Agriculture Reference
In-Depth Information
As shown in Figure 1 BA and BD soils are extremely contaminated with Pb and As,
imposing a enormous degree of stress in these soils.
Table 1. Physicochemical analysis of the soils from different locations. Data are the
mean
±
SE from three replicate measurements. Different superscripts letters (a-c)
represent significant differences (P < 0.05) between values at same column
Soil
Physicochemical analysis
Water content (%)
pH
Redox potential ( Eh) Organic matter (%)
16.4 ± 0.26
a
4.50 ± 0.08
a
127.7 ± 40.38
a
9.35 ± 0.35
a
BC
7.8 ± 0.18
b
4.15 ± 0.13
b
123.3 ± 4.04
a
4.72 ± 0.53
b
BA
24.1 ± 1.20
c
5.21 ± 0.005
c
121.7 ± 2.89
a
10.81 ± 1.06
a
BD
Several authors reported that long-term metal deposition into soils results in high metal
concentrations, which affects negatively soil microflora (Smith and Giller, 1992; Matsuda et
al., 2002). In this work, we were able to isolate rhizobia from all soils, indicating that these
bacteria were capable to survive under the metal concentrations determined. According to
Ibekwe et al. (1997) survival can be related to the physical protection of clay minerals and
organic matter or to the existence of microsites where metal contamination may be minimal.
These “niches” may harbour rhizobia that are not resistant to heavy metals and may explain
the presence of sensitive genotypes in contaminated soils.
Gross measurements of microbial diversity have been used to assess environmental stress
(Atlas, 1984), but such studies are hampered by problems of sampling, extraction and
culturing leading to bias towards certain groups within microbial communities. Pollution may
lead to a decrease in microbial diversity due to the extinction of species which lack sufficient
tolerance to the stress imposed, and enhanced population of other species which thrive under
stress (Atlas, 1984). Microorganisms within species of the same genus or within strains of the
same species can differ in their sensitivity to metals. Giller et al. (1993) demonstrated that
Sinorhizobium meliloti
was less sensitive, in terms of growth, to Cd than
R. leguminosarum
and
R. loti.
Therefore, the use of microbial populations which are easily culturable,
ubiquitous, sensitive to several contaminants, and whose intraspecific diversity is enough to
point out tolerance differences to contaminated soils, can be a way to overcome the problems
enumerated above.
Ubiquitous species, such as rhizobia, allow a wider comparison of heavy metal effects
between different environments.
Rhizobium
can probably be used as an effective tool in
ecotoxicity assays since these bacteria are known to be sensitive to a wide variety of
pollutants and are very important to soil fertility in a wide range of environments (Figueira,
2000; Figueira et al., 2005; Pereira et al., 2006; Vig et al., 2003).
In order to evaluate the influence of heavy metal contamination by a lead abandoned
mine in the soil microflora, forty-one isolates of
Rhizobium leguminosarum
bv.
trifolii
were
obtained from nodules of white clover (
Trifolium repens
, L.) plants grown in BA and BD
contaminated soils and also in control soil (BC).
