Abstract
In the last three decades, the concept of soil humic substances has been questioned in two main directions. Misinterpretations of CP MAS13C NMR spectroscopy led to the conclusion that soil organic matter is mainly aliphatic, questioning the theory of polymerization of humic substances from phenolic molecules. Conversely, some critics of humic substances assume that a great proportion of aromatic soil organic carbon originates from fire-affected carbon, often termed as black carbon (BC). However, the determination of BC in soil by two widely applied methods, the benzene polycarboxylic acid marker method and the UV method, is not reliable and seems to strongly overestimate the BC content of soils. The concept of humic substances continues to be relevant today. The polymerization of phenolic molecules that originate from the degradation of lignin or synthesis by microorganisms may lead to humic substances which can incorporate a variety of organic and inorganic molecules and elements. The incorporation, e.g., of triazines or surfactants into the humic matrix, leading to bound residues, illustrates that humic substances are important to explain central reactions in soil. Humic substances are also important to understand the availability of plant nutrients in soil, including P, Fe, and Cu, and they may have a direct effect on the growth of higher plants in soil. Therefore, there are good reasons to reformulate or to further develop the concepts and models of humic substances introduced and developed by M. Schnitzer, W. Flaig, W. Ziechmann, and F.J. Stevenson.
Highlights
For a long time, humic substances were considered as the stable part of soil organic matter with a yellow to dark brown color
The polymerization of phenolic molecules that originate from the degradation of lignin or synthesis by microorganisms may lead to humic substances which can incorporate a variety of organic and inorganic molecules and elements
These groups findings on the low aromaticity of soil organic matter, which were obtained with the use of CP MAS 13 C NMR spectroscopy
Summary
Humic substances were considered as the stable part of soil organic matter with a yellow to dark brown color. In the beginning of the 1980s, a relatively new, direct, non-destructive, and quantitative spectroscopic method was introduced to soil organic matter research, solid state cross polarization magic angle spinning 13 C nuclear magnetic resonance spectroscopy (CPMAS 13 C NMR spectroscopy). This method allowed new aspects of research in organic soil chemistry and has been extensively used to quantify the proportions of functional groups as well as the aliphatic and aromatic contents of soil organic matter and humic substances. Hasselmann [8] stated in his PhD thesis ([8], p. 71) concerning CP MAS 13 C NMR spectroscopy that ether-, sugar-, and carboxylate-C are overestimated by this method, whereas aromatic- and chinoid-C are strongly underestimated (translation from German to English by J.G.)
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