ABSTRACT
The process by which humic matter is formed has been called humification, which involves a number of biochemical reactions. It is closely connected to the organic and nitrogen cycles in the environment. Though some people are of the opinion that the mechanisms for synthesis are not clear, a number of hypotheses have in fact been presented on how humic matter is formed. In general, these theories differ in the way the sources of original or raw materials are utilized in the synthesis of humic substances. Whereas one group of theories is based on depolymerization of biopolymers causing their direct transformation into humic substances, the other group envisages polymerization of small molecules, liberated by complete decomposition of the biopolymers, in the formation of humic matter. All agree that the materials for formation originate mostly from plant material, though in practice animal residue can also be transformed into humic matter. The depolymerization theory, called biopolymer degradation by Hedges (1988), assumes that the biopolymers in plants are gradually transformed into humin, which eventually will be degraded successively into humic acids and fulvic acids. The lignin theory of Waksman (1932) and its modern version are considered examples of the biopolymer degradation theory. In contrast, the polymerization theory claims that the plant biopolymers are decomposed first into their monomers or smaller organic components. Humic substances are then formed by interaction reactions between these small components. This theory assumes fulvic acid to be formed first, which by polymerization or condensation can be transformed into humic acids. The polyphenol or phenol, quinone, and sugar-amine condensation theories belong to the category of the polymerization theory. This second pathway of humification has recently also been called the abiotic condensation process (Hayes and Malcolm, 2001). The lignoprotein theory of Flaig et al. (1975; 1988), focusing on the breakdown of lignin and further oxidation of the degradation units into quinone derivatives, is an excellent example of the polymerization or abiotic condensation theory. Hayes and Malcolm (2001) believe that the rate of depolymerization depends on the oxygen content, and humification will be retarded in anaerobic conditions. It is true that a lot of oxygen is required for oxidation reactions, but the issue can be raised whether a lack of oxygen will severely inhibit the humification process. As discussed in Chapter 2, huge deposits of peat and bogs, rich in humic matter, are instead formed in wetlands, where anaerobic conditions prevail.
