Humic Fractions and the Nature of Organic Materials in Intimate Association with Soil Clays
Michael H.B. Hayes, Guixue Song, Andre J. Simpson, 2009. "Humic Fractions and the Nature of Organic Materials in Intimate Association with Soil Clays", Carbon Stabilization by Clays in the Environment: Process and Characterization Methods, David A. Laird, Javiera Cervini Silva, Yona Chen, Claire Chenu, Françoise Elsass, Javier M. Gonzalez, Michael H.B. Hayes, David A. Laird, Alain Plante, Andre J. Simpson, Guixue Song, Jorge Tarcjotzly, Michael L. Thompson, I. Virto, Robert L. Wershaw
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For any consideration of the organic matter in intimate association with the surface of soil clays it is appropriate to consider first the totality of the organic matter in the soil. The term “soil organic matter” (SOM), according to Stevenson (1994), refers to “the whole of the organic matter in soils, including the litter, the light fraction, the microbial biomass, the water-soluble organics, and the stabilized organic matter (humus)”. Nowadays, the term natural organic matter (NOM) is widely used for the natural organic components in soils, sediments, and waters.
On the basis of earlier definitions by Kononova (1966), Hayes and Swift (1978) considered the complete soil organic fraction to be made up of live organisms and plants within the soil, and their undecomposed, partly decomposed, and completely transformed remains. However, they regarded SOM to be a more specific term for the non-living components that may be described as “a heterogeneous mixture composed largely of products resulting from microbial and chemical transformation of organic debris.” The transformation, or the humification process, gives rise to the final product, humus, a mixture of substances with some resistance to further degradation. This definition is similar to that of Stevenson who considered humus to be the total of organic compounds in soil exclusive of undecayed plant and animal tissues, their partial ‘decomposition’ products, and the soil biomass. Stevenson partitioned SOM into the “active” (or labile) and the “stable” pools. The “active fraction” contains the macro-organic (or particulate) matter and the
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Organic matter (OM) in soil plays vital roles with respect to global climate change, as the largest terrestrial reservoir of organic carbon, and with respect to soil quality through the stabilization of soil structure and the retention and cycling of plant nutrients. The interactions between lay minerals and OM are central to most of these functions. Clays may catalyze formation of new humic substances, inhibit the degradation of existing humic substances through physically sequestration, and clay-humic associations are at the very heart of aggregation and soil structure stabilization. In this book we seek to explore the state of knowledge related to these topics and the analytical tools used to investigate them. In chapter 1, Hayes et al. describe chemical fractionation techniques and relate clay bound soil OM to the “humin” fraction. Chen and arcjotzly (Chapter 2) discuss the role of humic substances and polysaccarides in formation and stabilization of soil structure. Gonzalez (Chapter 3) considers the potential catalytic role of clays in the formation of new humic materials. Wershaw (Chapter 4) considers the nature of soil OM and clay-humic complexes as revealed by NMR and other techniques. The last two chapters, Chenu et al. (Chapter 5) and Laird and Thompson (Chapter 6), focus directly on understanding the nature of clay-humic complexes as revealed by electron microscopic techniques. It is hoped that this volume will provide the reader with both advanced understanding of the current state of knowledge and an appreciation for the gaps in that knowledge. The knowledge gaps represent challenges for future generations of scientists.