Degradative studies on a soil humin fraction-sequential degradation of inherited humin

Abstract

Degradation products of soil humin preparations from three different soil types were studied. Inherited humin was isolated by ultrasonic treatment and densimetry of the heavy fraction of soil, and purified by treatment with dilute alkali and acid and exhaustive lipid extraction. These purified humins, which represented 10-26% of the total soil C (34-55% of total alkali-insoluble humified C), were degraded with potassium persulfate followed by alkaline permanganate oxidation. The degradation products obtained (identified by GC-MS as methyl derivatives) were fatty acids, dicarboxylic aliphatic acids, benzenecarboxylic acids, phenolic acids and alkanes. An unexpectedly high proportion of polymethylene compounds (mainly fatty acids) were observed in the humins of the three soils, in both loosely and strongly associated forms. It is considered that these aliphatic chains are stable humin constituents, physically or chemically associated with a highly oxidized, demethoxylated and disordered matrix entrapping a significant proportion of non-polar compounds. Some of the aliphatic components seem to be inherited from cuticular waxes of higher plants, but it is tentatively suggested that during the microbial reworking of lignins the lipid polymers were altered, then contributing to the formation of humin.