ABSTRACT
The process by which humic matter is formed has been called in soil science humification, which involves a number of biochemical reactions. Several versions of biochemical reactions are present, recognized as different pathways in the condensation-polymer concept, and with the advancing knowledge in the twenty-first century several more new pathways have been added on the basis of the self-assemblage concepts. Traditionally a subject in soil science, humic matter is now also a field of study in other scientific disciplines, e.g., geology, hydrology, and limnology. Such an expansion into different directions has brought new ideas and concepts, undoubtedly enriching the knowledge of humic substances. Terms used in geology are now also applied in humic acid science, and among them, the term diagenesis seems to be applied quite often to refer to the formation of humic substances, as employed in particular by geologists, geochemists, hydrologists, limnologists, and some chemists (McSween et al., 2003; Hedges and Keil, 1999; Hatcher et al., 1985). However, the term has the connotations of geological processes related closely to consolidation and lithification of unconsolidated material into “solid” rocks. In pedology, soils are unconsolidated materials weathered from rocks, and humic matter is synthesized during soil formation as one of the major constituents essential in the creation of soils. The humic substances are also closely connected to the organic and nitrogen cycles in the environment, and it is perhaps questionable to assume the two cycles above to be geological processes, although today many soil,
environmental, and chemical reactions seem to be readily claimed as belonging to the domain of geology by using the term biogeochemistry. This has, of course, created a lot of controversies in soil science as has the case of soil chemistry versus geochemistry (Tan, 2011). The issues and advantages of diagenesis in humic acid chemistry will be examined in more detail in the following section.
