MODIFICATIONS INDUCED IN SOIL PHYSICO-CHEMICAL PARAMETERS BY EXPERIMENTAL FIRES AT DIFFERENT INTENSITIES

Abstract

Field studies on the effects of heating soil are rare. Studies in which the temperature was varied and measured are even rarer. To cover this lack of knowledge, we conducted some experimental fires in which the fuel was controlled to obtain different fire intensities. Induced temperatures were measured by k-type thermocouples placed at various soil depths. These induced temperatures were highly variable at the soil surface, whereas below 2.5-cm depth, the temperature rise was negligible. Modifications of various soil physico-chemical parameters caused by fire were related to increasing temperatures at the soil surface. The passage of fire promoted the aggregation of clay particles into sand-sized particles, which become even more resistant to the disrupting action of water. The soil pH decreased at temperatures up to 395°C, followed by a clear increase at higher temperatures; however, both the cation exchange capacity and the organic matter content decreased continuously with increasing soil temperature. Total nitrogen content remained surprisingly constant as the soil surface temperature increased, whereas ammonium nitrogen peaked at 184°C. The organic phosphorus decreased continuously, and the available fractions increased. These results are in agreement with the results of our previous research on the effects of the artificial heating on soil properties and confirm that the processes occurring in soil are related to the temperature reached during the fire. The ashes, at least immediately after fire, do not affect these processes.