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"Adapted from Haider (1992).

"Adapted from Haider (1992).

been termed the "Lignin Theory" of SOM formation (Stevenson, 1994). The shortcoming of the Lignin Theory is that it cannot account for the increased N content of SOM relative to that of the initial residues.

Maillard (Stevenson, 1994) suggested that the accumulation of N occurred through the interaction of reducing sugars and amino acids to form dark brown polymers similar to humic acids through a process termed the "Browning" reaction. The Browning reaction gained recognition because microorganisms readily produce sugar and amino compound substrates. In addition, the mechanisms could explain the formation of humic substances in aquatic environments, such as the ocean, where no lignin exists. The mechanism involves rearrangement and fragmentation of compounds to form three intermediates: (1) aldehydes and ketones, (2) reductones, and (3) furfurals. All of these react readily with amino compounds to form dark-colored end products through the formation of double bonds. Jokic et al. (2004) suggest that the action of manganese(IV) oxide on sugar-amino acid condensation under ambient conditions results in the formation of heterocyclic N compounds, suggesting that soil redox reactions may be sufficient to promote this reaction.

The polyphenol theory has recently become popular for describing the formation of SOM (Fig. 12.16). Polyphenols result from the decomposition of lignin or are microbially synthesized. Oxidation and demethylation of hydroxy phenols and aromatic acids result in the formation of quinines. Quinones polymerize into dark-colored humic substances. The presence of amino acid greatly enhances the polymerization and condensation process. These form aminoquinone intermediates

Lignin

Lignin

Glycine long chain aliphatic compounds

FIGURE 12.16 Hypothesized formation of a humic substance by reaction of a phenol decomposition product between an amino acid and an aliphatic substance.

Glycine long chain aliphatic compounds

FIGURE 12.16 Hypothesized formation of a humic substance by reaction of a phenol decomposition product between an amino acid and an aliphatic substance.

upon oxidation to quinones and can condense to form brown, high-molecular-weight nitrogenous humates. The polyphenol reaction is considered important for forming SOM from lignin or melanin-degradation products (Haider et al., 1975; Stevenson, 1994).

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