Abstract 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 CO 2 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)

Abstract 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

Abstract Most arable soils contain 0.1 to 5% organic matter (OM) by weight. The lowest figures represent sandy soils of arid zones, whereas the higher values are typical of clayey temperate zones. The vast majority of these soils are physically and chemically influenced by the OM which they contain. In addition to the nutritional value of the OM, it plays a critical role in the formation and stabilization of soil structure, which in turn produces desired tilth and drainage as well as resistance to erosion. To a remarkable degree, increased OM can counteract the diverse structure effects that may prevail in either highly sandy or clayey soils. Increasing soil organic matter (SOM) content usually results in a decrease in bulk density and increase of total porosity. Over a wide range of 10 to 60 g organic C per kg soil, a curvilinear decrease in bulk density from 1.7 to 0.8 Mg m −3 has been observed ( Franzluebbers et al., 2001 ). A curvilinear positive dependence between the C content of soils in New Zealand and aggregate stability was also shown by Haynes (2001) . Friability of soils, namely, their tendency to form clods that easily crumble into their constituent natural aggregates is most commonly related to OM content as well as aggregate stability and bulk density ( Macks et al., 1996 ). Soil mineral particles usually aggregate into granular structures. The stability of soil aggregates (or micro-aggregates – the small particle size fraction of the aggregates – see below) is

Abstract Clay minerals are important from the agricultural, environmental, and industrial point of view because of their chemical and physical properties. Surface acidity of clays is of special interest given that clays catalyze several chemical organic reactions. In natural systems, research has suggested that clays play important roles in the formation of soil organic matter; however, the mechanisms of formation and the roles of clays are poorly understood and/or often controversial. Although, several different pathways of secondary synthesis leading to the formation of humic substances have been proposed, none are universally accepted ( Stevenson, 1982 ). The Maillard reaction, condensation of reducing sugars and amino acids, is one of the proposed pathways for formation of humic substances that has received relatively little attention. The starting materials of the Maillard reaction exist in soils; the questions are whether, and to what extent, clay minerals catalyze the Maillard reaction in soil environments? The objective of this chapter is to compile a synthesis of published research on the formation of soil organic matter with emphasis on the Maillard reaction as a potential pathway for the formation of humic substances and the role of clay minerals as catalysts for Maillard reactions.

Abstract 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.

Abstract The chemical and physical activity of clay minerals in soils, particularly in surface horizons, is significantly mediated by interactions with organic components. And the reactivity of soil organic matter, including its resistance to decomposition, is regulated by interactions with clay minerals. This marriage of organic and inorganic soil components has profound implications for our ability to quantitatively predict the consequences of alternative soil management practices that could improve food and energy production, protect water and air quality, or mitigate the impacts of greenhouse gases on global climate. In this paper, we discuss observations of clay-humic complexes made by electron microscopy and other complementary techniques. By “clay,” we mean secondary layer silicate and oxide minerals that are common to soils. By “humic materials,” we mean soil organic matter that was derived from biological tissue but has been so transformed in the soil environment that it lacks any recognizable biological structures. Our definition of humic materials precludes any genetic or functional relationship with humic and fulvic acids, which are procedurally defined extracts of soil organic materials that we view as heterogeneous mixtures of hydrolyzed humic material, biological tissue, and black C. Data reported in this paper come from various analyses of a single smectite-rich Mollisol in Iowa. Because we investigated only one soil, we cannot generalize our results beyond saying that the studied soil was typical of smectite-rich Mollisols in the upper Midwest. Interactions between soil organic matter (SOM) and soil minerals are primarily responsible for the formation and stabilization of soil structure

Abstract 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.