Agriculture Reference
In-Depth Information
possibility of heavy metals to exert a selective pressure in bacteria of contaminated soils,
resulting in the presence of more tolerant organisms. These tolerant genotypes persisted until
now in the population, evidencing an overlap of time between actual heavy metal
concentrations and
Rhizobium
tolerance, probably due to the time needed for the population
to adapt to the new conditions.
Interestingly, results show that
Rhizobium
population with higher resistance to Pb (BC
isolates) also have tolerance to other metals such as As, Cu, Ni ad Co. This fact suggests that
the selective pressure induced by Pb allowed the development of resistance mechanisms that
bestow tolerance to different metal stresses, which is corroborated by Díaz-Raviña et al.
(1994) and Nies (1992), who reported the existence of heavy metal multiple tolerance patterns
in soil microorganisms.
3.
S
OIL
E
NZYME
A
CTIVITIES
Soil enzymes are important to several crucial functions. They are intimately involved in
the cycling of nutrients, they affect the efficiency of fertilization, reflect the microbiological
activity in soil and act as indicators of soil change (Gianfreda et al., 2005; Giller et al., 1998).
Soil enzymes are the catalysts of important metabolic process functions including the
decomposition of organic inputs and the detoxification of xenobiotics (Gianfreda et al., 2005).
Soil enzymes activity is used as a sensitive indicator of the effect of pollutants, including
metals in soils (Bayer et al., 1982; Dick, 1997; Giller et al., 1998; Top et al., 1999). As
previously pointed out (Brookes, 1995), applications of European Community standards for
heavy metal concentrations result in significant negative impacts on microbial biomass and
activity, indicating the greater sensitivity of the soil to these impacts in comparison with
plants or animals. Important questions are what constitutes a significant environmental impact
on soils and when is reclamation complete. Indicators are needed, not only as surrogates for
reflecting the functionality of soils, but also to guide reclamation. The soil microbial
component and soil enzyme activities are attractive as indicators for monitoring disturbance
or pollution of soils because of their central and crucial role in the functions of the soil
ecosystem.
Enzymes may rapidly respond to the changes caused by both natural and anthropogenic
factors (Gianfreda et al., 2005). The strong inhibition of the activities of a variety of enzymes
has been reported in metal polluted soils over the past years (Doelman and Haanstra, 1986;
Marzadori et al., 1996; Mathur et al., 1980; Tyler, 1974; Tabatabai, 1977) and these effects
vary considerably.
Heavy metals may inhibit enzyme activities by masking catalytically active groups,
having denaturing effects on the conformation of proteins, competing with the natural ions
involved in the formation of enzyme-substract complexes (Gianfreda et al., 2005; Hinojosa et
al., 2004; Nannipieri, 1994), or by affecting the synthesis of the enzymes within the microbial
cells . For these reasons soil enzymes activities have been suggested as suitable indicators of
soil quality since they have been considered sensitive indicators to measure the degree of soil
degradation in both natural and agro-ecosystems, being thus well suited to measure the impact
of pollution on the quality of soil (Dick, 1997; Giller et al., 1998; Trasar-Cepeda et al., 2000).
