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

iomass. Similar declines in the activities of carbon-, nitrogen-, and phosphorus-acquiring
enzymes were also observed. Such dramatic effects have only been observed in highly
contaminated ecosystems. In a less contaminated grassland site near a Pb factory in Germany,
Chander et al. (2001) observed a lower ratio of microbial biomass carbon to soil organic carbon
in polluted soils. The ratio of basal respiration to microbial biomass (the “metabolic quotient,”
qCO2) declined with increasing metal concentration, though this observation depended on the
procedure for measuring microbial biomass (substrate-induced respiration versus fumigationextraction).
The combined effect of lower microbial biomass per unit soil carbon and similar or
lower qCO2 on polluted sites indicates that the ability of soil microorganisms to process carbon
inputs is compromised by metal pollution.
The type of ecosystem also plays a role in determining the effects of Pb and other metals
on the microbial processing of litter. Forest soils in temperate zones accumulate organic matter
at the soil surface to a greater degree than in grasslands. This organic-rich O horizon can support
a large microbial biomass; but it is also an effective trap for Pb inputs, because of the association
between Pb and soil organic matter. At highly contaminated forest sites, microbial biomass and
enzyme activities may be depressed (Fritze et al., 1989; Bååth et al., 1991), causing slower
decomposition of the litter.
In addition to effects on decomposition and carbon transformations, Pb and other trace
metals can also influence key nitrogen cycling processes. Studies in the 1970s demonstrated that
Pb and other metals inhibit the mineralization of nitrogen from soil organic matter and
nitrification (Liang and Tabatabai, 1977, 1978), resulting in lower nitrogen availability to plants.
More recent research has documented significant inhibitory effects of Pb and other metals on the
activities of several enzymes believed to be crucial to nitrogen mineralization in soils (Senwo
and Tabatabai, 1999; Acosta-Martinez and Tabatabai, 2000; Ekenler and Tabatabai, 2002). This
suggests that the inhibitory effect of Pb and other metals is broad-based, and not specific to any
particular metabolic pathway. In reducing environments, the rate of denitrification is also
depressed by trace metals. Fu and Tabatabai (1989) found that 2.5 µmol g !1 of Pb (ca.
500 mg/kg !1 ) was sufficient to cause 0, 27, and 52% decreases in nitrogen reductase activity in
three different soils.
Metal pollution can also affect soil invertebrate populations. Martin and Bullock (1994)
observed lower abundances of a variety of woodlice, millipedes, spiders, insects, and earthworms
AX7-101