NMR Characterization of Natural Organic Matter and Clay-Humic Complexes
Robert L. Wershaw, 2009. "NMR Characterization of Natural Organic Matter and Clay-Humic Complexes", 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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Soils and sediments are composed of complex mixtures of inorganic and organic components. The organic components of soils (soil organic matter) constitute a carbon pool that is about 2.5 times that of living vegetation, 1.5 times that of surface ocean, and about twice that of the atmosphere (González-Pérez and others, 2004). This carbon pool is particularly important because soil properties such as buffering capacity, metal-binding capacity, stability of aggregates of soil particles, water-holding capacity, and sorption of hydrophobic organic compounds are dependent, to a large extent, on the amount of natural organic matter (NOM) in a soil. All of these properties, with the exception of the last one, are important in controlling soil fertility.
The maintenance of soil fertility is of paramount importance for the survival of human life on our planet. In the rich, industrialized countries of the world soil fertility is maintained by application of chemical fertilizers that are produced using large amounts of fossil fuels. The ready availability of the chemical fertilizers has encouraged farmers to employ intensive agricultural practices such as growing a single crop year after year (monoculture), irrigation, and yearly tillage. Unfortunately, however, intensive agriculture can lead to deterioration of soil structure and aggregation (Pimentel and others, 1995; Tilman, 1999, Bongiovani and Lobartini, 2006).
Mäder and others (2002) showed that farming systems that make use of organic amendments (designated organic farming by Mäder and others, 2002) are much more sustainable than conventional intensive agriculture.
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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.