Characterisation of organic matter from organo-mineral complexes in an Andosol from Reunion Island

Organo-mineral complexes play a major role in the stabilisation of organic matter (OM) within soils, but only few data are presently available about the nature of the OM bound to mineral phases. In this study, we aimed at determining the nature of the OM present in various organo-mineral complexes in a tropical soil (Andosol). Organo-mineral complexes were isolated thanks to densimetric separation techniques. The mineral phases of organo-mineral complexes were identified using XRD, and OM was analysed by Py-GC-MS after carbon enrichment obtained through 5% hydrofluoric acid hydrolysis. We thus studied OM bound to poorly crystallised aluminosilicates (PC-AlSi), gibbsite and iron oxides, as well as free-OM (<1.9 g cm(-3) fraction), which represents OM that is not bound to mineral phases. In the organo-mineral complexes of the studied Andosol, lipids were mainly present with PC-AlSi while polysaccharides and proteins/amino acids were the main organic compounds associated with gibbsite. Lignin compounds were found in high quantities in free-OM but were not found in organo-mineral complexes. Only few organic compounds were bound to iron oxides. Furthermore, proteins/amino acids and lipids were also observed in all organo-mineral complexes, but polysaccharides were not. The present results show that mineral phase type influences not only the proportion but also the nature of the organic compounds present in organo-mineral complexes. This study provides new information about the nature of the OM bound to mineral phases, and will thus contribute to improving models of organo-mineral complexes in order to better understand OM stabilisation in soils. (c) 2012 Elsevier B.V. All rights reserved.