STRUCTURAL STUDY ON THE SOIL HUMIN FRACTION - BORON TRIFLUORIDE-METHANOL TRANSESTERIFICATION OF SOIL HUMIN PREPARATIONS

The boron trifluoride-methanol transesterification method was used to depolymerize soil humin preparations from three contrasted soil types in order to examine the presence of biopolyester moieties in these non-colloidal soil humic fractions. The humin samples were isolated physically from the soil (ultrasonic treatment and flotation) and purified by extractions with organic and inorganic solvents. Such humin fractions amounted to between 10-20% of the soil organic C and their CHCl3-soluble transesterification products (6-8.5% wt) were analyzed by combined gas chromatography-mass spectrometry. When subjected to transesterification, the humins yielded approximately equal amounts of aromatic and alkyl compounds. The former fraction consisted mainly of phenolic acids. The major constituents of the alkyl fraction were omega-hydroxy fatty acids, di- and trihydroxy monobasic acids and long-chain dibasic acids, in addition to epoxy monobasic acids, mono and dienoic fatty acids, and a minor proportion of n- and b-fatty acids and beta-hydroxy monobasic acids. The results obtained suggest that an important amount of lipid polymers, such are cutins and suberins, tends to accumulate in the soil humin fraction incorporating a series of compounds derived from microbial metabolism.