Air Quality Criteria for Lead Volume II of II - (NEPIS)(EPA) - US ...

and, therefore, increased water flux to the soil surface. Also, until a new canopy closes, the soil
surface is exposed to increased solar radiation and higher temperatures. Together, the higher
moisture and temperature in surface soils tend to increase the rate of organic matter
decomposition. Several studies have estimated decreases of up to 40% in the organic matter
content of forest floor soils after clear-cutting (Covington, 1981; Federer, 1984; Johnson et al.,
1995b). This loss of organic matter from the forest floor could result in the mobilization of
organically complexed Pb. However, observations from clear-cut sites in the United States and
Europe indicate that forest harvesting causes little or no mobilization of Pb from forest soils.
At the Hubbard Brook Experimental Forest in New Hampshire, whole-tree harvesting, the
most intensive form of clear-cutting, resulted in very small increases in Pb concentrations in soil
solutions draining the Oa soil horizon despite substantial reductions in the organic matter mass of
that horizon (Fuller et al., 1988; Johnson et al., 1995b). These increases were associated with
similarly small increases in dissolved organic carbon (DOC) concentrations in the Oa horizon
soil water. Output of Pb from the watershed stream was unaffected by clear-cutting. Similarly,
Berthelsen and Steinnes (1995) observed small decreases in the Pb content of the Oa horizon
(“H” in the European system of soil classification) in clear-cut sites in Norway, compared to
uncut reference sites. This mobilization of Pb from the Oa horizon was accompanied by an
increase in the Pb content of the upper mineral soil horizons. The Pb decline in the Oa horizon
was accompanied by a decrease in the organic matter content, leading the authors to attribute the
Pb dynamics to leaching with DOM. In a study conducted in Wales, Durand et al. (1994)
observed lower Pb outputs from a stream draining a clear-cut watershed than from where the
stream drained the upper reaches of the watershed, which were uncut. The DOC and H + outputs
were also lower in the clear-cut area. These patterns persisted in all 5 years of the study.
Forest harvesting is a severe form of ecosystem disturbance, and, thus, it is somewhat
surprising that studies of clear-cutting have shown little or no effect on Pb mobility or loss from
forest ecosystems. Perhaps the strong complexation behavior of Pb with natural organic matter
results in the retention of Pb in forest soils. Even in cases where Pb is mobilized in forest floor
soils (Fuller et al., 1988; Berthelsen and Steinnes, 1995), there is no evidence of loss of Pb from
the ecosystem, indicating that mineral soils are efficient in capturing and retaining any Pb that is
mobilized in the forest floor. Therefore, the principal risk associated with forest harvesting is the
loss of Pb in particulate form to drainage waters through erosion. In some relatively remote
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