Неалу metals, sources of contamination in soils and effects on plants

Heavy metals (and metalloids) are those which are denser than water, have greater atomic weight, and show biological toxicity (Tchounwou et al. 2012, Li et al. 2019). The most important heavy metals from toxicological perspectives (and some for nutritional essentiality) are mercury (Hg), copper (Cu), nickel (Ni), lead (Pb), cadmium (Cd), chromium (Cr), strontium (Sn), zinc (Zn), iron (Fe), manganese (Mn), molybdenum (Mn), arsenic (As), aluminum (Al), cobalt (Co), and silver (Ag). Some heavy metals like Zn, Fe, Mn, Mo, Ni, Co, and Cu are essential micronutrients required for the proper functioning of both plants and animals but in lesser amounts (Zwolak et al. 2019). The essential heavy metals in trace amounts (depending on living beings) play major roles in the regulation of structural and functional diversity of enzymes, vitamins, photosynthetic pigments, and many other metabolic activities in both humans and plants. In plants, their role in cellular metabolism, enzymatic activities, electron transporters, synthesis of biological molecules and membranes, and photosynthetic pigments has been acknowledged in different studies and reviews (Arif et al. 2016, Oves et al. 2016). However, excess amounts of even essential heavy metals may cause several physiological and metabolic abnormalities in plants and humans leading to specific hyper-concentration-symptoms in them. Non-essential heavy metals, particularly Pb, Hg, Cd, Cr and As, are highly toxic to living organisms. Since their biological role is unknown, they are regarded as ‘hazards’ by many environmental protection agencies among which the United States Environmental Protection Agency considers them as highly toxic and carcinogenic (Tchounwou et al. 2012).

Soil pollution emerges when heavy metals and several other organic and inorganic contaminants are introduced to the soil (Cachada et al. 2018). Among the inorganic pollutants, heavy metals contaminations in soils are of significant concern for living beings because of their toxic effects on flora, fauna and microbes. Leading sources of soil contamination are human activities which include industrial processes, the use of agrochemicals, and waste disposal (Shankar 2017). Natural agents and anthropogenic activities are the leading sources of the incursion of heavy metals to soils. 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 origins of soil’s contamination with heavy metals are largely linked with industries, mining and processing (Islam et al. 2018). Fertilizers and pesticides used in agriculture for dealing with nutrient deficiencies hi soil and insects and pests atrocities are also considered as main anthropogenic agents which have increased heavy metal concentrations in soils. The distribution and occurrence of heavy metals in soils depend on the site, nature of industrial operations and mining activities.

Soils contaminated with heavy metals adversely affect the soil’s microbial communities, plants, animals and humans and the whole ecological system (Edwards 2002, Padmavathiamma and Li 2007, Khan et al. 2009). In a managed ecosystem, polluted soils offer problems of diverse nature and intensity to crop growers as well as to consumers. Generally, polluted soils can directly challenge the germination and subsequent growth of cultivated crops by altering their metabolic activities, reducing water and nutrient absoiption, and root injury (Khan et al. 2008). The rendered functionality of important physiological processes such as photosynthesis and respiration in response to metal and pollution stress would result in deficit accumulation of photosynthate and subsequent breakdown of the respiratory substrate in different plant organs. This can lead to stunted growth and a reduction in yield outputs of the crops. Other drastic consequences of polluted soils are the negative impacts on or even elimination of microbial communities (He et al. 2005, Wang et al. 2007), which are particularly alarming in the case of soil beneficial microbes because of their contributions toward plant growth promotion. Several studies have outlined the adverse effects of both nonessential heavy metals and essential heavy metals (in exceeding concentrations) on germination, growth, physiology and developmental aspects of plants. Inhibited germination and impaired seedling growth in Arabidopsis thaliana were observed in response to Cd, Pb and Hg stress (Li et al. 2005). In many other works, negative consequences of both essential heavy metals (Ni, Cu, Co, Al, etc.) at higher concentration and non-essential heavy metals (Pb, Hg, Cd, As, Cr, etc.) have been documented in different plant species in terms of abnormalities in their nutrient and mineral uptake, physiology, biochemistry, and photosynthetic activities (Table 1). Thus, reclamation of soils polluted with heavy metals in agro-farming systems is vitally important for mitigation of the adverse effects on soil microbes, growth and yield of crops.

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