The soil lipid fraction consists of a complex mixture of low-polarity compounds of microbial or plant origin [1]. Homologues series of alkanes are present in all soil types: long-chain homologues are frequently considered to derive from epicuticular waxes of vascular plants, whereas short-chain homologues are thought to derive mainly from microorganisms. In general the free lipids (i.e., directly extracted with organic solvents) amount to a little proportion of the soil organic matter (2¿ 50 g/kg), but may have a direct influence in soil processes viz, enhancing soil water repellency or promoting soil aggregation. On the other hand, alkanes as many other soil lipids, are also a valuable source of environmental information: In fact, homologous series of alkanes may vary in terms of the vegetation and microbial activity, but also reflect different soil abiotic and ecophysiological properties with a bearing on the selective accumulation of the individual alkane molecules. In this exploratory research we focus exclusively on the n-alkanes found in the free soil lipid fraction, which are studied by gas chromatography¿mass spectrometry (GC¿MS), using the characteristic ion trace at m/z 85 to identify and quantify the alkane series. For this study we collected 35 topsoil samples from Spanish soils (0¿10 cm depth) in contrasted environments. The lipids were Soxhletextracted for 12 h with petroleum ether (40¿60 ºC). Thehomologous series were considered in the range between C15 (pentadecane) to C35 (pentatriacontane). The proportions of the different homologues were used as descriptors to explore possible correlations with a large series of soil properties. Complementary chemometric methods were used to examine the information latent in the alkane patterns viz, partial least squares regression (PLS), multidimensional scaling (MDS) and principal component analysis (PCA). We found a large forecasting potential of the n-alkanes mainly as regards soil physical properties (i.e., available water content, total soil water storage capacity, bulk density, water holding capacity, and moisture in the wilting point), followed by some structural characteristic of the soil humic acids. This was also the case with the origin and degree of decomposition of the soil organic matter (particulate organic matter) and the saturation of the exchange complex (mainly concentration of available Ca and Mg). The above results suggest the high potential of the alkane signature as source of information mainly on hydrophysical soil quality. Further research in progress would shed light on possible cause to effect relationships between the alkane signature and physical, chemical and biological soil properties involved in emergent soil properties and complex biogeochemical processes e.g., soil quality and soil C storage potential.

Peer Reviewed