Summary
A scheme on humus formation has been proposed. This is as follows: Lignin/carbohydrates, the chief sources of C for the microorganisms are first broken down by extracellular enzymes into smaller units. Soluble units are absorbed into the microbial cell where part of them are converted to phenols/quinones. These together with oxidising enzymes are discharged into the environment where they polymerise by a free radical mechanism. The formation of FA (fulvic acids) has also been explained and it has been concluded that the difference between FAs and HAs (humic acids) is merely in the degree of polymerisation and that FAs are not necessarily more aliphatic than HAs.
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Varadachari, C., Ghosh, K. On humus formation. Plant Soil 77, 305–313 (1984). https://doi.org/10.1007/BF02182933
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DOI: https://doi.org/10.1007/BF02182933