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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 …