The chemical composition of organic matter was studied in the whole spodosol Bh horizon soil and in five particle-size fractions (clay, fine silt, medium silt, coarse silt, sand) by doing organic carbon (C(org)) and total nitrogen (N(t)) analyses and pyrolysis-field ionization mass spectrometry (Py-FIMS). Compared with the whole soil, the silt and clay fractions were increasingly enriched in C(org) and N(t), with decreasing particle size, whereas the sand-size fraction was depleted in these elements. This distribution of soil organic matter (SOM) was significantly reflected by the proportions of volatilized matter (r = 0.998***) and the total ion intensities (r = 0.878**) per mg sample. Assuming a complete removal of adsorbed water, it was calculated that 85% of the organic matter in the whole soil, 40% each in silt and clay fractions, and 100% of the organic matter in the sand was volatilized during Py-FIMS. On the basis of these amounts of volatilized SOM, the following order could be computed for the proportions of assigned biomarkers of SOM in the samples: whole soil (48%) > sand (43%) > fine silt (26%) > medium silt (17%) = coarse silt (17%) > clay (14%). In the whole soil, silts, and clay, the accumulated organic matter showed characteristic signals of lipids, alkylaromatics, carbohydrates, phenols and lignin monomers, and lignin dimers. N-containing compounds were of lesser abundance. Relatively intensive signals of dehydroergosterol and ergosterol referred to soil fungi in the spodosol Bh-horizon. For the sand fraction, the divergent total ion intensity per mg C(org) the lower average molecular weight of the pyrolysis products, and the Py-FI mass spectrum indicated a different composition of organic matter compared with the finer particle sizes. Thus, higher relative abundances of carbohydrates, phenols and lignin monomers and lower relative abundances of dimeric lignin units and N-containing compounds were observed. The high accumulation of SOM in clay, fine silt, and medium silt in combination with the thermal evolution profiles of the biomarkers in these fractions indicated a higher importance of intramolecular SOM bonds compared with intermolecular organo-mineral bonds for the stabilization of organic matter in the particle-size fractions <20 mum. With decreasing particle size, carbohydrates, phenols and lignin monomers, and N-containing compounds were stabilized increasingly in thermolabile bonds. Except for sand, more thermostable bonds were observed for lignin dimers and alkylaromatics in all particle fractions. The bimodal curves of lipids referred to two bond types for this biomarker in the organic matter of the spodosol Bh-horizon.
