**4.1 Methods**

Samples of both soils (calcareous and gypsiferous soil) were collected randomly and analyzed separately with 4 replicates for each soil type. Soil samples were heated for 30 minutes in a muffle furnace at temperatures of 25°, 150°, 250° and 500°C. These temperatures cover the range that is habitually reached in surface soils affected by fires (Chandler et al., 1983; Giovannini & Lucchesi, 1997; Walker et al., 1986). Soil sample treated at 250ºC was selected for adding black ashes, in a quantity related to plant biomass growing on each soil (Martí, 1998). The amount of ashes added to calcareous soil was twice (10 g kg-1) that added to gypsiferous soil (5 g kg-1).

Fig. 6. Appearance of a recently burned soil surface in Mountains of Zuera and Castejón de Valdejasa (Zaragoza).
