Soil contamination and human health
Heavy Metals
This group of contaminants includes metals (e.g., Cadmium, Cobalt, Mercury, Lead) and metalloids (e.g. Selenium, Antimony, Aresenic) with an atomic number greater than 20 (Sanaei et al., 2020) and a density greater that 5 g/cm¯³. Heavy metals are persistent and non-biodegradable, with some of them being toxic even at low concentrations.
Heavy metals naturally occur at site-specific (low) concentrations in soils, and some of them - such as Iron, Zinc, Nickel, Copper, or Magnesium - are essential micronutrients for plants, animals, and humans (Balseiro-Romero & Baveye, 2018). However, pollution or unsuitable agricultural management may also lead higher concentrations and or increased bioavailability, thus causing toxic effects and threats to both soil health and human health (Demková et al., 2017).
Reasons for elevated heavy metal concentrations in agricultural soils are mainly intentional applications of compounds containing high levels of heavy metals. These can be fertilizers (e.g., phosphate or nitrate fertilizers, livestock manure, composts, biosolids, or sewage sludge), lime, pesticides, or wastewater for irrigation (Alengebawy et al., 2021; Nicholson et al., 2003; Srivastava et al., 2017; Rai et al., 2019; Abreu-Junior et al., 2018; Giannakis et al., 2020).
For example, Phosphate (P)-based fertilizers can be a source of heavy metals such as Zinc or Cadmium (Rai et al., 2019) and may cause soil pollution, crop uptake, and bioaccumulation along the food chain, resulting in environmental and human health risks (Niño-Savala et al., 2019). P-fertilizers are estimated to contribute 45% to the total Cadmium-load in European croplands, while 55% of the total dietary intake of Cadmium by consumers is associated with Cadmium accumulation in soils (Marini et al., 2020).
The mobility and bioavailability of heavy metals is strongly influenced by soil conditions. Detailed risk assessments are essential if potentially contaminated material is to be applied to soil (Giannakis et al., 2020).
Mitigating the risk of soil pollution with heavy metals includes avoidance of excessive fertilizer use (Nag & Cummins, 2021), monitoring of fertilizer composition (Srivastava et al., 2017; Nag & Cummins, 2021), and periodic soil sampling and testing to assess the concentration and bioavailability of heavy metals (Nag & Cummins, 2021). Since heavy metals in animal manure originate from livestock feed, feed should be controlled and intentional feeding with heavy metals restricted to optimal doses rather than maximum permitted levels (Hejna et al., 2019).
Heavy metals incorporated within agricultural particulate matter (Li et al., 2015) can be extensively dispersed throughout the atmosphere (Hassanien et al., 2010). The lifetime of heavy metal-carrying particulate matter and their possible toxicity is a function of their size. Generally, the smaller and lighter a particle is, the longer it will stay in the atmosphere, while larger particles (>10 μm in diameter) tend to be deposited by gravity in a matter of hours (Hassanien et al., 2010).
As heavy metals can be absorbed by food plants or aerially deposited on their surface, the food chain has been reported to account for approximately 90% of total heavy metal intake (Demková et al., 2017; Sanaei et al., 2020). Additionally, they can be inhaled together with soil particles or (in specific cases or at high concentrations) absorbed through the skin.
Itis important to note that the chemical species or compounds in which heavy metals occur determine their toxicological impact and may be more important than the total uptake. Studies report that heavy metals can impair neurological, cardiovascular, urogenital, endocrine, immune, digestive, respiratory, and detoxification functions of the body, causing a variety of health problems (Karimyan et al., 2020; Balali-Mood et al., 2021; Hassanien et al., 2010; Doabi et al., 2018). Additionally, some heavy metals (e.g., Arsenic, Cadmium, Chromium, Nickel) have carcinogenic effects (Karimyan et al., 2020; Balali-Mood et al., 2021).
Heavy metal concentration, exposure duration, and ingestion rate, as well as nutritional status and immune system and individual detoxification organ strength are factors affecting human health impacts (Sanaei et al., 2020; Hassanien et al., 2010).
Heavy metal accumulation can reduce a soil’s biological activity, causing reduced fertility and plant growth, loss of soil organic matter (Feszterová et al., 2021), loss of soil structure, and increased soil acidity (Alengebawy et al., 2021). Heavy metals can induce oxidative stress in plants, affect their photosynthetic processes, their respiration and transpiration, as well as the uptake and transport of essential micronutrients. Hence, they may inhibit plant development and growth, including germination and root elongation, resulting in a decrease in biomass and a potential reduction in productivity and yields (Srivastava et al., 2017; Niño-Savala et al., 2019; Khan et al., 2015; Feszterová et al., 2021; Abdel-Rahman et al., 2021). High concentrations of heavy metals can also cause plant death and motivate the selection of species with resistance mechanisms and high tolerances (Niño-Savala et al., 2019).
Heavy metal type, concentration, and bioavailability determine to what extent they will be absorbed by plants (Khan et al., 2015; Feszterová et al., 2021). Additionally, heavy metal uptake by plants also differs between plants species, as different plants have different uptake rates for specific metals (Khan et al., 2015; Gruszecka-Kosowska et al., 2019).
Studies show that heavy metals in soils can have inhibitory effects on soil microbial and enzymes activities, microbial biomass, populations, growth, and metabolic processes (Alengebawy et al., 2021; Srivastava et al., 2017). Therefore, the abundance, diversity, and structure of the microbial communities can be affected (Hu et al., 2021), potentially causing functional changes in soil flora with implications for C or N cycling (Hodson et al., 2013). Overall though, despite the toxic effect of heavy metals, microorganisms such as bacteria, fungi, or algae can survive in their presence thanks to several evolutionary strategies enabling them to reduce or tolerate toxicity. Additionally, heavy metals in soil can act as evolutionary drivers, as studies show the ability of soil organisms to develop mechanisms by which they can tolerate or resist the effects of heavy metal-induced stress (e.g. acclimation and adaptation) (Hodson et al., 2013).
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