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

iomass carbon. Because of this link between photosynthesis and energy flow, any effect that Pb
has on the structure and function of terrestrial ecosystems (as discussed in Section AX7.1.5.3.)
influences the flow of energy into the ecosystem. This section focuses on how Pb influences
energy transfer within terrestrial ecosystems, which begin with the decomposition of litter and
other detrital material by soil bacteria and fungi, and cascade through the various components of
the detrital food web. Because the mobility of Pb in soils is closely tied to organic matter
cycling, decomposition processes are central to the biogeochemical cycle of Pb. This section
concludes with a discussion of how biogeochemical cycling of Pb has changed in response to the
changing Pb inputs to terrestrial ecosystems.
Effects of Lead on Detrital Energy Flows
Lead can have a significant effect on energy flows in terrestrial ecosystems. At some sites
severely affected by metal pollution, death of vegetation can occur, dramatically reducing the
input of carbon to the ecosystem (Jordan, 1975; Galbraith et al., 1995). Subsequently, wind and
erosion may remove litter and humus, leaving bare mineral soil, a nearly sterile environment in
which very little energy transfer can take place (Little and Martin, 1972; Galbraith et al., 1995).
At Pb-affected sites that can retain a functioning forest stand, the rate of decomposition of
litter may be reduced, resulting in greater accumulation of litter on the forest floor than in
unpolluted stands. Numerous investigators have documented significant declines in litter
decomposition rates (Cotrufo et al., 1995; Johnson and Hale, 2004) and/or the rate of carbon
respiration (Laskowski et al., 1994; Cotrufo et al., 1995; Saviozzi et al., 1997; Niklínska et al.,
1998; Palmborg et al., 1998; Aka and Darici, 2004) in acid- and metal-contaminated soils or soils
treated with Pb. The resulting accumulation of organic matter on the soil surface can be
dramatic. For example, an oak woodland 3 km from a smelter in Bristol, England had a litter
layer mass 10 times greater than the mass in a similar stand 23 km from the smelter (Martin and
Bullock, 1994).
Reduced decomposition rates in polluted ecosystems are the result of the inhibition of soil
bacteria and fungi and its effects on microbial community structure (Bååth, 1989). Kuperman
and Carreiro (1997) observed 60% lower substrate-induced respiration in heavily polluted
grassland soils near the U.S. Army’s Aberdeen Proving Ground in Maryland. This decline in
carbon respiration was associated with 81% lower bacterial biomass and 93% lower fungal
AX7-100