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However, when the conditions are changed and this population is submitted to a selective
pressure, the genotypes presenting tolerance ability can survive, hence yielding a tolerant
population, with lower genetic variability. Hinojosa et al. (2004) referred that indicators of
soil quality and health, to be useful and practical, must meet certain criteria.
These include: sensitivity to perturbation or contamination; a relationship to soil function;
reproducibility; low temporal and spatial variability; and have simple sampling and analytical
methods. Both enzymes activity and Rhizobium tolerance showed to meet some of these
criteria. Our results evidenced that dehydrogenases and catalase, which are intracellular
oxirreductases, showed a good correlation with the heavy metal contamination of soils.
Nevertheless, there are several works (Gianfreda et al., 2005; Kizilkaya et al., 2004) alerting
to several factors (pH, organic matter, clay etc.) that affect soil enzymatic activity, and they
should be taken into consideration, if we want to extrapolate or compare values from
environments with distinct edaphic, climatic or contamination features. Rhizobium isolates
showed a wide range of heavy metals tolerance, confirming the responses showed to lower
levels of heavy metal contamination (Pereira et al, 2006) and other environmental stresses
(Figueira, 2000), which indicates that, in addition to their ubiquity, culturability, and
involvement in the N cycle, Rhizobium populations are a good indicator of soil quality and
heath. However, it should be taken in consideration that rhizobia populations take some time
to adapt to the habitat conditions, which can limit their ability as immediate indicators of the
soil's health. Even so, this feature presents the advantage of reflecting the circumstances that
conditioned the population's evolution in the past.
A study, like this chapter, that simultaneously determine the soil enzyme activity and the
tolerance Rhizobium populations towards the heavy metal contamination, will provide an
indication of not only the perturbations that a specific pollutant is causing in the present, but
also the effects induced in the past, which is of extreme importance in long-term
contaminations or with pollutants that are recalcitrant like heavy metals, and may be useful
for the evaluation of natural and agricultural soils pollution, and for the improvement of soil
productivity or the reclaim of contaminated soils.
5. R EFERENCES
Adam, G; Duncan, H. Development of sensitive and rapid method for the measurement of
total microbial activity using fluorescein diacetate (FDA) in a range of soils. Soil Biology
and Biochemistry , 2001 33, 943-951.
Al -Khafaji, AA; Tabatabai, MA. Effects of trace elements on arylsulfatase activity in soils.
Soil Science, 1979 127, 129-133.
Alloway, BJ. Introduction. In: Alloway BJ editor. Heavy Metals in Soils . New York: Blackie
Academic and Professional; 1995a. 3-9.
Alloway, BJ. Soil processes and the behaviour of metals. In: Heavy Metals in Soils . New
York: Blackie Academic and Professional; 1995b. 11-37.
Arshad, MA; Coen, GM. Characterization of soil quality: physical and chemical criteria.
American Journal of Alternative Agriculture , 1992 7, 25-33.
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