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

Land Use and Industry
Changes in land use also represent potentially significant changes in the cycling of
organic matter in terrestrial ecosystems. Conversion of pasture and croplands to woodlands
changes the nature and quantity of organic matter inputs to the soil. In temperate climates, forest
ecosystems tend to accumulate organic matter in an O horizon on the forest floor, whereas
organic matter in grasslands and agricultural fields is concentrated in an A horizon at the soil
surface. Andersen et al. (2002) compared the trace metal concentrations in arable fields in
Denmark to nearby sites that had been converted to forest land. After 34 years of afforestation,
the soils showed no significant difference in Pb concentration or fractionation, despite significant
acidification of the soils. Afforestation had no effect on the soil carbon concentration,
suggesting that land use change may have little effect on Pb cycling unless soil carbon pools are
affected.
Similarly, the introduction of industrial activity may have consequences for organic
matter cycling, and subsequently, Pb mobilization. In a rare long-term study of polluted soils,
Egli et al. (1999) studied the changes in trace metal concentrations in forest soils at a site in
western Switzerland between 1969 and 1993. The site is 3 to 6 km downwind from an aluminum
industrial plant that operated between the 1950s and 1991. In the 24-year period of study, the
site experienced significant declines in organic carbon in surface (0 to 5 cm depth) and
subsurface (30 to 35 cm) soils. In the 30 to 35 cm layer, the organic carbon concentration
declined by more than 75%. Extractable Pb (using an ammonium acetate and EDTA mixture)
declined by 35% in the same layer. The authors suggested that the Pb lost from the soil had been
organically bound. While this study indicates that loss of soil carbon can induce the mobilization
and loss of Pb from terrestrial ecosystems, it is also worth noting that the decline in soil Pb was
considerably smaller than the decline in organic carbon. This suggests that Pb mobilized during
organic matter decomposition can resorb to remaining organic matter or perhaps to alternate
binding sites (e.g., Fe and Mn oxides).
The effects of industries that emit Pb to the atmosphere are discussed in Sections
AX7.1.5.2 and AX7.1.5.3 below.
Forest harvesting represents a severe disruption of the organic matter cycle in forest
ecosystems. Litter inputs are severely reduced for several years after cutting (e.g., Hughes and
Fahey, 1994). The removal of the forest canopy results in reduced interception of precipitation,
AX7-92