Agriculture Reference
In-Depth Information
micronutrients for plants, animals and microorganisms, whereas others such as Cd, Hg and Pb
have no known biological and/or physiological functions being toxic even at very low
concentrations (Gadd, 1992).
The increasing occurrence of heavy metal-contaminated areas and the consciousness of
their highly toxic effects to humans, as well as to animals, plants and microorganisms explain
the growing concern about heavy metal pollution. Hence, the pressure to decontaminate
heavy metal polluted soils has increased recently, however concern with the cost of soil
remediation lead to explore not only cost effective technologies but also alternative
monitoring tools. The immediate concern of rehabilitation practitioners is the availability and
capacity or degradative potential of the autochthonous microbial communities. The adverse
effects of heavy metals can lead to a reduction in biodiversity and resultant functions in the
soil, for this reason it seems deem necessary to use more relevant ecological test species.
Rhizobium
are ubiquitous bacteria that have a profound agronomic importance, since the
symbiotic association between legumes and rhizobia is by far the most important contributor
to the world's supply of biologically fixed N
2
(Somasegaran and Hoben, 1994). For this
reason and taking into consideration the importance of legumes in animal and human
consumption, some attention has been given to the effects that these elements exert on
Rhizobium
(Ibekwe et al., 1995). Several authors reported that leguminous species grown on
contaminated soils, exhibited reduced yields and N content (Chaudhary et al., 2004), since
rhizobia isolated from host plants root nodules were completely ineffective to fix N
2
(Giller et
al., 1989; Hernandez et al., 2002). In others studies (Pereira et al., 2006)
Rhizobium
has
shown to be a genus very sensitive to environmental stresses, evidencing to be a good
indicator of soil contamination.
Contradictions on heavy metal ecotoxicity can be attributed to overgeneralization of the
outcomes from short-term laboratory studies that focus on a single soil type under controlled
conditions. Field data on the effects of heavy metal are limited, and most of the information
has become from experiments using sludge containing several metals. However, heavy metal
toxicity requires assays with sensitive, reliable and ecologically relevant biological tools.
Parameters involving species abundance and diversity along with functional parameters can
give a better understanding of heavy metal toxicity.
The presence of heavy metals in the soil may influence the biochemical process by
affecting both microbial proliferation and enzyme activities (Gianfreda et al., 2005). Visser
and Parkinson (1992) have suggested that the biological and biochemical properties that are
most useful for detecting the deterioration of soil quality are those that are most closely
related to nutrient cycles, including the activities of soil enzymes. Soil enzyme activities are
the driving force behind all the biochemical transformations occurring in the soil since they
catalyse all biochemical reactions and are an integral part of nutrient cycling and soil fertility
(Bandick et al., 1999). Soil enzymes are believed to be primarily of microbial origin but also
originate from plants and animals (Tabatabai, 1994). They are usually associated with viable
proliferating cells, but enzymes can be either excreted from a living cell or released into soil
solution from dead cells (Tabatabai, 1994). Soil heavy metal contamination exerts an
influence on the microbiota, which manifests itself in changes in the enzyme activity (Baran
et al., 2004; Gianfreda et al., 2005). There is growing evidence that soil biological parameters
may have a potential as early and sensitive indicators of soil ecological stress and restoration
(Dick and Tabatabai, 1992).
