PRESERVATION OF ALIPHATIC MACROMOLECULES IN SOIL HUMINS

Two series of alkali-insoluble preparations of soil organic matter (hu mins) were isolated from three representative types of terrestrial hum us by two methods, respectively based on (i) ultrasonic disaggregation followed by flotation in a bromoform-ethanol mixture and (ii) partiti on in water-methylisobutylketone. The humins amounted to between 7-42% of the total carbon in the soil heavy fraction and were characterized by pyrolysis followed by gas chromatography-mass spectrometry. The py rolysis patterns of these particulate soil organic fractions differed quantitatively and qualitatively from those typical for terrestrial hu mic acid or lignocellulosic materials. The humins isolated by both met hods yielded major series of alkyl products (56-81% of the total chrom atographic area) including very variable amounts of alkanes, fatty aci ds and olefins. The aromatic pyrolysis products consisted mainly of al kyl-substituted mono and di-phenols, naphthalenes, phenanthrenes, fluo renes and methoxyphenols. The pyrograms of the humins isolated by part ition were much more varied and produced a greater yield of alkanes th an those isolated by flotation. The differences between humins from th e three types of humus were small and tended to coincide in both serie s of preparations. Both the C-13 NMR spectra and the atomic ratios sug gested the presence of stable carbohydrate-like material, which was no t evident in the pyrograms or in the total yield of hydrolyzable sugar s. The results suggest that humins represent the high-stability aggreg ate compartments of active terrestrial soils where structural rearrang ement of aliphatic biogenic macromolecules may take place, simultaneou sly to the condensation of recalcitrant macromolecular material tightl y bonded on reactive mineral surfaces. Copyright (C) 1996 Elsevier Sci ence Ltd