Agriculture Reference
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contamination in agricultural soils. On the other hand, under heavy metal stress catalase
activity of microorganisms, such as
Rhizobium
(Corticeiro et al., 2006), increase markedly.
Thus, catalyse activity appears to be influenced in different ways by heavy metals, explaining
why the results between BA and BD soils are difficult to interpret.
Phosphatases are broad enzymes that catalyze the hydrolysis of both esters and
anhydrides of H
3
PO
4
(Eivazi and Tabatabai, 1977). These enzymes are responsible for soil
organic phosphorous mineralization and the release of inorganic phosphorous needed by
microorganism and plants. The enzymes are classified as acid and alkaline phosphatases due
to their optimal activities at (< 6.5) and (11) pH ranges, respectively. Phosphatases activities
have been described as a good indicator for heavy metal contamination (Wyszkowska, 2002).
However, Al-Khafaji and Tabatabai (1979), Bardgett et al. (1994) and Yeates et al. (1994)
described that acid phosphatase activity was less affected by heavy metals than arylsulfatase.
Acid and alkaline phosphatase activities were determined according to Eivazi and Tabatabai
(1977). Results showed in Figure 5 indicate that acid phosphatase activity varied significantly
(P<0.05) between locations. However, it was not detected activity of alkaline phosphatase in
all of the soils tested. According to Gianfreda et al. (2005) soil enzyme activities are usually
significantly correlated to soil pH. The investigated soils showed low pH values (Table 1),
which could affect the activity of alkaline phosphatase. Arguably, the field soil pH would not
directly affect the enzyme assay as the assays are run using buffers at their optimal pH.
However, the long-term effect of low pH would probably cause shifts in microbial
community composition and size and this, in turn, would affect enzyme dynamics at the time
of sampling (Hinojosa et al., 2004). Furthermore, according to Eivazi and Tabatabai (1977)
acid phosphates is predominant in acid soils and alkaline phosphatase is predominant in
alkaline soils. Acid phosphate activity was higher in the most contaminated soils. However,
according to Wyszkowska (2002) the activity of acid and alkaline phosphatases decreased in
soils contaminated with Cr (IV), and in fact our work shows that acid phosphatase activity is
negatively correlated with Cr levels. On the other hand, phosphatases can be inhibited not
only by heavy metals (Tyler, 1974), but also by inorganic phosphate, which produces a
feedback inhibition of this enzyme (Nannipieri et al., 1979). Actually, in this study acid
phosphatase activity is higher in the BA soil with the lower level of phosphorus (7 mg kg
-1
)
and lower in BC, which has a higher level of phosphorus (131 mg kg
-1
).
Lipase activity was determined according to Margesin et al. (2002). Lipase activity is
often associated to oil degradation; however information about its behaviour in the presence
of metals is limited. In this work, lipase activity was similar in all soils (Figure 5), showing
that its activity is not differentially influenced by heavy metals concentrations present in the
three soils.
4.
C
ONCLUSION
Due to their small size, which provides a large contact area that can interact with the
surrounding environment, microorganisms are the first biota showing the impact of toxic
compounds. For this reason they are considered to be the best indicators of soil pollution.
Metals exert a selective pressure on organisms, resulting in microbial populations with higher
tolerance, but with lower diversity, when compared to unpolluted neighbouring areas (Bååth,
