Variation in solvent-extractable lipids and n-alkane compound-specific carbon isotopic compositions with depth in a southern China karst area soil

Abstract

Because literatures about the lipid compositions of modern soils in karst areas are scarce, we have studied the soil horizons overlying the Heshang (和尚) Cave that has provided paleoclimate records from speleothem lipid contents. Our analysis reveals a series of n-alkanes, free fatty acids, n-alkanols and n-alkan-2-ones distribution and relative abundance changing with the depth, and in which the ratios of the lower molecular weight to higher molecular weight n-alkanes, free fatty acids, n-alkanols and n-alkan-2-ones have a peak at a subsurface depth of 5 to 10 cm. An accompanying peak in 17β(H), 21β(H)-hop-22(29)-ene (diploptene) and a shift to less negative n-alkane carbon isotopic values also identify this layer in the karst soil. This pattern indicates the existence of a subsurface soil layer in which the microorganisms that produce these compounds are especially abundant. The carbon isotopic values of individual plant wax n-alkanes are about 3‰ greater at the base of the 30- to 40-cm soil profile than in the surface layer, probably as a result of selective microbial degradation of n-alkanes from different primary sources. The lipids and carbon isotopic values of individual plant wax n-alkanes study of the overlying soil show a strong microbial activity in this karst soil and help in interpreting the lipid compositions and specific carbon isotopic value of n-alkanes of the stalagmites of the Heshang Cave for paleoenvironmental reconstructions.