Heavy Metal Pollution in Agricultural Soils: Consequences and Bioremediation Approaches
Metals and metalloids which have densities higher than water (generally having atomic density > 4 g/cm) are regarded as heavy metals (Nagajyoti et al. 2010), although many other physical and chemical propexties of metals are also taken into consideration while defining the term ‘heavy metals’ (Appenroth 2010). Naturally, the main sources of heavy metals are in the Earth’s crust, rocks, volcanoes, and sea-water, where they are found as oxides, sulfides, phosphates and in several other forms (Masindi and Muedi 2018). Weathering processes of mineralized rocks, volcanic eruptions and flooding from oceanic water bodies, rivers and lakes containing heavy metals are some of the natural drivers which add metal contaminants to soils (Alloway 2012, Majeed et al.
2019). Anthropogenic oxigins of soil’s contamination with heavy metals are largely linked with industries, mining and processing (Islam et al. 2018). Feitilizers and pesticides used in agriculture for dealing with nutrient deficiencies in soil and insects and pests atrocities are also considered as main anthropogenic agents which have increased heavy metal concentrations in soils.
Plants’ growth and development are affected by several factors among which the chemistry and compositional aspects of soil are important. Availability of necessary nutiients and water and stable communities of microbes in soil govern physiological growth and developmental features of plants. Discrepancies in soil’s nutritional and microbial components decline the health and fertility of soils that correspond to the overall disturbance of plants. The presence of heavy metals in soils beyond threshold levels induces stress and toxic environment which significantly influence plants’ potential to absorb nutrient and water leading to reduced physiological and metabolic activities in them. Negative interactions of heavy metals with soil’s beneficial microbes can contribute to reduced soil fertility and impaired plants-microbe relations. Consequently negative effects of heavy metal stress on morphology, development and yield attributes of plants depends on the type of heavy metals, their concentrations and the responding plant species. Some heavy metals like lead (Pb), cadmium (Cd), mercury (Hg), and arsenic (As) that occur in many physical and valiancy forms are not required by plants for their growth or other physiological activities and hence they are highly toxic in very limited concentrations (Saxena et al. 2019). The occurrence of these metals in soils affect plants’ germination, growth, and development due to poor water and nutrient uptake, and metabolic abnormalities.
Several techniques have been used as cleanup strategies for the reclamation of polluted soils with variable outcomes and cost efficiency (Kong and Glick 2017). Physical, chemical and thermal methods are employed to remove heavy metal-pollutants from soil and water; however, the costs, teclmical feasibility, health hazards, and the emergence of secondary' pollution make these strategies less effective (Lone et al. 2008, Ashraf et al. 2017, Kong and Glick 2017). Instead, bioremediation techniques (the use of microorganisms and plants) for extraction of soil contaminants has been well received during the last few years because of their potential sustainability, low costs and risks they pose to the environment and human health (Ma et al. 2011). Although plants mediated remediation (phytoremediation) alone has been documented to yield promising results, microbial assistance of plants in the phytoremediation processes may further accelerate the extraction of pollutants and can enhance the efficiency of bioremediation (Ma et al. 2011, Asln af et al. 2017).
Key components in a successful biorernediation approach involve the identification of bio-remediating agents, then tolerance level to heavy metals, and their suitability in the polluted environment. Further exploration of metal-tolerant species of bacterial genera and their use as co-inoculants with phyto-extractant plants would lead to reduced reliance on traditional approaches used for soil reclamation. In this chapter, sources of heavy metal pollution in soils, the effect of heavy nretal stress on plants, and potential biorernediation approaches for reclamation of heavy metal polluted soils are addressed.