Spectroscopic and pyrolytic features and abundance of the macromolecular refractory fraction in a sandy acid forest soil (Landes de Gascogne, France)

The insoluble, non-hydrolysable, macromolecular fraction was isolated from an acid sandy forest soil from Cestas (Landes de Gascogne, south west France) through successive hydrolysis with TFA and HCl. This refractory organic matter (ROM) accounts for a large part, ca. one third, of the total organic matter of the soil. Characterisation via a combination of elemental analysis, FTIR and solid-state 13C NMR spectroscopy, conventional pyrolysis and thermally assisted hydrolysis and methylation with tetramethylammonium hydroxide showed a complex composition for the ROM, including contributions from lignin, suberan, condensed tannins, higher plant waxes, suberin and polysaccharides attributed to cellulose. The latter probably survived the extensive acid treatment used to isolate the ROM, thanks to protection within the ligno-cellulose complex and/or to a high degree of crystallinity. Suberin also likely benefitted from some physical protection. In contrast, protein-derived moieties and bacterial lipids do not seem to be significant contributors. The occurrence of some of the above components is more or less efficiently revealed (or even not for cellulose and suberin) depending on the particular analytical method.