The spontaneous secondary synthesis of soil organic matter components: A critical examination of the soil continuum model theory

The “Soil Continuum Model” questions the occurrence of any independent natural process of secondary synthesis that generates compounds structurally distinct from plant or microbial metabolites. This review shows that a vast volume of interdisciplinary scientific evidence supports the formation of relevant non-pre-existing complex molecules exhibiting various types of structures. These molecules form during degradation and decay of biological cell components. The spontaneous abiotic and enzymatically catalysed reactions of components of organic residues and of their oxidative decomposition products suggested by state-of-the-art studies are indeed those proposed by most of the classical descriptions of humification. The review also highlights the chemically active role of pedofauna, explaining why the apparently harsh conditions of alkaline extraction of HS cannot be considered un-natural. Many insects and larvae feeding on foliage of plants with a high content of tannins have a midgut pH above 9. Albeit, reducing conditions are often maintained to avoid oxidation, peroxidases are active in the intestinal tract and pass on to feces. Polyphenols are then immediately enzymatically oxidized to their reactive quinone form, once feces are excreted and exposed to oxygen. Implications of our current knowledge on the reactivity of plant components in soil are discussed in relation with the present state of the art research on humic substances. Contrary to claims by the “Soil Continuum Model” theory, complimentary modern approaches need to be used to understand the complexity of soil organic matter.