Unveiling the molecular composition of the unextractable soil organic fraction (humin) by humeomics

We applied humeomics to a soil unextractable humin fraction (HUM1) and its derived humin (HUM2) after removal of minerals by an HF/HCl treatment. Humeomics implies progressive separation and structural characterization of molecules released from complex humin matrices as (i) unbound, (ii) weakly ester-bound, (iii) strongly ester-bound, and (iv) ether-bound. Molecular characterization of fractions was achieved by GC-MS, thermochemolysis-GC-MS, and 13C-CPMAS-NMR. Both weight and chromatographic yields were higher for the clay-depleted HUM2 than those for HUM1, and this increased molecular detection in HUM2. Saturated and unsaturated alkanoic, α,ω-alkanedioic, hydroxyalkanoic acids, alkanols, and hydrocarbons were found in both HUM1 and HUM2. Abundant odd-C numbered n-alkanoic acids in unbound fractions indicated accumulation of free microbial metabolites, whereas plant-derived acids remained in fractions more tightly bound to the humin matrix. Unsaturated, n-alkanedioic, and hydroxyalkanoic acids were detected after hydrolysis of complex esters. The aromatic character in humin residues progressively increased with humeomics sequential steps, while alkyl and hydroxy-alkyl compounds were reduced. Humins contained similar components as a humic acid extracted from the same soil, implying that traditional humic pools differed in supramolecular arrangement rather than in molecular composition. The humeomic approach enables the determination of the molecular composition of humic matter and may improve knowledge of the structure-activity relations of organic matter in soil.