Soil Bioremediation. Группа авторов

is a comprehensive technique, which offers heavy metal (HM) contamination remediation with an innovative and cost‐effective option. The use of plants to bring back contaminated sites is termed as phytoremediation as it uses the plant’s natural characteristics to up take, accumulate, store, degrade, and remediate heavy metals [4]. With the green revolution, use of pesticides and fertilizers has polluted the soil with HMs like Cd, Pb, Ni, and Hg. Pesticides, beside their biocidal and fertilizing effects, contain considerable concentrations of HMs. Undeniably, pesticides can be very toxic and are responsible for farming diseases such as cancers and neurodegenerative diseases. However, in developed countries, there is a rapid change from subsistence farming to intensive farming, in order to feed more people. There are also some issues like lack of selectivity, overuse, and over exploitation that leads to risk for living organisms and humans by contaminating drinking water, food, and soils. Their presence in soil, water, plants, and even the atmosphere, together with their potential pharmacodynamic properties, can have harmful effects on the environment and on human health [4, 5]. This problem can be overcome by phytoremediation, which can reduce HM pollution and decrease their impact on the environment [5]. These techniques are exciting prospects for reducing environmental pollution. Plants can bioaccumulate, biotransform, and bioremediate HMs [6]. Unlike organic compounds, heavy metals cannot be degraded. Hence, an effective clean‐up strategy via immobilization is required to reduce or remove toxicity. In recent years, scientists have commenced generation of cost‐effective technologies that comprise use of microorganisms/biomass or live plants to clean polluted areas. These technologies are best applied at sites with shallow contamination of organic, nutrient, or metal pollutants that are acquiescent to one of the five applications, i.e., phytotransformation, rhizosphere bioremediation, phytostabilization, phytoextraction, and rhizo‐filtration. The technology involves efficient use of plants to remove, detoxify, or immobilize environmental contaminants in a soil, water, or sediment mix through the natural, biological, chemical, or physical activities or processes of the plants [7]. The exploitation of plants to remediate soils contaminated with trace elements could offer a cheap and sustainable technology for bioremediation. Many modern tools and analytical devices have provided insight into the selection and optimization of the remediation process by plant species. Metal‐hyperaccumulating plants, desirable for heavily polluted environments, can be established by the insertion of novel traits into high biomass plants in a transgenic approach, which is an encouraging strategy for the development of effective phytoremediation technology. The inherited manipulation of a phytoremediator plant needs many optimization processes, including mobilization of trace HM ions, their uptake into the root, stem, and other viable parts of the plant and their detoxification and allocation within the plant [8, 9].

2.2 Sources of Heavy Metals

      HMs are the natural elements that have higher atomic weights and density above 5 g/cm³. These are inevitable and cannot be avoided as they are found naturally, through weathering [10], volcanic eruptions [11], and fossil fuels [12]. Then they were used raw and as processing materials such as, Pt in hydrogenation [13], As in pesticides [14] Cd in fertilizers [15], and in a range of other industrial, domestic, agricultural, and medical applications [16]. It is assumed that almost all the heavy metal concentrations are higher and widespread in the environment due to road dusts [17–19].

      2.2.1 Natural Sources

      Heavy metals are found naturally in the environment as result of volcanic eruptions, and sedimentary and metamorphic rock deposits and their releases during weathering and pedogenic processes. These HMs are introduced into soil and groundwater and reaches the human food chain [20, 21]. All HMs in the environment originate in natural phenomena and human activities distribute them to other parts of the ecosystem [22, 23]. In addition, gases and fluid emissions from the earth’s surface, atmosphere, sea floor, and volcanoes are additional important sources of HMs.

      2.2.2 Anthropogenic Sources

HMs are released into the environment by industrial activities, ore mining, and through other product uses. Further anthropogenic sources are agricultural activities such as: fertilizer use, animal manures, and pesticides; metallurgical activities, smelting, metal finishing; dyes; energy production; transportation; and microelectronic products [24]. Fertilizers are used to provide essential nutrients to soil and crops for sustainable production and improved quality and pesticides are applied to protect crops from pest and diseases. Both products contain HM. Moreover, soil amendments, derived from sewage sludge also contain HM, which is mobilized during crop growth due to irrigation [25, 26]. It is believed that Pb, Hg, As, Cr, Cu, and Ni mainly came from anthropogenic sources with complex distribution and exposures [27]. Fossil fuel combustion is again one of the major sources of HM and there is a need for the reduction of this source by adopting other fuel technologies [28]. Urban areas are generally contaminated with Pb, Zn, Cd, and Cu due to fossil fuels emissions by traffic and the paint industry [29].

2.3 Impacts of Heavy Metals on Soil and Microbial Activity

      2.3.1 Soil Microbial Community

      Different HMs have different effects on soil microbes and their processes however, some of these microbes might have evolved tolerance and adaptation mechanisms against HM in the soil environment [30]. Soil microbes are also biomonitors of HM pollution in soil. To check HM effects on soil microbes, microbes under study are divided into three groups, i.e., sensitive, tolerant, and resistant. It was established that resistance is evolved in microbes for HMs by the passage of time after repeated exposure [31]. Microbial colonies in Zn and Ti contaminated soils have decreased C contents, Zn shows biocidal effect and decreased microbial activity [32]. Soil microbial properties, microbial biomass, respiration, N‐mineralization, soil enzymes involved in cycling of C, N, P, and S were proven in response to different levels of HM pollution and it was revealed that microbial biomass and enzyme activities were reduced with increasing HM concentration. This implies that HM concentration in soils can severely decrease the functions of the soil microbial community and impair specific pathways of nutrient cycling [33]. In another study, results showed that microbial biomass in terms of C was depressingly affected by different levels of HMs. Enzyme activity was significantly dejected by HM stressed conditions. Soil phosphatase enzyme activities were found in the soils 200 m away from the HM [34, 35].

      2.3.2 Soil Organic Matter

Heavy metals also have negative effects on Soil Organic Matter (SOM) as it decreased up to 1% in those soils contaminated with Zn and Cu by hindering the microbial activity and plant growth [36, 37]. In another study, HMs showed a positive relationship with SOM because an adverse HM effect on soil microbes appeared to increase the accumulation of organic matter but the relationship was negative in terms of soil respiration and microbial biomass since the bacteria were less effective in mineralizing SOM under these conditions [38, 39]. Microbial activities such as respiration, C and N mineralization, biological N2 fixation, and soil enzymes were assessed for this [40]. SOM degradation leads to changes in the soil, these are assessed by their change with regards to Zn, Pb, and Cu that caused a decrease in SOM after soil has received long‐term application of sewage sludge [41].

      2.3.3 Plants

      HM’s availability in high concentrations from mining, industrial, and disposal of industrial sewage sludge is increasing the pollution problem in agriculture and in the crops grown on these soils that receive HM contaminated materials. HMs cause numerous negative effects on plants that limit their growth and yield [42, 43]. Generally, HMs affect plant growth by causing cytological disorders, disturbing metabolic processes and physiological growth [44]. Heavy metals also involved in the production of ROS (reactive oxygen species), cause blocking of essential functional groups in biomolecules and displacement of essential metal ions [45, 46].

      2.3.4 Water

      HM contamination issue is increasing all over the world in every component of the environment including groundwater resources. Heavy metals, such as Cd, Cu, Pb, Cr, and Hg are major pollutants that