Humic Substances, Part 4: Where Do They Come From

WHERE DO HUMIC SUBSTANCES COME FROM?

Although the formation process of HS has been studied hard and for a long time, their formation is still the subject of long-standing and continued research. Some theories have lasted for years; for example, the ‘sugar-amine condensation’ theory, the ‘lignin’ theory or the ‘polyphenol’ theory. A review of such theories can be found in a monograph of Davies and Ghabbour 1999. Nowadays, most investigators suppose that humic substances originated in lignin (Oglesby et al. 1967).

Polyphenols come mostly from lignin during its biodegradation, and probably play a key role in the formation process. Polyphenols are also regarded as the main agents in the formation of humic substances from some plants that do not contain much lignin and/or from non-lignin containing plants. Polyphenols can be considered as humic acid precursors. They themselves possess enough reactive sites to permit further transformations, for example some condensation reactions. The humic substances system is created. by the association of various components present in the humification process, such as amino acids, lignins, pectins or carbohydrates, through intermolecular forces (donor-acceptor, ionic, hydrophilic, and hydrophobic) It is evident that the mechanisms of the formation of humic substances can be slightly different, depending on geographical, climatic, physical and biological circumstances, respectively. These compounds can be formed in several ways, and the role of lignin is important in the majority of these processes (Burdon 2001, Davies et al. 2001). Burdon (2001) proposed that humic organic matter consists mainly of a mixture of plant and microbial constituents plus the same constituents in various stages of decomposition (i.e. plant/microbial
mixtures of carbohydrates, proteins, lipids and partially degraded lignins, tannins, melanins, etc.).