Molecular and isotopic study of lipids in particle size fractions of a sandy cultivated soil (Cestas cultivation sequence, southwest France): Sources, degradation, and comparison with Cestas forest soil

Lipid analysis was performed for three particle size fractions (i.e., sand-, silt- and clay-sized) isolated from a sandy spodosol of the Cestas cultivation sequence (Landes de Gascogne, France). The forest was cleared 22 years ago and since then it has been continuously used for maize cropping. Lipid identification was carried out before and after saponification of lipid extracts. Numerous compound classes (free and esterified fatty acids, esterified ω-hydroxy acids, esterified diacids, esterified aromatic acids, free and esterified alkanols, alkanes and alkanones) were identified in all the particle size fractions and their distributions determined. Large differences in lipid molecular composition were observed between the fractions of the cultivated soil, as well as with previously analysed fractions of the Cestas forest soil. Detailed information was thus obtained about the different lipid sources in these fractions and the influence of change in land use, illustrating the importance of studying lipids from size separates. Stable carbon isotope ratios were determined for the cultivated and forest soils and their particle size fractions, for total organic carbon (Corg) and lipid carbon; measurements were also performed for individual long chain n-alkanes. Isotopic data generally also showed large differences between the fractions of the cultivated soil and the fractions of the forest soil as well. Data are discussed in relation to the contribution of carbon from maize after 22 years of cultivation, the preservation/degradation of the carbon from the initial forest soil, the differences between total and lipid carbon, and the nature of the carbon retaining the isotopic signature of the forest in the cropped soil.