Study of the composition of the macromolecular refractory fraction from an acidic sandy forest soil (Landes de Gascogne, France) using chemical degradation and electron microscopy

The composition of the insoluble, non-hydrolysable, macromolecular fraction from an acidic sandy forest soil (Cestas, Landes de Gascogne, S.W. France) was examined via chemical degradation and high resolution transmission electron microscopy (HRTEM). Chemical degradation was performed using four reagents (sodium persulfate followed by potassium permanganate, sodium perborate and ruthenium tetroxide). Large differences in the level of degradation of this refractory organic material (ROM) and in the nature and relative abundance of the GC-amenable reaction products were observed between these reagents. Chemical degradation supported the presence of condensed tannins in the ROM, previously suggested from NMR data, and showed the contribution of lipid-derived moieties condensed to the macromolecular refractory material. The contribution of black carbon was revealed with ruthenium tetroxide oxidation and by HRTEM observations that showed the presence of charcoal particles probably related to fire events. All the oxidation treatments point to the lack of a significant contribution of bacteria-derived moieties to the Cestas ROM. The different oxidants revealed different components of the ROM. Thus, the occurrence of suberin was better indicated by the perborate treatment, the contribution of moieties originating from the oxidative cross-linking of unsaturated acids by alkaline permanganate and RuO4, and the presence of condensed tannins and black carbon was only revealed by the RuO4 treatment.