Organic Matter Fractions Controlling Soil Water Repellency in Sandy Soils From the Doñana National Park (Southwestern Spain)

The relationship between soil organic matter (SOM) content and organic molecular assemblages (n-alkane/alkenes pairs and n-fatty acids) with soil water repellency (WR) in different sieve fractions (1–2, 0·25–1, 0·05–0·5 and <0·05 mm) was studied in sandy soils from the Doñana National Park (SW Spain) under four vegetation species: Quercus suber (QS), Pteridium aquilinum (PA), Pinus pinea (PP) and Halimium halimifolium (HH). The severity of soil WR [assessed according to the water drop penetration time (WDPT) test] was ordered according to the sequence QS > PA > PP > HH. A positive correlation was found between soil WR from each sieve fraction and SOM content. Most severe soil WR was detected in QS for all sieve fractions, followed by the finer fractions (<0·05 mm) from PA, PP and HH samples. The finer fraction had the highest SOM content, ranging between 20·9% (PP) and 46·9% (QS). Coarser PA, PP and HH soil fractions (1–2 mm) showed the highest long-chain-even C numbered fatty acids in the order PP > PA > HH. A significant correlation was observed between SOM content and the severity of soil WR in QS samples and finer fractions of soil under other vegetation (PA, PP and HH). Coarser fractions (1–2 mm) from PP, PA and HH 1–2 mm sieve fractions showed high severity of WR and relatively low SOM contents that could be explained by a low degree of evolution of the organic residues as seen by analytical pyrolysis, that is, higher alkane/alkene carbon preference index values and the presence of a higher diversity of fatty acid structures. Copyright