DITHIONITE-CITRATE-BICARBONATE-EXTRACTABLE ORGANIC-MATTER IN PARTICLE-SIZE FRACTIONS OF A HAPLAQUOLL

The influence of the dithionite-citrate-bicarbonate (DCB) extraction on soil organic matter (SOM) in five particle-size fractions of a Haplaquoll was studied by pyrolysis-field ionization mass spectrometry. By treatment with DCB, 4 to 19 g kg(-1) of organic C (C-dcb; 17.8-44.4% of total C), 2.2 to 20.3 g kg(-1) of Fe (Fed(dcb); 13.6-29.7% of total Fe), and 0.4 to 8.4 g kg(-1) of Al (Al-dcb; 0.6-8.0% of total Al) were extracted from the size fractions. The pyrolysis-field ionization mass spectra recorded before and after DCB extraction and the proportions of six important compound classes of SOM showed that lipids, alkylaromatics, and lignin dimers were preferentially extracted. In the extraction residues of clay, fine silt, and medium silt, increases in total ion intensities, weight-averaged molecular weights of the pyrolysate, and characteristic shifts in the pyrolysis thermograms toward lower temperatures were observed. These properties indicated a general destabilization of the SOM structure due to DCB extraction. We explained these results by the solubility of these SOM constituents in alkali or by associations with pedogenic oxides that stabilized the SOM structure. The results pointed also to unknown, possibly selective, losses of C species when DCB extraction is used to improve the quality of solid-state (cross polarization/magic angle spinning)-C-13 nuclear magnetic resonance spectra.