Preservation of aliphatic macromolecules in soil humins

Two series of alkali-insoluble preparations of soil organic matter (humins) were isolated from three representative types of terrestrial humus by two methods, respectively based on (i) ultrasonic disaggregation followed by flotation in a bromoform-ethanol mixture and (ii) partition 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 pyrolysis patterns of these particulate soil organic fractions differed quantitatively and qualitatively from those typical for terrestrial humic acid or lignocellulosic materials. The humins isolated by both methods yielded major series of alkyl products (56–81% of the total chromatographic area) including very variable amounts of alkanes, fatty acids and olefins. The aromatic pyrolysis products consisted mainly of alkyl-substituted mono and di-phenols, naphthalenes, phenanthrenes, fluorenes and methoxyphenols. The pyrograms of the humins isolated by partition were much more varied and produced a greater yield of alkanes than those isolated by flotation. The differences between humins from the three types of humus were small and tended to coincide in both series of preparations. Both the 13C NMR spectra and the atomic ratios suggested the presence of stable carbohydrate-like material, which was not evident in the pyrograms or in the total yield of hydrolyzable sugars. The results suggest that humins represent the high-stability aggregate compartments of active terrestrial soils where structural rearrangement of aliphatic biogenic macromolecules may take place, simultaneously to the condensation of recalcitrant macromolecular material tightly bonded on reactive mineral surfaces.