Historical and current progress in understanding the origin and structure of humic substances

Abstract

Research into humic substances (HS) has focused selectively on the search for aromatic structures. All aromatic compounds are regarded as eligible HS constituents, whereas aliphatic, polysaccharide structural constituents are considered non-HS. Wet chemical methods for analysing HS produce 2–5% aromatic compounds from the original starting material. Only those compounds in the hydrophobic extraction phases are studied carefully. Compounds in the hydrophilic extraction phases, containing mainly aliphatics, are discarded. Solid-state NMR spectra from HS show 30–40% of carbon–carbon unsaturation, which is selectively interpreted as ‘aromatic’. No recognition is given to the fact that NMR spectroscopy of HS cannot distinguish olefinic unsaturation from aromatic unsaturation in molecules. By considering this unsaturation to be more or less olefinic, solid-state NMR shows the structure of HS to be mainly aliphatic. Polysaccharides are recycled in humification via microbial metabolism. Lignin is not formed again in humification. During evolution, polysaccharides appeared first and lignin much later, and it is reasonable to suppose that the existing humification process accommodated lignin as an additional material to consume. Humification has evolved to become a key process in the production of fertile soils, and dearomatisation may be an important component of this role.

Keywords:

• humic substances
• humification
• dearomatisation
• functional recalcitrance
• NMR
• olefinic unsaturation
• non-humic aromatics

Acknowledgements

Professor Roger Jones is thanked for revision of the English of the text. Kari Hänninen is presently working as associate professor at the University of Jyväskylä. He has spent 30 years researching humus chemistry. In 1987 he finalised his dissertation in that field. Presently he is teaching and researching environmental chemistry (stability of the climate) and environmental biotechnology (composting, anaerobic digestion).