WATER REPELLENCY IN SOILS AFFECTED BY WILDFIRES: A CASE STUDY IN CALAMUCHITA, CORDOBA, ARGENTINA

Natural and anthropogenic wildfires are common in the province of Cordoba, Argentina, and its effects on soil water repellency, aka hydrophobicity, are poorly understood. This property impacts the ability of water to infiltrate soil, and therefore has critical implications on soil erosion. The aim of this study was to analyze the physical and chemical properties of pre- and post- fire soils, as well as soils heated under controlled laboratory conditions. This allowed us to characterize the changes in hydrophobicity that occur with increasing temperature and determine their causes and the soil temperature at which they occur. Previously burned and unburned samples of the superficial soil horizon from a pine afforestation in the Calamuchita Valley were collected. Unburned samples were heated under laboratory conditions between 175 °C and 270 °C to simulate various temperatures that occur during a fire event. The samples were analyzed for hydrophobicity by the Water Drop Penetration Time (WDPT) test. Granulometry (Robinson pipette method), color (Munsell notation), pH (potentiometry) and organic matter (total, young and old fractions by the Walkley-Black method) were also measured.

The results indicate a decrease of water repellency both in previously burned samples (WDPT values between 5-60 s) and heated samples (WDPT < 5 s at 220 °C) when compared to unheated samples (WDPT > 3600 s). Additionally, the proportion of old to young organic matter, the ratio of clay to sand, and the pH tend to increase with heating and fire temperature. The darkening of samples with temperature is evidenced by lower value and chroma.

The decrease in water repellency is mainly associated with the breakdown of the original organic matter into ashes and organic substances of hydrophilic nature. Another factor that diminishes the hydrophobic nature of the soil is the higher soil specific surface linked to the decrease in granulometry. Additionally, the increase of the soil pH would foster fungal activity, a source of hydrophilic compounds. By comparing changes in soil properties between heated and burned samples, it is inferred that these changes took place at a temperature in the range 190 °C – 270 °C. The hydrophilic condition of the burned soil would have protected it against erosion, at least in the outermost layer of the superficial horizon.