Index

Abstract

Soil organic matter (SOM) is the substrate and habitat of soil microorganisms and fauna. These biotic pools, along with inorganic soil components, contribute to the degradation and synthesis of humic acids (HAs) and SOM through chemical and biochemical reactions. During the past 225 years, most researches on the molecular structure and reaction of HA and SOM were done by soil chemists, with limited contribution from soil microbiologists. This may be one reason why progress in our knowledge of the molecular structure of SOM and HA has been slow. Closer cooperation between chemists and microbiologists would have been certainly beneficial. In this chapter, we summarize current knowledge of 2D and 3D molecular structures and propose new chemical reactions to synthesize HAs and SOM. We indicate how the application of recent advances in microbial biochemistry could assist soil chemists in their work on elucidating the molecular structures of HAs and SOM. We show that the continuous production of complex polyketides (PKs) from small soil oxoacids by soil microorganisms and associated polyketide synthases catalyzes the second biotic stage of humification in soils. The PKs involve complex alkylaromatic, aromatic, polyaromatic, phenolic, and polyphenolic structures. Due to their bioactivity, rapid adsorption to clay colloids, and high energy content in their chemical bonds, biologically and chemically formed PK structures represent kinetically passive soil carbon pools, and so lend themselves as carbon skeletons that contribute to the formation of a stable central unit structure in humic substances and SOM.