Effects of Low Temperature Fire On Soil Aggregate Stability.

The use of prescribed burning to reduce fuel loads in forests, or to clear land and reduce weed populations in agricultural systems has increased in recent years. This study was motivated by field observations of marked increase in soil erodibility following low-temperature (<200 °C) burns, which have been previously regarded as harmless to soil structure. The study was designed to test our hypothesis that evaporation of soil pore water leads to high vapor pressure formation within soil aggregates. If this vapor pressure exceeds the tensile strength of the soil, the soil aggregates breakdown resulting in increased susceptibility to erosion. In this research, the interactions between heating temperature, soil water content and stability of soil aggregates were studied. Fire was simulated in the laboratory for soil aggregates with size range between 0.25-1, 1-2 and 2-4 mm from three different types of soils using a muffle furnace equipped with a timer and heating rate control under controlled laboratory conditions. Five heating temperatures were selected for this study, entailing maximum temperatures of 75, 100, 125, 150 and 175 °C. The experiments were done on soils with three water contents. The stability of the heated aggregates was measured by wet sieving to determine the resistance of soil structure against mechanical or physicochemical destructive forces. The total organic carbon content was determined based on Loss-On-Ignition method (LOI) by exposing the samples to 400^oC for 16 hours. A significant loss of aggregate stability was observed above the boiling point of water, without detectable loss of soil organic matter, confirming our hypothesis.