Clay-associated organic matter in kaolinitic and smectitic soils

In the global carbon cycle, soil organic matter (SOM) is a major source/sink of atmospheric carbon. Clay minerals stabilize part of the soil organic matter through mineral-organic matter binding. Stabilization of organic matter is essential for tropical soils. Since the climatic conditions of the tropics favor decomposition of organic matter, tropical soils would be very poor in organic matter without this stabilization process. This research aims at determining the effect of clay mineralogy on the amount and composition of organic matter that is bound to the mineral surface. We focused on organic matter that is associated with kaolinite and smectite. We characterized kaoliniteand smectite-associated SOM in soils from seven countries, employing C NMR spectroscopy and Py-GC/MS. The content of carbon in the total clay-size fraction showed no significant difference between kaolinitic and smectitic soils. This suggests that the total amount of organic carbon in the clay-size fraction is independent of the clay mineralogy. We first extracted the clay fraction with NaOH and thereafter with Na4P207. About half of the kaolinite-associated SOM was extractable by NaOH. In the smectitic soils, pyrophosphate extracted more organic carbon than did NaOH. The Py-GC/MS and NMR results indicate that kaolinite-associated SOM is enriched in polysaccharide products, while smectite-associated organic matter contains many aromatic compounds. We suggest that different clay minerals use different binding mechanisms to complex SOM. As a result, the composition of clay-associated organic matter would be influenced by the type of clay that is dominantly present in the soil. Introduction Of the inorganic constituents in soil, clay minerals are particularly important in the stabilization of organic compounds (Greenland, 1971; Martin and Haider, 1986; Theng and Tate, 1989; Hassink, 1995). Clay minerals have a high specific surface area and carry a charge, enabling them to bind, and thereby chemically stabilize, organic matter. Clay aggregates also provide micropores for the physical protection of SOM. The interactions between organic matter and clays have been reviewed by several authors (e.g. Mortland, 1986; Huang and Schnitzer, 1986; Oades et al, 1989; Theng and Tate, 1989; Christensen, 1996, Schulten et al., 1996). Various terms have been used to describe the resultant products, the most common being organo-mineral complexes (Christensen, 1996; Schulten et al, 1996), and associations (Schnitzer et al, 1988). These terms are frequently used interchangeably. For consistency the following definitions will be used here: Clay-associated organic matter is all organic matter present in the "clay-size" separate, both free and bound. Clay-complexed organic matter refers to organic matter bound to clay mineral surfaces, e.g. by Ca-bridging, or intercalation between clay layers (Theng and Tate, 1989). It is practically defined as the organic matter present in the heavy clay-size fraction after density fractionation of the clay-size fraction. Secondary organo-mineral associations, often referred to as aggregates (Christensen, 1996), are involved in the physical protection of organic matter by occlusion. These associations are not further discussed in this paper. Previous research has mainly been concerned with characterizing the amount and composition of the organic matter in the clay-size fraction relative to that present in the other size fractions (Christensen, 1992 and references cited therein). Recently, Leinweber et al.