Forest fire effects on soil chemical and physicochemical properties, infiltration, runoff, and erosion in a semiarid Mediterranean region

Forest fires are a major environmental concern, especially in the semiarid Mediterranean regions, where the long dry and hot summers and mild winters favor outbreaks of wildfires. The objective of this work was to study the effects of different fire treatments on physical, chemical, and physicochemical properties of Pale rendzina, and their impact on infiltration rate (IR), runoff and soil loss under consecutive rainstorms. After a wildfire in a forest located in northern Israel, soil samples were taken from an area that was directly exposed to fire (direct fire treatment) and from adjacent unburned (unburned soil treatment). Part of the unburned soil was heated in a muffle at 300°C (heated soil treatment). Runoff, soil loss and IR values were measured for the various samples using a laboratory rainfall simulator, and aggregate stability was determined using slaking and dispersion values. The organic matter, clay, and sand content, and cation exchange capacity were significantly lower in the heated soil than in the unburned soil. The CaCO₃ content in the heated soil was significantly higher than in the unburned and direct fire soils. In general, the IR values were highest, intermediate, and lowest and the runoff and soil loss amounts were lowest, intermediate, and highest in the heated, direct fire, and unburned soils, respectively. However, these differences decreased with progression of the consecutive rainstorms. Heating the soil to 300°C enhanced soil-structure stability, most likely due to increased dehydration of 2:1 clay minerals and transformation of iron and aluminum oxides which acted as cementing agents. In addition, soil heating increased the electrical conductivity (EC) and decreased the sodium adsorption ratio in the heated soil solution in the first rainstorm. These processes limited clay dispersion and seal formation in the heated soil, leading to high IR values and low runoff and soil loss. In the second and third rainstorms, EC of the soil solution decreased, which in turn increased clay dispersion. This lessened the differences in the IR values and runoff and soil loss amounts between the fire treatments in these rainstorms compared to the first rainstorm.