Clay-Size Organo-Mineral Complexes in Temperate Soils: Relative Contributions of Sorptive and Physical Protection
C. Chenu, I. Virto, A. Plante, F. Elsass, 2009. "Clay-Size Organo-Mineral Complexes in Temperate Soils: Relative Contributions of Sorptive and Physical Protection", 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 contain the largest C pool at the surface of the continents with more than 1500 Gt C (IPCC 2001). Managing soil C has been proposed as a way to reduce the increase of CO2 concentration in the atmosphere. However, choosing the best management option requires an appropriate knowledge of the mechanisms responsible for organic matter stabilization in soils in the long term.
Several processes can explain the stabilization of organic matter in soils (Figure 1), including (i) the selective preservation of recalcitrant molecules, (ii) chemical stabilization, which involves all intermolecular interactions between organic substances and inorganic ones leading to a decrease in availability of the organic matter, i.e. surface adsorption and precipitation and (iii) physical stabilization that is related to the decrease in the accessibility of organic substrates to micro-organisms due to occlusion in small pores (Sollins et al., 1996; Baldock and Skjemstad, 2000).
One of the major controls of soil C stocks is soil texture. Indeed, clayey soils are generally richer in C than coarser-textured soils, and several authors have established correlations between soil texture and C contents. For example, Hassink et al. (1997), Feller and Beare (1997) and later (Six et al., 2002) showed that the amount of C stored in the silt + clay fraction of soil (i.e. < 20μm particle size or < 50μm particle size) increased with the abundance of this fraction. Keil et al. (1994) and Mayer (1994)
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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.