Organic geochemical studies of soils from the Rothamsted Classical Experiments—I. Total lipid extracts, solvent insoluble residues and humic acids from Broadbalk Wilderness

Abstract

Total lipid extracts and insoluble organic matter, i.e. solvent insoluble matter and humic acids, were studied from soil samples taken from the three adjacent plots comprising the Broadbalk Wilderness at Rothamsted Experimental Station, Harpenden, Hertfordshire, U.K. Analyses involved high-temperature gas chromatography (HT-GC) and HT-GC-mass spectrometry (HT-GC-MS) to investigate trimethylsilylated total lipid extracts and Curie-point pyrolysis-GC (Py-GC) and Py-GC-MS to investigate solvent insoluble fractions. The plots were chosen specifically for their different types of vegetation cover. Samples of the vegetation were examined in parallel with the underlying soils in an effort to follow the fate of the major plant components in soil. The application of HT-GC and HT-GC-MS allowed changes in high molecular weight lipids, particularly intact acyl lipids, such as triacylglycerols, wax esters, steryl and triterpenyl esters, to be studied in leaf and soil extracts. The total lipid extracts of the soil samples from the wooded area were dominated by the input from leaf-derived lipids. The lipid extracts of soils from the grazed and stubbed areas were markedly different from those from the wooded area, and reflected the mixed vegetation cover dominated by grass species. In marked contrast, the pyrolysis data from the insoluble organic matter and humic fractions of the soils did not reflect the composition of the lignin comprising the overlying vegetation, but rather showed evidence of amino acid moieties probably present as polypeptides. The absence of the lignin signal is possibly due to rapid diagenetic changes presumed to be influenced by the slightly alkaline pH of the soil. The ability to recover recognizable chemical signals from soil lipids has important implications for archaeological investigations aimed at revealing temporal changes in vegetation cover and/or differences in land use at specific site locations.