Thermal stability and composition of mineral-bound organic matter in density fractions of soil

Heavy density fractions of soil contain organic matter tightly bound to the surface of soil minerals. The chemical composition and ecological meaning of non-metabolic decomposition products and microbial metabolites in organi c-mineral bonds is poorly understood. Therefore, we investigated the heavy fraction (density >2 g cm(-3)) from the topsoil of a Gleysol (Bainsville, O ttawa, Canada). It accounted for 952 g kg(-1) of soil and contained 19 g kg (-1) of organic C. Pyrolysis-field ionization mass spectra showed intensive signals of carbohydrates, and phenols and lignin monomers, alkylaromatics (mostly aromatic) N-containing compounds, and peptides. These classes of co mpound have been proposed as structural building blocks of soil organic mat ter. In comparison, the light fraction (density > 2 g cm(-3)) was richer in lignin dimers, lipids, sterols, suberin and fatty acids which clearly indi cate residues of plants and biota. To confirm the composition and stability of mineral-bound organic matter, we also investigated the heavy fraction ( density > 2.2 g cm(-3)) from clay-, silt- and sand-sized separates of the t opsoil of a Chernozern (Bad Lauchstadt, Germany). These heavy size separate s differed in their mass spectra but were generally characterized by volati lization maxima of alkylaromatics, lipids and sterols at about 500 degrees C. We think that the observed high-temperature volatilization of these stru ctural building blocks of soil organic matter is indicative of the organic- mineral bonds. Some unexpected low temperature volatilization of carbohydra tes, N-containing compounds, peptides, and phenols and lignin monomers was assigned to hot-water-extractable organic matter which accounted for 7-27% of the carbon and nitrogen in the heavy fractions. As this material is know n to be mineralizable, our study indicates that these constituents of the h eavy density fractions are degradable by micro-organisms and involved in th e turnover of soil organic matter.