Effects of hydrofluoric acid dissolution on organo-mineral associations in Cambisols and Andosols of Reunion
Résumé
Interactions between organic matter (OM) and mineral phases represent a fundamental mechanism of carbon (C) sequestration in soils. They result in the formation of organo-mineral associations, which prevent rapid OM biodegradation. The density-separation technique enables effective isolation of different types of organo-mineral associations in soil. However, analyses of OM in these organo-mineral associations are limited because the C contents are very small (<10 mg OM g−1 of soil) and mineral phases complicate direct studies of OM. Previous studies have shown that hydrofluoric acid (HF) dissolution is an effective treatment for OM analysis since the mineral phases are dissolved and C is concentrated in the extract. Nonetheless, few studies on the effects of HF dissolution have been directly carried out on organo-mineral associations. The main objectives of this study were to determine the effects of HF dissolution on organo-mineral associations and to establish the best HF treatment conditions for OM analysis. We chose two specific tropical soils, an hydric Andosol and an haplic Cambisol (Réunion), that varied in their mineralogy. After densimetric separation, we tested three HF concentrations and four contact times. Mineral removal was 94% for both soils at the most concentrated HF treatments. Aluminium and silicon compounds were largely dissolved after the first treatment, while iron-rich residue accumulated after repeated HF treatments. XRD analyses confirmed the rapid dissolution of halloysite, gibbsite and quartz and the persistence of iron oxides. For the organo-mineral associations of the Cambisol, HF treatments did not enable us to obtain a usable signal with 13C NMR. For the organo-mineral associations of the Andosol, a four-times extraction over 2 hours with 5% HF allowed us to acquire an effective signal with 13C NMR for OM qualitative analyses. However, a new compound made of aluminium and fluorine appeared with the 10% HF treatments for both soils. In order to compare and analyse the OM present in organo-mineral associations of these two soil types, we thus recommend using four 2-hour treatments with 5% HF.