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

Abstract

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.

Subcritical water extraction to isolate kinetically different soil nitrogen fractions

“…At 150 and 200 • C, SCWE probably extracted increasingly more OM, which is strongly bound to minerals, i.e. belonging to the third pool defined by Schulten and Leinweber (1999). This was indirectly confirmed by correlation analysis with the clay percentage and contents of Fe ox and Al ox , as described above.…”

“…On the basis of the data in this study it seems plausible to hypothesize that SCWE at 100 • C primarily releases weak or intermediately bound N and OC, i.e. belonging to the second SOM pool identified by Schulten and Leinweber (1999). The first pool may well be of lesser importance because unbound undecomposed OM in fact comprises but a relatively small share of the soil N and is probably poorly extractable due to its particulate nature.…”

“…Schulten and Leinweber (1999) found that volatilization of mineral bound OM requires higher temperatures than unbound OM to enable thermal bond cleavage. They identified three thermal classes: (i) unbound undecomposed plant fragments relatively rich in aliphatics; (ii) a thermally labile fraction containing Ncontaining compounds and carbohydrates associated with humified OM and (iii) thermally stable mineral bound OM.…”

Pyrolysis-field ionization mass spectrometry of rhizodeposits – a new approach to identify potential effects of genetically modified plants on soil organisms

“…These were combined to obtain one thermogram of total ion intensity (TII) and an averaged mass spectrum. In addition, for each single scan, the ion intensities of marker signals for ten selected classes of chemical compounds in SOM were calculated (Schulten and Leinweber, 1999). All Py-FIMS data were normalized per mg sample.…”

Measuring and modeling continuous quality distributions of soil organic matter

“…Either method offers an easy way to measure the amount of carbon or mass lost at different temperatures producing a thermogram. The thermograms often exhibit different peaks for labile and more resistant SOM (Kristensen, 1990;Schulten and Leinweber, 1999;Siewert, 2004;LopezCapel et al, 2005b;Bruun et al, 2008b). Thermal treatments have also been used to isolate black carbon from soil (Gustafsson et al, 1997(Gustafsson et al, , 2001Gelinas et al, 2001).…”