Calcinated : SOM - , No-structure Undisturbed : SOM + , structure Sieved : SOM + , No-structure

1. MATERIAL & METHODS: Water Surfactant Ethanol Calcinated Undisturbed Sieved • Calcinated: SOM-, No-structure • Undisturbed: SOM+, structure • Sieved: SOM+, No-structure Water retention model with log-normal pore radius (r) distribution SOIL WATER RETENTION MODEL: Air entry value Extended Young-Laplace equation: Ψ= cosβ(Ψ)/r ; Ψ= Soil suction cos β(Ψ)=a1 exp(-a2Ψ); β=contact angle Air entry “like” value Wettable Hydrophobic Effective saturation pF RESULTS: CONCLUSIONS: • WATER RETENTION OF A HYDROPHOBIC SOIL: DISENTANGLING THE EFFECT OF STRUCTURE, ORGANIC MATTER AND SOIL WATER REPELLENCY • Carlos M. Regalado - Axel Ritter • (cregalad@icia.es) Soil water retention is affected by hydrophobicity. Both soil organic matter content (SOM) and structure also affect water retention. However these three effects, hydrophobicity, SOM and structure, are coupled such that one may be affected by the others. Thus, for example, SOM contributes to soil structure but also to soil water repellency. We measured the water retention curve of a forest organic soil (48% SOM) under different treatments in order to evaluate in isolation the contribution of hydrophobicity, SOM and structure. We propose a mechanism to explain how the various components involved in the water retention of the studied soil affect its moisture characteristic curve. Experiment II ( 3 humectation liquids ) Experiment I ( 3 soil treatments ) Pressure cells • Water: SOM+ & Hydrophobic • Surfactant: SOM+ & Wettable • Ethanol: SOM+ & Wettable • Determination of the drying water retention curve (0-90kPa) of the soil samples from experiments I & II Water Surfactant Ethanol Experiment I SOM-, S-, H- Calcinated X X Undisturbed SOM+, S+, H+ SOM: Soil organic matter S: Structure H: Hydrophobicity -/+: Absence/Presence X X SOM+, S-, H+ SOM+, S-, H- SOM+, S-, H- Sieved Experiment II + We studied the water retention curve of a forest organic soil. The experiments showed that the presence of organic matter increased the soil water retention capacity. This increase is due largely to a higher contact angle (hydrophobicity) provided by the soil organic matter, and more specifically to its effect on the value of the air entry point or bubbling pressure. Soil structure was also relevant, reducing the water retention in the range pF> 1 partly offsetting the hydrophobicity character of the soil, suggesting that previous results obtained from disturbed soil samples should be taken with caution when extrapolated to under field conditions. The data were fitted satisfactorily to the retention curve model originally developed by Kosugi (1994) and extended in this work in order to consider the case in which the contact angle varies with suction.