Insoluble alkyl carbon components in soils derive mainly from cutin and suberin

Insoluble alkyl carbon components were isolated, identified and quantified in soil samples of a Haplic Podzol (from the Oi to the Bs horizons) and a Folic Histosol (from the Oi to the Oa2 horizons) to obtain information about their fate during litter decomposition and humification. The compounds were released by a sequential chemical degradation which comprised acid and base hydrolysis, cleavage with iodotrimethylsilane (ITMS) and oxidation with ruthenium tetroxide (RuO4). The identification and quantification were carried out using gas chromatography–mass spectrometry (GC–MS). Alkyl C components were detected as fatty acids and alcohols with varying structures and chain length in the range from 7 to 32 C atoms. They derived mainly from the hydrolysable plant polyesters cutin and suberin preserved in the soils, and containing ether and double bonds. The total concentration ranged between 26.3 mg g−1 total organic carbon (TOC) in the Bs horizon and 44.8 mg g−1 TOC in the Oa horizon of the Haplic Podzol. In the Folic Histosol the contents decreased from 28.9 mg g−1 TOC in the Oi horizon to 8.9 mg g−1 TOC in the Oh2 horizon. For both soils, the depth functions depended mainly on the cutin and suberin input from litterfall and roots as well as their translocation and enzymatic degradation. The contribution of the insoluble alkyl C identified by GC–MS to the total alkyl C determined by NMR decreased from about 10% in the Oi horizon to 2% in the Oa2 horizon of the Folic Histosol.