The association between three dominant shrub species and water repellent soils along a range of soil moisture contents in semi-arid Spain

Soil water repellency (SWR) is a widespread phenomenon associated with decreased infiltration rates, fingered flow infiltration, and increased runoff. In Spain, research into the causes of SWR has focused on fire-induced effects. This research investigates the impact of different plant species and soil moisture contents (SMCs) on the water repellency of the surface soil horizon in a semi-arid shrubland ecosystem. The plant species were selected for their different wax input rates into the soil. In situ (field) SWR was investigated spatially, whereas in the laboratory it was assessed over a soil moisture range. The selected plant species were Stipa tenacissima, Anthyllis cytisoides and Rosmarinus officinalis. In the field, water repellency was found to be limited strictly to an organic soil layer ('mulch') between the soil crust and the ectorganic profile (litter, fragmentation and humification layers). When comparing the three species, soil under Stipa was found to be wettable or slightly water repellent, soil under Anthyllis and Rosmarinus severely water repellent. The water repellency of the surface soil appeared to be related to the thickness of the ectorganic profile. The soil organic carbon (SOC) contents correlated with water repellency (r(2) = 0-74, p = 0.01). Water repellency displayed a two-peak distribution over a soil moisture range. The first peak ranged from 22 to 26% SMC (w w(-1)), the second peak from 45 to 47%. The integrated area in a 'WDPT versus SMC' graph was calculated. Soil under Stipa, Anthyllis and Rosmarmus was water repellent in the ratios 1: 2-3: 4-5. The integrated area measurement had a better reproducibility and was more stable than SWR measurements at specific SMCs. It is concluded that, in the investigated ecosystem, SWR occurs without the influence of fire but underneath individual plants (fertile islands) only; plant species affect SWR substantially differently; and SWR occurs over a wide range of SMCs. Appropriate measures of SWR are discussed. Copyright (c) 2007 John Wiley & Sons, Ltd.