Effects of wildfire and laboratory heating on soil aggregate stability of pine forests in Galicia: the role of lithology, soil organic matter content and water repellency.

Abstract The present work aims to assess the effects of wildfires on soil aggregate stability and the role therein of contrasting lithologies as well as of fire-induced changes in organic matter content and soil water repellency. To this end, a pair-wise comparison of neighbouring burned and unburned soils was carried out and complemented by laboratory heating experiments to clarify the role of fire intensity. In total, 18 pairs of adjacent burned and unburned pine forest soils were sampled within one month after wildfire. At each site, five samples were collected of the top 5 cm of the A horizon at randomly selected sample points and were mixed in the field to obtain one composite sample per site. Three additional samples were collected at each site but stored separately, and those of three sites were selected for the laboratory heating experiments. Laboratory heating involved five different temperatures ranging from 25 to 460 °C. Aggregate stability of the field and laboratory samples was determined using the water drop impact test, organic carbon content using a modified Sauerlandt method and soil water repellency using the ‘Molarity of an Ethanol Droplet’ test. The wildfire effects on field aggregate stability were highly variable and results indicated that these changes depend primarily on organic matter combustion and, thus, fire intensity. Controlled heating up to 220 °C either did not alter aggregate stability or increased it with increasing temperature, possibly due to the development of a protective coating of organic compounds inducing water repellency. Heating at 380 and 460 °C, by contrast, produced considerable to massive combustion of organic matter and, thereby, very pronounced reduction of aggregate stability as well as water repellency.