hemicellulose, cellulose; Linhin compounds, wax resins are more difficult to decompose.
Environmental conditions such as humidity, temperature, air regime, soil solution composition and properties also have a strong influence on the rate of mineralization. Normally at 70% soil moisture, pH 6.5 – 7.5, temperature 25 – 30oC and sufficient air is suitable for soil microbial activity and therefore mineralization process also occurs strongly. . Under such conditions, organic matter is decomposed rapidly and humus is less accumulated. Therefore, the decomposition of organic matter in soils with light mechanical composition (such as sandy soil) also takes place faster than in soils with heavy mechanical composition (heavy soil and clay).
The path of organic matter accumulation after 1 year of application to the soil (Brady 1990)
Humidification is the process of decomposing and re-synthesizing organic substances to form humus with the active participation of soil organisms.
Humus is a complex macromolecular organic compound that is the product of the humification process of common organic matter. It is said that all organic components in the soil (proteins, linins, lipids, amino acids, carbohydrates, etc.) can be involved in the formation of soil humus. However, there are still different opinions on the nature of the humus formation process.
Chemicalists believe that humus formation is merely a chemical reaction. Representing this view as Vacsman, Scheffer. According to Vacsman (1936), the nucleus of humus is formed by lignin combining with alkaline minerals in the soil, then the oxidation reactions will attach other organic acids to form humus. In addition, during the decomposition of organic matter, an amorphous black product with a complex composition is formed called humus.
Schefer suggested that the formation of humic acids could be by biochemical pathways and also by purely chemical pathways. Chemically, humic acids are formed from phenols, quinols and amino acids through oxidation and polymerization reactions.
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Today, there is much evidence that humus formation is actively involved in biochemical processes, especially soil microorganisms. The formation of humus by chemical means is very limited, it can only be encountered in places where conditions are unfavorable for biological processes such as soils that are too acidic or too toxic. They inhibit biological processes from occurring.
The biochemical view of humus formation holds that humus is formed from the breakdown and resynthesis of common organic matter with the active participation of biochemical reactions, especially enzymes. secreted by microorganisms.
The process of humus formation from a modern point of view
Chiurin is a person who has made many contributions to the study of soil humus. He suggested that the basic feature of humification is the gradual biochemical oxidation of various cyclic macromolecular compounds, in which proteins and linins play an important role. These oxygen reactions occur upon decomposition of plant remains under the influence of atmospheric oxygen, oxidase enzymes and other inorganic catalysts.
These macromolecular compounds link together and then polymerize into humus substances. During their life, soil microorganisms use organic decomposition products, metabolic products and synthesize amino compounds, aromatic compounds also participate in the formation of humus.
Describe the pathways that form humus from common organic matter in the soil (Stevenson, 1982).
Pathways of humus formation
From the above diagram, it can be seen that the origin of substances involved in the formation of humus may include all organic substances that are intermediate decomposition products, products of re-synthesis of organisms. They can be sugars, polyphenols, quinols, amines, lingin compounds, etc. In which aromatic compounds such as phenols, polyphenols, quinol, polyquinols, and linins play an important role.
In general, the process of humus formation can be divided into three basic steps as follows:
– From organic remains, which are mainly decomposed plant remains with the active participation of soil microorganisms to form organic compounds, which are intermediate products such as sugars, polyphenols, quinols, substances amines,…
– The action of intermediate compounds, either further decomposed or linked together to form more complex substances.
– Polymerization and linkage of the above intermediate compounds to form humus substances.
According to Stevenson, there are 4 different ways of forming humus: Condensation association between sugar compounds and amines (path 1); between polyphenols which are decomposition products of organic matter and amines (pathway 2); between substances that are products of the breakdown of linin and amine compounds (pathway 3); and linins modified with amines (pathway 4). These pathways all involve biological processes.
According to Selman Waksman, humus is formed mainly from Linhin compounds (path 4), so it is also known as the theory of humus formation. According to this theory, first of all, the modified lignin compounds lose their methoxyl groups (OCH3). In the presence of orthohydroxylphenols and the oxidation of fatty compounds to the formation of carboxyl groups (COOH). These lignin compounds are gradually transformed to form humic acids. The formation of humus by pathway 1 is negligible.
Some other authors believe that soil humus is formed by pathways 2 and 3 as the main and called the polyphenol theory of humus formation. According to this theory, lingin is also considered to be the first important source for the formation of humus. Under the action of biological enzymes, lingin is broken down into phenol aldehydes and organic acids. They are then converted to quinol compounds which polymerize to form humus.
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It is now recognized that all four pathways of humus formation occur simultaneously. However, depending on the specific conditions and properties of each soil type, a certain road may prevail. Normally humus formed from modified linseeds (path 4) predominates in poorly drained soils; while polyphenol formation (pathways 2 and 3) is predominant in forest soils.