Heavy metals such as cadmium, copper, lead, chromium and mercury are important environmental pollutants, particularly in areas with high anthropogenic pressure. Their presence in the atmosphere, soil and water, even in traces can cause serious problems to all organisms and heavy metal bio accumulation in the food chain especially can be dangerous to the human health. If heavy metal polluted soil is used for crop cultivation then the heavy metals deposited in soil enter into food chain and at higher concentration create severe human health problems. On the contrary, at permissible limit, metals are important for enzymatic activity and genetic material integrity in biological system. To understand the importance and risk associated with heavy metals, a genuine attempt is made to present different aspects of metal contamination in soils. Furthermore, nutritive value of heavy metals and toxicity to bacteria and plants is discussed. Finally, different strategies adopted by biological systems to detoxify heavy metals are critically highlighted. This review is likely to help to better understand the over does risk of heavy metals and its biological detoxification strategies.
- BACKGROUND OF STUDY
Due to consistently increasing human populations, the current agricultural systems are under tremendous pressure basically for two reasons: i) cultivable land is declining very rapidly and ii) The human food demand is on the rise. Therefore, well-directed and concerted efforts are needed in order to use the full potential of agro-ecosystems efficiently and to overcome these problems. However, the plant nutrients like nitrogen (N), phosphorus (P), potassium (K), and some other minor nutrients play important roles in crop improvement in conventional agriculture practices. In contrast, the deficiency in even micronutrients, which are typically present at <100 mgkg-1 dry weight, may significantly limit the crop yields in many production systems (Ilemobayo and Kolade, 2008). Some of micronutrients such as Cu, Fe, Mn, and Zn, are essentially required for various physiological functions of plants and animals. Although the majority of plants require these elements in minimal quantities, agricultural soils are often deficient in one or more of these micronutrients. In general, the concentration of such nutritional contents in plant tissues falls below the optimum levels. There are also minor elements, known as trace elements, or other metalloids which play important roles in functioning of living organisms including those of microbioma. In addition, these elements could also participate in other activities:
- Forming the structure of proteins and pigment
- Redox processes
- Regulation of the osmotic pressure
- maintaining the ionic balance and
- Acting as enzyme component of the cells .
Among these elements, Al, Co, Se, and Si play a role in promoting plant growth and may be essential for particular taxa. Likewise, Zn plays a significant role in cellular division and amplification, protein synthesis, and contributes in carbohydrate, lipid, and nucleic acid metabolism. On the other hand, structure and composition of microbiota and plant growth are reported to be significantly affected when the concentrations of such trace elements exceed the normal level. Moreover, the concentration of these trace elements also varies from soil to soil and/or region to region. For instance, multiple surveys conducted to determine the status of nutrient in agricultural soils in China and India revealed that Zn is commonly the most deficient micronutrient in soil. While the nutritional deficiency levels in Chinese soils were (%): Zn 51, Mo 47, B 35, Mn 21, Cu 7, and Fe 5 , the deficiency levels in Indian soils were: 49 Zn, 33 B, 12 Fe, 11 Mo, 5 Mn and 3 Cu. Thereby, identifying the elements of soil nutrient pools and their consequential effect on both microbes and plants are necessary for enhancing crop production and plant nutritional value. (Iwegbue 2007).
The term “heavy metal” is linked in many people’s minds to metals that are toxic. However, this is not always the truth. The effect of any substance on a living system is always dependent on its available concentration to cells. Also, several heavy metal ions are crucial in metabolic processes at low concentrations but are toxic at high concentrations.
1.2 STATEMENT OF PROBLEM
Diverse methods are being used to clean up the environment from these contaminants, but most of them are costly and far away from their optimum performance. For instance, the chemical technologies generate large volumetric sludge and increase the costs , chemical and thermal methods are both technically difficult and expensive that all of these methods can also degrade the valuable component of soils. Conventionally, remediation of heavy-metal-contaminated soils involves either onsite management or excavation and subsequent disposal to a landfill site. This method of disposal solely shifts the contamination problem elsewhere along with the hazards associated with transportation of contaminated soil and migration of contaminants from landfill into an adjacent environment. ( R. Rakhshaee,2009). Also, soil leaching is an alternative way to excavation and disposal to landfill. This method is very costly and produces a residue rich in heavy metals, which will require further treatment. Moreover, these physio-chemical technologies used for soil remediation render the land usage as a medium for plant growth, as they remove all biological activities
1.3 OBJECTIVES OF THE STUDY
The objectives of this study is to:
- Test various soil microbial parameters as indicators of contents as well as of exchangeable and organically bound fractions of these metals.
- Check the distribution of micro flora in the presence of heavy metals
- Compare the different contaminated soil as regard the mechanic site used.