|Abstract or Summary
- The hydrophobicity of soils of the Deschutes National Forest was
studied. The soils are Cindery Typic Cryorthents, formed in cinders
and ash from Mt. Mazama. Ponderosa pine is the dominant overstory
vegetation. Of particular interest was the effect of prescribed
burning on hydrophobicity. Fire has been shown to cause a normally
hydrophilic soil to become hydrophobic. This non-wettability reduces
water infiltration into the soil. As a result, the potential for
erosion increases and less water is available for plant growth.
The objectives of the study were to determine (1) whether or
not prescribed burning causes the formation of a water repellent
layer, (2) which variables affect the hydrophobicity of the soil
following burning, (3) the horizontal and vertical extent of the
hydrophobic layer, and (4) how long the hydrophobicity persists in
Critical Surface Tension (CST) was measured to characterize
hydrophobicity. A site burned 25 June 1982 and a site burned 15
September 1982 were sampled to meet objectives (1), (2), and (3).
Objective (4) was met by sampling six additional sites where the
time since burning ranged from 9 to 51 months.
The presence of pre-burn hydrophobicity, believed to be caused
by fungal products, complicated determining the effects of burning
on the hydrophobicity of the soil. Pre-burn hydrophobicity was more
extensive on the site which was sampled in September than the site
sampled in June. Ninety-six % of the sampling points were hydrophobic during September and 42% during June. Two possible
reasons were postulated for this difference. First, the amount of
hydrophobicity due to the presence of fungal hyphae may vary
seasonally; fungal products may accumulate during summer and then
leach out of the profile with fall rains and spring snowmelt.
Second, avoiding fungal pockets may not have been as successful when
September sampling occurred as in June. Soil infected with fungal
hyphae was avoided when CST was measured, because the fungal pockets
did not form a continuous layer parallel to the surface. Fungal
pockets were avoided by observing the light color of the dry fungal
soil and the presence of hyphae. The soil had a light color because
the water content was low. The soil was drier in September than in
June. Distinguishing between fungal and non-fungal soil based on
color differences was relatively easy in June, because the
non-fungal soil was moist. However, the color difference between
fungal and non-fungal soil was not as distinct during September
sampling. The difference in color due to water content between
fungal and non-fungal soil was small. As a result, the effort to
avoid fungal caused water repellent areas was not as successful.
More of the sampling points were hydrophobic in September.
The June burn caused an increase in the hydrophobicity of the
soil. The increase was greatest at the 2-3 cm depth. The
hydrophobicity of the soil following burning in the June burn was
explained by the degree of litter combustion. Hydrophobicity was
produced where complete combustion occurred but not with incomplete
combustion of the litter.
Pre-burn hydrophobicity of the soil sampled in June occurred
more often in the upper 2 cm than at the lower depths. Pre-burn
hydrophobicity occurred at 42.5% of the sampling points. Post-burn
hydrophobicity occurred randomly at all depths and occurred at 60.5%
of the sampling points.
On the site burned in September, most sampling points were hydrophobic before burning because of the presence of fungal products.
Hydrophobicity decreased in the upper 2 cm of the soil. It was
postulated that the hydrophobic fungal products were volatilized by the high temperatures of the prescribed burn and diffused deeper
into the soil where they then condensed.
The hydrophobicity of the soil following burning in the
September burn was correlated with hydrophobicity of the soil be
fore burning. Soil was found to be hydrophobic after burning if it
was hydrophobic before burning. Measurements of litter depth, water
content, and degree of combustion did not explain the variation in
post-burn hydrophobicity of the soil at either site.
Pre-burn hydrophobicity of the soil sampled in September
occurred more near the surface than deeper in the soil. Pre-burn
hydrophobicity was found at 96% of the sampling points. Post-burn
hydrophobicity was not quite as extensive; 92% of the sampling
points were hydrophobic. Post-burn hydrophobicity occurred deeper
in the soil than pre-burn hydrophobicity in September, but the
difference between depths was not significant.
The percentage of hydrophobic sampling points decreased as time
since burning increased. The relationship was significant at the
95% confidence level.