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Thermal stability and composition of mineral‐bound organic matter in density fractions of soil
Author(s) -
Schulten H. R.,
Leinweber P.
Publication year - 1999
Publication title -
european journal of soil science
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.244
H-Index - 111
eISSN - 1365-2389
pISSN - 1351-0754
DOI - 10.1046/j.1365-2389.1999.00241.x
Subject(s) - chemistry , organic matter , lignin , environmental chemistry , topsoil , soil organic matter , gleysol , soil water , organic chemistry , soil science , geology
Summary 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 organic–mineral bonds is poorly understood. Therefore, we investigated the heavy fraction (density > 2 g cm –3 ) from the topsoil of a Gleysol (Bainsville, Ottawa, 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 compound have been proposed as structural building blocks of soil organic matter. In comparison, the light fraction (density > 2 g cm –3 ) was richer in lignin dimers, lipids, sterols, suberin and fatty acids which clearly indicate 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 topsoil of a Chernozem (Bad Lauchstädt, Germany). These heavy size separates differed in their mass spectra but were generally characterized by volatilization maxima of alkylaromatics, lipids and sterols at about 500°C. We think that the observed high‐temperature volatilization of these structural building blocks of soil organic matter is indicative of the organic–mineral bonds. Some unexpected low‐temperature volatilization of carbohydrates, 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 known to be mineralizable, our study indicates that these constituents of the heavy density fractions are degradable by micro‐organisms and involved in the turnover of soil organic matter.