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

In ecosystems where Pb concentrations in soil solutions are low, toxicity levels for
vegetation are not likely to be reached regardless of the soil Pb concentration. Furthermore,
mycorrhizal infection of tree roots appears to reduce the translocation of Pb from roots to shoots
(Marschner et al., 1996; Jentschke et al., 1998). In a study of mycorrhizal and non-mycorrhizal
Norway spruce (Picea abies (L.) Karst.), mycorrhizal infection of roots was not affected by Pb
dose. Some, but not all, species of mycorrhizae showed reductions in the amount of
extrametrical mycelium with Pb exposure but only at solution concentrations of 5 µM, a level at
least 50 times greater than typical concentrations in forest soils. In a related study, the growth
rate of mycorrhizal fungi was unaffected at solution Pb concentrations of 1 and 10 µM, but
decreased at 500 µM (Marschner et al., 1999).
Low soil solution Pb concentrations and the influence of mycorrhizal symbionts also
result in low uptake of Pb by terrestrial vegetation. The net flux of Pb into vegetation in the
northern hardwood forest at Hubbard Brook in the 1980s was estimated as only 1 g ha !1 year !1
(Johnson et al., 1995a), representing 3% of the precipitation input. Klaminder et al. (2005) also
measured a Pb uptake of 1 g ha !1 year !1 in a spruce-pine forest in northern Sweden. Despite plant
uptake fluxes being very low, they are sensitive to differences and changes in Pb deposition.
Berthelsen et al. (1995) observed decreases in the Pb content of stem, twig, leaf, and needle
tissues of a variety of tree species in Norway between 1982 and 1992, when atmospheric Pb
deposition declined by approximately 70%. They also observed significantly lower Pb
concentrations in tree tissues collected in northern Norway versus southern Norway, where
atmospheric Pb deposition is greater.
Even at subtoxic concentrations, Pb and other metals may influence species diversity in
terrestrial ecosystems. However, little work has been done on the effect of low-level metal
concentrations on species diversity. In one study, plant species diversity was positively
correlated to the concentration of available Pb in natural and artificial urban meadows in Britain
(McCrea et al., 2004). The authors hypothesized that Pb may inhibit phosphorous uptake by
dominant species, allowing less abundant (but more Pb-tolerant) ones to succeed.
AX7.1.4.3 Effects of Lead on Energy Flows and Biogeochemical Cycling
In terrestrial ecosystems, energy flow is closely linked to the carbon cycle. The principal
input of energy to terrestrial ecosystems is through photosynthesis, in which CO2 is converted to
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