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 contaminate water used for drinking and agricultural purposes. Plants growing on HM‐polluted sites take in this contaminated water and accumulate HM in them and when these plants are used as food, these HM are transferred to the organism's bodies accumulating again [43]. Once fresh water organisms are exposed to HMs, like Cr, Ni, Cd, and Pb, accumulation is increased with increased exposure time, harming the consumer's kidneys and other body organs [47]. Food crops consumption that are irrigated with HM‐contaminated water is also a major route of human exposure to HM [48, 49]. Metals are being utilized in a range of ways in industries and agriculture; particularly heavy metals such as Hg, Cd, Pb, and As. These constitute a significant potential threat to human health via contaminated water [50]. Water pollution remains a major source of morbidity and mortality in different parts of the world. Water environment treatment has led to improved health outcomes among Chinese people. Reduced water pollution mediated the associations between water environment treatment and health outcomes [51].
2.3.5 Humans
HMs are among one of the widespread pollutants in the world, which cause severe threats to almost every organism including humans. In agricultural soils, HMs like Cd, Cu, Pb, and Zn concentrations are 0.097, 22.6, 26.0, and 74.2 mg/kg, respectively and can even reach 3.16, 99.3, 84.1, and 147 mg/kg, respectively. In water, Cd, Cu, Pb, and Zn concentrations are 0.080, 7.91, 15.7, and 18.7 ug/l respectively and cause contamination in drinking water and food that threatens human health [52]. Occupational exposures to Mn, Cu, Fe, Hg, Zn, and Al pose a risk factor for Parkinson's disease [53, 54]. Rice grown on Cd contaminated soils causes Cd loaded grains and poses kidney failure in humans consuming the contaminated rice [55–57]. Table 2.1 defines HM effects on plants and humans.
Table 2.1 Effect of different heavy metals on plants and humans.
| Heavy metals | Effects on plants | Citations | Effects on Humans | Citations |
| As | Growth, leaf gas‐exchange, water potential, protein content, and biomass. | [58–61] | Carcinogen, cyto, and genotoxic, cause diabetes, cardiovascular diseases, and disrupts DNA repair. | [62–66] |
| Al | Decreases in biomass, photosynthesis, protein contents, disrupts NPK, Ca, Mg uptake | [67–70] | Neuro, geno, and cytotoxic, damages membrane and DNA, cause chromosomal and lymphocyte aberrations. | [71, 72] |
| Cr | Damages rhizobia, production of ROS and oxidative stress, low seed germination and reduced nutrient uptake | [73–76] | Carcinogen, damages DNA and reproductive system with birth and growth defects. | [73–77] |
| Ni | Reduces growth and photosynthetic pigment contents, low crop growth, increases imbalance between K and Ca | [78–80] | Cardiovascular diseases, haemo, immuno, neuro, geno, nephron, and hepatotoxic, carcinogen and cause reproductive system illness. | [81, 82] |
| Cu | Low yield, failure in making seed, less photosynthetic rate, ROS production, cell damage, disturbs photosynthesis and electron transport. | [83, 84] | Cardiovascular system and disrupts enzymes activity. | [85, 88] |
| Pb | Stunted growth, chlorosis and blackening of root system, inhibit photosynthesis, upset water balance, changes affects membrane structure. | [89, 90] | Mental retardation, growth impairment, comma, convulsions, anemia, hypertension, and immunotoxicity. | [91, 92] |
