The effect of fire on lipid composition of soil size fractions and Soil hydrophobicity

Abstract

Low soil‐water affinity and soil water repellency (SWR, hydrophobicity) prevents water from wetting or infiltrating soils in burnt and unburnt ecosystems, triggering hydrological, geomorphological, geochemical, and biochemical changes. Fire may destroy, develop or even enhance SWR in previously wettable or water‐repellent soils. SWR is partially due to a lipid like cover, rich in fatty acids (FAs). Here we report the first results of a study on the effect of fire on the distribution of soil lipids and their role in the SWR. Two samples of sandy soil under Quercus suber canopy at the Doñana National Park (SW‐Spain) were taken. One come from a burnt site and the other from an adjacent unburnt (control) one, with the same physiographic characteristics. SWR was determined using water‐drop‐penetration‐time test in the <2 mm sieved (bulk) soils and in six size fractions: 1‐2 mm, 0.5‐1 mm, 0.25‐0.5 mm, 0.1‐0.25 mm, 0.05‐0.1 mm and <0.05 mm. Lipids were extracted from all samples (n = 14); the FAs and neutral lipids were identified and quantified by GC/MS and GC/FID. The carbon isotope ratios (δ13C values) for the individual fatty acids were determined by GC/C/IRMS. The SWR values of soil samples and fractions were statistically different (p < 0.01), for both, the fire‐affected and control soils, and different grain‐size fractions. SWR values in burnt bulk soil and 0.05‐0.1 mm fraction were higher than in unburnt homologues. The coarsest and finest soil fractions (1‐2 mm and <0.05 mm, respectively) of the unburnt soil were the most hydrophobic; in contrast, the finer fractions (0.05‐0.1 mm and <0.05 mm) were the most hydrophobic in burnt soils. The total amount of lipids and total FAs were higher in burnt bulk sample and all the size fractions, except the coarser one, which had twice the amount of lipids, compared to the burnt one. All samples showed a similar distribution of saponifiable lipids, characterized by straight‐chain saturated acids in the C14‐C32 range and only differing in their relative abundances. In bulk soil and <0.5 mm fractions the concentrations (in μg FA/g soil) of the FAs were higher in burnt compared to the unburnt soil (this difference was small or absent in C22). For the coarser fractions, the opposite trend was observed in most FAs, except C18, and in the 0.5‐1 mm fraction for C<20 acids. Principal component analysis (PCA) performed on lipid concentration, concentration ratios, and SWR indicated that hydrophobicity of soils was positively correlated to total amount of lipids, normal C>24 Fas and branched C>24 FAs, and negatively correlated with the even/odd FAs ratio. The 1 biosynthetic origin of these lipids and their transformation pathways during a fire will be discussed with the results of the ongoing measurements of the δ13CFA values.