Structural characterization of soil organic matter and humic acids in particle-size fractions of an agricultural soil

Organic matter (OM) in agricultural soils consists mainly of partially decomposed plant residues, microbial biomass, and humic substances. The advanced analytical techniques of ¹³C nuclear magnetic resonance (NMR), tetramethylammonium hydroxide (TMAH) thermochemolysis-gas chromatography (GC)/mass spectroscopy (MS), and pyrolysis-GC/MS (Py-GC/MS) were employed to study the chemical structure of bulk soil organic matter (SOM) and its corresponding humic acid (HA) extracts from different aggregate-size fractions (<2 and >250 μm) of a dark brown Mediterranean soil. The main products released by TMAH thermochemolysis of the bulk soil and HA samples were: lignin-derived compounds (LG) and nonlignin-derived aromatic compounds, heterocyclic N, fatty acid methyl esters (FAMEs), and dicarboxylic acid dimethyl esters (DAMEs). The TMAH chromatogram of the >250-μm size fraction revealed more LG than that of the <2-μm fraction. The relative intensity of the long-chain FAMEs and DAMEs peaks decreased with aggregate size, but their presence highlights the contribution of aliphatic biopolymers to the structure of the SOM. Both Py-GC/MS and TMAH-thermochemolysis data suggest that the HAs contain large portions of lignin and cuticular materials in their structure. With decreasing particle size, the HA contained more lignin-derived units in the final stages of oxidation, more fatty acids originating from microbial activity, and higher contents of aromatic nonlignin-derived structures. Our data suggest that a steady-state situation exists for the presence of HA, where fresh OM is being decomposed and humified into the HA-like structures, but this fraction is subject to further decomposition (mainly through loss of O-alkyl and transformation of O-substituted aromatic carbons) as humification proceeds.