The Transfer of Heat and Hydrophobic Substances During Burning

TLDR

Hydrophobic substance translocation and resulting water repellency depend on changes in polarity and oxidation, and dry‑soil fire models may not capture movement in moist soils. The experiment burned pine litter for 5 or 25 minutes over wet or dry sand to evaluate the resulting water‑repellent layers. A 5‑minute burn on dry sand produced the strongest repellency, whereas 25‑minute burns moved organics deeper but weakened surface repellency, and in wet sand repellency was limited to the top 1.5 cm, suggesting prescribed burns should be performed on moist soil to reduce water‑repellency problems.

Abstract

Abstract Wet and dry sand was tested for water repellency, after burning pine litter ( Pinus coulteri D. Don). Four conditions were studied: a burn of 25 min over dry or wet sand and a burn at 5 min over dry or wet sand. The thickest and most intense water‐repellent layer was produced by a 5‐min burn over dry sand. Although organic materials were translocated deeper in the dry sand during the 25‐min burn (down to 4 cm), some of the water repellency was destroyed in the upper 1‐cm layer. In wet sand, water repellency was concentrated in the upper 0‐ to 1.5‐cm layer. The translocation of hydrophobic substances and resulting water repellency depends on changes in their polarity and oxidation state. Relationships developed for fire over a dry soil may not adequately account for movement of organic substances in a moist soil. These results suggest prescribed burning should be done when the soil is moist on areas where water repellency is a problem.