Fire passage is accompanied by a heat wave and ash deposition affecting the upper soil layer. Changes in soil properties are directly related to heat intensity, the amounts of ashes deposited, and soil type. We subjected two soils (gypsiferous soil and calcareous soil) to artificial heating and ash incorporation and compared changes in select chemical and physical properties. The two soils studied were selected to provide a wide range of characteristics in soils of the semiarid Ebro Valley (NE Spain). Samples of both soils were heated for 30 minutes in a muffle furnace at temperatures of 25°, 150°, 250° and 500°C. Ashes were added only on soil samples heated at 250°C, in a quantity related to plant biomass growing on each soil (twice the amount in calcareous soil than in gypsiferous soil). Increasing heat intensity increased organic matter combustion as well as nutrient availability. 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. Total N content decreased at temperatures greater than 250°C, with about one-third being volatilized. Changes in chemical properties were similar for both soils although quantitative differences between soils were found. Cation exchange capacity (CEC) was reduced for gypsiferous soil heated to 500°C and to 250°C for calcareous soil. Heating increased sand-sized particles by fusion of clay, greatest 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. A negative correlation was observed among SAS and soil erodibility (K-USLE). Bulk density and particle density increased in both soils when heated to 500°C. Water availability increased when soils were heated to 500°C, likely due to texture and structural modifications. Addition of the ashes increased organic matter content, C/N ratio, and pH in both soils and increased nutrient availability. These responses were greater in calcareous than in gypsiferous soil. Physical soil properties were not significantly modified by ash addition.

The effect of heating and ash deposition on microbiological properties was studied in two arid soils in an incubation experiment, simulating the effects of bushfire. Top soil (0-15 cm) was heated to 150°C, 250°C and 500°C for 30 minutes; unheated soil was taken as a control (25°C). Samples of the soil heated to 250°C were mixed with black ash (1%, w/w) for analysis of their effect on microbial population and activity. Soils were incubated for nine months and their basal respiration was assessed three times a month. Normalized and specific respiration, biomass-C, bacteria, and fungi numbers were analyzed one month after heating and in the final month of incubation to assess short-term and long-term effects of fire on soil microbiology. At the start of the incubation period and for intermediate heating treatments (150°C and 250°C), basal and specific soil respiration and biomass-C were enhanced in calcareous soil but depleted in gypsiferous soil. At the highest temperature (500°C), these biological properties, as well as specific respiration and colony forming units, were significantly reduced in both soil types and for each period of sampling. At the end of the incubation period, all biological properties maintained the differences between treatments but with lower values than at the beginning of the incubation. 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 on C dynamics as a function of soil properties.