O

Lignin and microbial products phenolic, quinoid, cell wall etc.

Physically protected particulate organic matter condensation Humic precursors

Humic substances

Lignin and microbial products phenolic, quinoid, cell wall etc.

Mineral associated humic substances

Soil aggregate formation

Mineral associated humic substances

Soil solution Dissolved organic carbon and nitrogen

FIGURE 12.15 Hypothesized mechanism for the formation and protection of humic substances showing the importance of microbial products and altered lignin components in condensation reactions and formation of soil aggregates.

CH3OH

N content of humic substances. Humic substances consist of approximately 50-55% C, 5% H, 33% O, 4.5% N, 1% S, and 1% P (Table 12.3). Other metals and micro-nutrients, such as Al, Ca, Zn, and Cu, are present in much smaller amounts.

The humification process is analogous to lignin formation in that precursors are enzymatically produced or altered and the synthesis is characterized by condensation into larger molecules or structures. As with lignin, no two humic substances are alike in structure, but they behave similarly in function. The general chemical and structural properties of SOM are similar in soils found around the earth and appear to be a universal outcome of microbial decomposition processes in association with humification process and interactions with secondary minerals. As humans begin to explore the solar system the discovery of humic substances on distant planets may suggest past or present life. Since extraterrestrial simple organic compounds such as amino acids are found in meteorites it would be of interest to determine if prebiotic humic compounds also occur on asteroids.

There has been considerable debate about whether plant materials are directly incorporated into humic substances or are altered through microbial activity (Shevchenko and Bailey, 1996). Waksman (1936) theorized that lignin and proteins condensed to form humic acids, with the latter being of microbial origin. This has

TABLE 12.3 Elemental and Functional and Constituent Group Components of Humic and Fulvic Acids"
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