Agriculture Reference
In-Depth Information
For this reason, one of the objectives of this chapter was to evaluate the impact of heavy
metals on
Rhizobium
populations isolated from a real world pollution site in an abandoned
mine deactivated 50 years ago. In order to reach this goal some physicochemical parameters
that influence metal bioavailability were determined, as well as total heavy metal
concentrations in soils. Metal tolerance of
Rhizobium
isolates was screened in artificial media
supplemented with different metals (Pb, As, Cd, Ni, Cu and Co) and their tolerance related to
soil contamination. Since soil enzymes are very sensitive to soil contaminants, such as heavy
metals, soil enzyme activities have been proposed as biological indicators of pollution (Dick
and Tabatabai, 1992; Nannipieri, 1994). Therefore, this chapter also aimed to determine the
activity of soil enzymes such as dehydrogenases, hydrolases, phosphatases, catalase and
lipase in order to evaluate the degree of soil contamination.
2.
H
EAVY
M
ETAL
T
OLERANCE OF
R
HIZOBIUM
S
TRAINS
I
SOLATED FROM
B
RAÇAL
M
INE
The soil is a dynamic, multi-component system, whose properties are continually
modified by microbial, chemical, geological and anthropogenic processes (Saeki et al., 2002).
In addition, soil undergoes short-term fluctuations, such as variations in moisture, pH and
redox conditions. These changes in soil properties may affect the form and bioavailability of
metals. So, the amounts of metals accumulated in soils are dependent on the emission levels,
the transport of the metal from the source to the accumulation site and the retention of the
metal once it has reached the soil (Alloway, 1995b).
Three composite soils were obtained from a lead mine (Braçal) in Portugal (Aveiro
region), deactivated 50 years ago. BA and BD soils were collected in the most contaminated
area of the mine, while BC soil was collected in the periphery. This soil was used as control,
since its contamination is lower (Figure 1).
Table 1 summarizes the physical and chemical properties of investigated soils. The pH
values determined varied significantly (P< 0.05) between locations, being in most soils below
the optimal pH (6.8) for rhizobia growth. BC and BA soils showed a quite acid pH (4.5 and
4.15 respectively), while the BD soil has a pH of 5.2. According to Ibekwe et al. (1997,1998),
soil pH has a significant effect on rhizobia survival. These authors reported that in the more
acid soils clover nodules were ineffective for N
2
fixation irrespective of soil metal content.
Under lower pH conditions, rhizobia numbers and nodule number
per
plant were both
significantly reduced. Broos et al. (2005) also reported that
Rhizobium leguminosarum
bv.
trifolii
survival was reduced at pH <5.5 and can be completely inhibited at pH < 4.5.
Total heavy metal concentrations (Pb, As, Cd, Zn, Ni, Cu, Cr and Co), measured after
extraction with
aqua regia
, varied significantly between locations (P< 0.05) as shown in
Figure 1. Generally, contaminated soils (BA and BD) presented higher concentrations of
metals than the control soil (BC). However, Cr concentrations were higher in BC soil than in
the other soils, but were below the higher EC limits permitted for heavy metals (CEC, 1986)
The concentrations of non-essential metals (Pb and As) were higher in BA soil, while BD soil
showed higher levels of essential metals (Ni, Co and Cu). Cadmium concentrations were
similar in both soils (Figure 1). Maximum permitted levels imposed by the CEC (1986) were
exceed for almost metals in BA and BD soils. (Figure 1).
