**4.2 Results and discussion**

Similar changes in chemical properties were found for both soils at highest temperatures but differences between soils were observed at intermediate temperatures (Table 2).

Wildfire passage is accompanied by a heat wave and a deposition of ash as a result of biomass combustion (Fig. 6). Fire effects are directly related to the soil type affected as well as fire severity, i.e. the heat reached and the quantity and quality of ashes deposited on soil surface. To test these variables we treated two contrasting soils under laboratory conditions, the most frequently found soils in the semi-arid Central Ebro Valley (NE-Spain), a Calcaric Regosol developed on marls and a Haplic Gipsisol, developed on

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)

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

Similar changes in chemical properties were found for both soils at highest temperatures but

differences between soils were observed at intermediate temperatures (Table 2).

**4. Fire effects on soil properties under laboratory–controlled heatings** 

gypsiferous marls.

Valdejasa (Zaragoza).

**4.2 Results and discussion** 

that added to gypsiferous soil (5 g kg-1).

**4.1 Methods** 


Table 2. Changes induced by simulated fire at different temperatures and ash addition (A) in the physical, chemical and biological properties of gypsiferous (G) and calcareous (C) soils in the semi-arid Central Ebro Basin. In each row, treatment data with the same letters are not significantly different (LSD test, P>0.05). Basal respiration (mg CO2 soil kg-1 day-1)

Among the results it should be noted that heating soil to 250°C caused a decrease in pH and an increase in electrolytic conductivity (ECe) and soluble Ca. Heating soil to 500°C caused an increase in pH and a decrease in ECe and soluble Ca. Increasing heat intensity increased organic matter loss by combustion, which was accompanied by an increase of available nutrients content. Total N content decreased at temperatures greater than 250°C, with about one-third being volatilized. Cation exchange capacity (CEC) was reduced for gypsiferous soil heated to 500°C and to 250°C for calcareous soil. As for physical soil properties, heating increased the quantity of sand-sized particles by fusion of clay with

Soil Erosion and Conservations Measures in Semiarid Ecosystems Affected by Wildfires 97

**Hypercalcic Calcisol** 

Calcareous coluvium

(Pleistocene)

Evergreen oakwood Meadow

**Haplic, Eutric Cambisol** 

Coluvium (Pleistocene)

**Phaeozem** 

with moss

(Miocene)

Table 3. Some characteristics of the studied soils and their ecosystems.

Soil moisture regime Xeric-aridic Xeric Udic Soil temperature regime Thermic Mesic Mesic

Organic matter in A (%) 9.8 % 4.1 % 7.0 % Textural class in A (%) Clay Loam Loam Loam

UTM 30T (X-Y) 671-464 708-467 733-472 Altitude (m) 630 581 1218

Fig. 8. Soil blocks were burned with a blowlamp reaching 250ºC at 1 cm soil depth.

Location: municipality Zuera Arascués Linás de Broto

Non-altered blocks of soil have been obtained in the field in every area. All treatments have been carried out three times. Once in the laboratory, all the soil blocks have been air-dried and they have been burnt with a blowlamp (Llovet et al., 2008) up to reaching a maximum

**Soil Unit (FAO) Rendzic** 

of 250ºC at 1 cm depth (Fig. 8).

Plant community Aleppo pinewood

Soil parent material Marls & limestones

the greatest increase occurring in soil heated to 500°C. Soil aggregate stability (SAS) of both soils was reduced by heating to 250°C with greater reductions at 500°C, likely due to a reduction in organic matter and clay size particle content. Bulk density and particle density increased in both soils when heated to 500°C. Water availability (difference between field capacity and permanent wilting point) increased when soils were heated to the highest temperature, likely due to texture and structural modifications. The addition of ashes increased organic matter content, C/N ratio, and pH in both soils and increased nutrient availability, especially in the calcareous soil where soil addition was higher than in gypsiferous one.

Soil respiration were quickly enhanced in calcareous soil but depleted in gypsiferous soil for intermediate heating treatments (150°C and 250°C). At the highest temperature (500°C), these biological properties were significantly reduced in both soil types and on a long term basis. Black ash addition increased basal respiration in both soils but did not affect other biological properties. These results demonstrate the existence of both labile and permanent effects of soil burning and a differential response to C dynamics as a function of soil properties (Badía & Martí, 2003b).

The unburned or control soils (25ºC) have originally contrasting properties (C versus G soil) that are not strongly altered at intermediate temperatures (150ºC, 250ºC). But the highest heat treatment (500ºC) conducted on both soils (C500 vs G500) showed a lot of similarities as result of the degradation of their initial properties (Fig. 7).

Fig. 7. Dendrogram of experimental calcareous (C) and gypsiferous (G) soils at different temperatures (25ºC, 150ºC, 250ºC, 500ºC) and ash addition (A). Numbers 0 to 25 shows the rescaled distance cluster combine, and 1 to 10 are the case numbers.
