Nature and origin of lipids in clay size fraction of a cultivated soil as revealed using preparative thermochemolysis

Results of chemical treatments to isolate a pool of biochemically resistant soil organic matter (SOM) remain equivocal because they do not exhibit the expected relative increase in the proportion of resistant material with decrease in total SOM during long term biological mineralization. On the other hand, certain OM (considered to be enriched in aliphatic compounds) resists H2O2 oxidation as a result of association with minerals as well as its specific chemical recalcitrance, thereby protecting it against microbial degradation. Clay fractions isolated from soils under long term cultivation or long term bare fallow were examined using preparative thermochemolysis with TMAH (tetramethylammonium hydroxide, an alkylating agent) before and after peroxide treatment to characterise the molecular structure of the hydrophobic part (e.g. lipids). Results showed an increase in the proportion of some of the lipids after peroxide treatment, the lipids identified being mainly fatty acids (FAs) and hydrocarbons. The H2O2-resistant pools of lipids have an exclusively microbial signature but their quantity and relative distributions differed depending on land use. In the case of acids (as methyl esters), peroxide treatment appeared to mimic long term microbial oxidation, but this was not the case for n-alkanes. Chemical methods, such as H2O2, may not effectively mimic long term biological oxidation of clay-associated OM because, in isolation, they cannot account for the strong interaction between biochemical recalcitrance and physical protection, which exists even within the clay size fraction. (C) 2008 Elsevier Ltd. All rights reserved.