Synthesis of humic substances

Wang,T. S. C.,Wang,M. C., Ferng, Y.L., and Huang, P. M. (1983a). Catalytic synthesis of humic substances by natural clays, silts and soils. Soil Sci. 135, 350-360. 

According to one theory, the synthesis of humic substances is supposed to be brought about, primarily, by the condensation of the autolysis products from micro-organisms growing on carbohydrates (mostly cellulose). The biosynthesis of aromatic compounds from carbohydrates " may be of importance in the soil. 

Environmental organic matter is a composite of humic and nonhumic substances, which is formed through operation and interactions of various biotic and abiotic processes. Humic substances are formed through both selected preservation (residue) and catalytic synthesis mechanisms. Both enzymatic and mineral catalyses contribute to the formation of humic substances in the environment. The relative importance of these catalytic reactions would depend on vegetation, microbial population and activity, enzymatic activity, mineralogical composition and surface chemistry of environmental particles, management practices, and environmental conditions. Selective preservation pathways would play a more important role in humification processes in poorly drained soils and lake sediments, compared with more aerated environmental conditions. 

Elemental composition is an important chemical property which can be used to establish the nature and source of humic substances. When the percent composition data are displayed as the atom ratios H C, 01C, and NIC, some general characteristics become visible. Soil, coal, marine, and aquatic humates may be distinguished, one from the other. Structural trends may be identified in specific environments, such as lake sediments and soil profiles. Nonhumate contaminants can be detected. Atom ratios may also aid the investigator in proposing hypothetical structures for humic and fulvic acids and serve as a guide in the synthesis of artificial humates. 

Synthetic humic acids have been prepared by a number of investigators over the past 50 years. Most of these preparations are based on a preference for a specific hypothesis for the origin of humic substances (1) partial degradation of lignin (2) chemical polymerization of microbial phenols or lignin monomers and amino acids (3) cell autolysis products (4) microbial synthesis (see Stevenson, 1982, Chapter 8 for review). 

Phenolic compounds of low molecular weight, particularly P hydroxybenzoic, vanillic, p-coumaric and ferulic acids, are of widespread occurrence in soils and occur mainly but not entirely in chemically-bound forms. They are believed to be important intermediates in the formation of humic substances and, at certain concentrations, they may influence the growth of plants and the activities of soil micro-organisms (see chapter 6). There is evidence that the phenolic acids and related compounds in soils originate in part from the decomposition of plant residues and in part from synthesis by soil micro-organi sms. 

Humic substances A series of relatively high-molecular-weight, yellow- to black-colored substances formed by secondary synthesis reactions. The term is used as a generic name to describe the colored material or its fractions obtained on the basis of solubility characteristics. These materials are distinctive to the soil (or sediment) environment in that they are dissimilar to the biopolymers of microorganisms and higher plants (including lignin)... 

Trusov (1915,1916), as indicated in Section 1.4.1, introduced the concept that polyphenols and quinones contributed to the synthesis of HS. Subsequently, as the result of enzymatic oxidation (with oxidase enzymes from microorganisms), the phenols oxidize to quinones and these, through further condensation, are converted into dark-colored humic-type substances. 

Humic substances account for various definitions according to different authors, and also its definition changes in literature through the years. These substances are conventionally defined as a series of relatively high-molecular-weight, yellow-to black-coloured substances, formed by secondary synthesis reactions (Stevenson, 1982 Soil Science Society of America, 1996) or as a category of naturally occurring,... 

Generally, humic substances are defined as condensed polymers of aromatic and aliphatic compounds produced by decomposition of plant and animal residues and by microbial synthesis. They are amorphous, dark-colored, and hydrophilic, with a wide range in molecular weight from a few hundreds to several thousands. Furthermore, humic substances contain a large number of nonidentical functional groups, with different pKa values, and are partitioned into three main fractions based upon their solubility behavior (Fig. 3.16) (Evangelou, 1995b) ... 

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