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

The insoluble, non-hydrolyzable, macromolecular material isolated from a fo rest soil from Lacadee (south-west France) was examined via a combination o f various methods: FTIR spectroscopy, elemental analysis, "off-line" pyroly sis and high resolution transmission electron microscopy. Such a resistant material, which accounts for ca. 25% of total humin, was shown to be chiefl y composed of melanoidins and black carbon. Two types of black carbon parti cles were identified by dark field and lattice fringe electron microscopy. Contrary to previous observations, based on solid state C-13 NMR spectrosco py and Curie point Py/GC/MS, highly aliphatic moieties only afford a minor contribution to the refractory material of the Lacadee soil. Additional stu dies, using mixtures of model compounds, were carried out to examine the or igin of this conspicuous overestimation of the level of aliphaticity in suc h heterogeneous material when the latter two methods are used. (C) 2000 Els evier Science Ltd. All rights reserved.