Structural Characterization of Soil Organic Matter and Humic Acids in Particle‐Size Fractions of an Agricultural Soil

Abstract

Organic matter (OM) in agricultural soils consists mainly of partially decomposed plant residues, microbial biomass, and humic substances. The advanced analytical techniques of 13 C 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.