Plant Ash and Heat Intensity Effects on Chemicaland Physical Properties of Two Contrasting Soils

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.