Mechanism of Fire-Induced Water Repellency in Soil

Organic soil litter collected under Manzanita (Arctostaphylos sp.) was burned over a sand column. Ottawa sand was used in the column. Temperatures were monitored by means of thermocouples in the litter layer, at the sand surface, and 1 cm deep in the sand. Two basic types of experiments were conducted. In one type the burned litter layer was allowed to remain on the sand until cool. In the other type, the burned litter layer was removed immediately after the burning period. This second type of experiment was conducted to prevent heat moving into the sand from the litter after burning. The results showed that the maximum temperatures reached in the sand, and the water repellency produced in the sand, were lower in the experiment where the burned litter layer was removed. The substances causing the water repellency in the 0 to 1-cm depth were also more subject to extraction in the experiment where the litter layer was removed. The organic carbon content of the sand was quite similar in both experiments. When a recently ground quartz sand was used in the sand column, the substances causing water repellency were extractable. The fractionation of the extracted substances showed that the polarity of the molecules of the substances causing water repellency decreased with depth in the sand. Heating studies on three individual fractions indicated differences in volatility among the fractions. The chemistry of the substances in two fractions was altered by heating 20 min at 300C. The primary conclusions of the study were that the primary movement of organic substances from the burning litter layer into the underlying sand occurs during the fire. After the burning, heat moves down through the underlying sand “fixing” some of the more polar hydrophobic substances and revolatilizing the less polar substances, thereby broadening the water-repellent layer. The temperature required for the “fixing” and the revolatilization appears to be greater than 250C.