Chemical structure and sources of the macromolecular, resistant, organic fraction isolated from a forest soil (Lacadée, south-west France)

The insoluble, non-hydrolyzable, macromolecular material isolated from a forest soil from Lacadée (south-west France) was examined via a combination of various methods: FTIR spectroscopy, elemental analysis, “off-line” pyrolysis and high resolution transmission electron microscopy. Such a resistant material, which accounts for ca. 25% of total humin, was shown to be chiefly composed of melanoidins and black carbon. Two types of black carbon particles were identified by dark field and lattice fringe electron microscopy. Contrary to previous observations, based on solid state 13C NMR spectroscopy and Curie point Py/GC/MS, highly aliphatic moieties only afford a minor contribution to the refractory material of the Lacadée soil. Additional studies, using mixtures of model compounds, were carried out to examine the origin of this conspicuous overestimation of the level of aliphaticity in such heterogeneous material when the latter two methods are used.