The Supramolecular Structure of SOM

A major breakthrough in understanding SOM chemistry in the last decade came with the recognition that soil humus is a self-assembled supramolecular associations of small heterogeneous molecules held together mainly by weak hydrophobic linkages, rather than being composed of large molecular weight macropolymers (Piccolo 2001).

Humus, otherwise referred to as Humic Substances (HS), is the natural organic matter comprising up to 80% of SOM. Because of the beneficial effects that HS have on the physical, chemical, and biological properties of soil, their role in the soil environment is significantly greater than that attributed to their contribution to sustaining plant growth. The HS are recognized for their controlling both the fate of environmental pollutants and the chemistry of organic carbon in the global ecosystem (Piccolo 1996).

Despite their prominent importance, a better knowledge of the basic nature and reactivity of HS has been elusive for a long time because of their large chemical heterogeneity and geographical variability. Because it is a mixture that originates randomly from the decay of plant tissues or microbial metabolism-catabolism or both, the chemistry of humus is not only of utmost complexity but also a function of the different general properties of the ecosystem in which it is formed: vegetation, climate, topography, etc. The tremendous task of advancing the knowledge of humic chemistry and its consequences to other environmental domains still lies ahead of us. It should be obvious, to a world that appreciates the potentials of genetic engineering based on an understanding of DNA structure, that accurate predictions of reactivities and development of related technologies can only be made when there is a basic knowledge of the chemical structure of the reacting molecules.

Piccolo summarized his and other authors' experiments supporting the supra-molecular structure of humus in different reviews (Piccolo 2001, 2002; Piccolo et al. 2003). These experimental results cannot be explained by analytical interferences or the traditional macropolymeric model of HS. They can rather be interpreted with the concept of loosely bound humic supramolecular associations. By this concept one can imagine HS as relatively small and heterogeneous molecules of various origin which self-organize in supramolecular conformations. Humic superstructures of relatively small molecules are not associated by covalent bonds but stabilized only by weak forces such as dispersive hydrophobic interactions (van der Waals, p-p, and CH-p bondings) and hydrogen bonds, the latter being progressively more important at low pHs. Hydrophilic and hydrophobic domains of humic molecules can be contiguous to or contained in each other and, with hydration water, form apparently large molecular size associations. In humic supramolecular organizations, the intermolecular forces determine the conforma-tional structure of HS, and the complexity of the multiple noncovalent interactions controls their environmental reactivity.

By the concept of supramolecular association, the classical definitions of humic and fulvic acids are reconsidered. Fulvic acids may be regarded as associations of small hydrophilic molecules in which there are enough acidic functional groups to keep the fulvic clusters dispersed in solution at any pH. Humic acids are made by associations of predominantly hydrophobic compounds (polymethylenic chains, fatty acids, phenolic and steroid compounds) which are stabilized at neutral pH by hydrophobic dispersive forces (van der Waals, p—p, and CH-p bondings). Their conformations grow progressively in size when intermolecular hydrogen bondings are increasingly formed at lower pHs, until they f1occulate.

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