Effects of fire temperature on the physical and chemical characteristics of the ash from two plots of cork oak (Quercus suber)

Cork oak, (Quercus suber) is widely distributed in the Mediterranean region, an area subject to frequent fires. The ash produced by burning can have impacts on the soil status and water resources that can differ according to the temperature reached during fire and the characteristics of the litter, defined as the dead organic matter accumulated on the soil surface prior to the fire. The aim of this work is to determine the physical and chemical characteristics of ash produced in laboratory experiments to approximate conditions typical of fires in this region. The litter of Quercus suber collected from two different plots on the Iberian Peninsula, Mas Bassets (Catalonia) and Albufeira (Portugal), was combusted at different temperatures for 2 h. We measured Mass Loss (ML per cent), ash colour and CaCO₃ content, pH, Electrical Conductivity (EC) and the major cations (Ca²⁺, Mg²⁺, K⁺ and Na⁺) released from ash slurries created by mixing ash with deionized water. The results showed that ML per cent is higher at all temperatures in Albufeira samples compared to Mas Bassets samples, except at 550°C, and the rate of loss increases faster with temperature than the Mas Bassets samples. At 150°C the ash colour is yellowish, becoming reddish at 200-250°C and black at 300°C. Above 400°C the ash is grey/white. This thermal degradation is mostly observed in Albufeira litter. The formation of CaCO₃ was identified at a lower temperature in Albufeira litter. At temperatures < 300°C, pH and EC values are lower, rising at higher temperatures, especially in Albufeira slurries. The concentration of cations at lower temperatures does not differ substantially from the unburned sample except for Mg²⁺. The cation concentration increases at medium temperatures and decrease at higher temperatures, especially the concentration of divalent cations. The monovalent cations showed a larger concentration at moderate temperatures, mainly in Albufeira ash slurries. The analysis of the Ca:Mg ratio also showed that for the same temperature, a higher severity results for Albufeira litter. Potential negative effects on soil properties are observed at medium and higher temperatures. These negative effects include a higher percentage of mass loss, meaning more soil may be exposed to erosion, higher pH values and greater cation release from ash, especially monovalalent cations (K⁺, Na⁺) in higher proportions than the divalent ions (Ca²⁺, Mg²⁺), that can lead to impacts on soil physical properties like aggregate stability. Furthermore, the ions in ash may alter soil chemistry which may be detrimental to some plants thus altering the recovery of these ecosystems after fire. Low intensity prescribed fire can be a useful tool to land management in these sites, due to the reduced effects of fire temperatures on the physical and chemical properties of surface litter, and can reduce the risk of high temperature wildland fires by reducing fuel loadings. From the perspective of water resources, lower fire temperatures produce fewer impacts on the chemistry of overland flow and there is less probability that the soil surface will be eroded. Copyright © 2009 John Wiley & Sons, Ltd.