References

Microorganisms in Toxic Metal-Polluted Soils 329
Cd, from long-term inputs of contaminated sewage sludge may contain less
biomass and altered microbial functionality. A general order of inhibition
was found to be Zn >Cu >Cd for single metals while for metal combinations,
it was (Cd + Cu + Zn) > [(Cd + Zn) and (Cu + Zn)] > (Cd + Cu).
In both cases, grassland >arable (Renella et al. 2002). However, shortterm
incubations are a poor model of changes in microbial biomass or
activity due to chronic metal exposure (Renella et al. 2002). In general,
experimentation based around metal additions to soils is unpredictable
and may have little relation to field results (Giller et al. 1998). Undoubtedly,
metal effects on natural soil communities are complex and difficult
to characterize because of the complex array of contributing factors. Most
knowledge about toxic metal effects on soil microorganisms is derived
from data relating to only a few toxic metals, e. g. Cu and Zn, or from
studies on sewage sludge applications that contain metal mixtures at relatively
low concentrations (Shi et al. 2002). Many contaminated sites contain
mixtures of metals as well as organic pollutants: each may have reciprocal
physical and chemical effects on the other with complexation and other
phenomena affecting toxicity, bioavailability and degradation for example.
Toxic metals are often associated with organic pollutants in polluted
sites. Such mixtures may provide special problems for a functional soil
community. In one study, it was found that the soil microbial community
was predominantly affected by hydrocarbons rather than associated Pb
and Cr contamination (Shi et al. 2002). Therefore, although some gross
generalizations are possible regarding toxic metal influence on microbial
communities, individual cases are likely to be site-specific and extremely
complex.
Numerous studies have shown that microbial population responses to
toxic metals are characterized by a population shift from bacteria, including
streptomycetes, to fungi (Mineev et al. 1999; Chander et al. 2001a, b;
Kostov and van Cleemput 2001; Olayinka and Babalola 2001; Khan and
Scullion 2002). However, other studies have shown a higher metal sensitivityofthefungalcomponentofthemicrobialbiomass(Pennanenet
al. 1996). The time needed for development of community tolerance may
be highly variable, some studies detecting increased tolerance of bacterial
communities after a few days of metal exposure (Diaz-Ravina and Baath
1996), other studies demonstrating several years (Doelman and Hanstra
1979). An increase in metal tolerance of a bacterial community after metal
addition may be attributed to the immediate death of sensitive species
followed by differing competitive abilities and adaptation of bacterial survivors
(Diaz-Ravina and Baath 1996). For soil denitrifying communities, an
immediate metal effect (1 day) was a reduction in the denitrification rate,
but also a decrease in N2O reduction more than the N2O productionrate,
i.e. nitrate reductase activity was a sensitive metal target. Metal exposure