Abstract
Wildfires are a cause of soil water repellency (hydrophobicity), which
reduces infiltration while increasing erosion and flooding from
post-fire rainfall. Post-fire soil water repellency degrades over time,
often in response to repeated wetting and drying of the soil. However,
in mountainous fire-prone forests such as those in the Western USA, the
fire season often terminates in a cold and wet winter, during which
soils not only wet and dry, but also freeze and thaw. Little is know
about the effect of repeated freezing and thawing of soil on the
breakdown of post-fire hydrophobicity. This study characterized the
changes in hydrophobicity of Sierra Nevada mountain soils exposed to
different combinations of wet-dry and freeze-thaw cycling. Following
each cycle, hydrophobicity was measured using the Molarity of Ethanol
test. Hydrophobicity declined similarly across all experiments that
included a wetting cycle. Repeated freezing and thawing of dry soil did
not degrade soil water repellency. Total soil organic matter content was
not different between soils of contrasting hydrophobicity. Macroscopic
changes such as fissures and cracks were observed to form as soil
hydrophobicity decayed. Microscopic changes revealed by scanning
electron microscope imagery suggest different levels of soil aggregation
occurred in samples with distinct hydrophobicities, although the size of
aggregates was not clearly correlated to the change in water repellency
due to wet-dry and freeze-thaw cycling. A nine year climate and soil
moisture record from Providence Critical Zone Observatory was combined
with the laboratory results to estimate that hydrophobicity would
persist an average of 144 days post-fire at this well-characterized,
typical mid-elevation Sierra Nevada site. Most of the breakdown in soil
water repellency (79%) under these climate conditions would be
attributable to freeze-thaw cycling, underscoring the importance of this
process in soil recovery from fire in the Sierra Nevada.