Should heavy metals be monitored in foods derived from soils fertilized with animal waste?

Heavy metals (HM) represent a large group of elements with atomic density >5 g cm−3 or atomic number >20 (Saidur et al., 2017), among which some are essential to plants, such as iron (Fe), zinc (Zn), copper (Cu), nickel (Ni), and manganese (Mn). However, HMs may be contaminants and/or pollutants, depending on the concentration in soils. HMs such as Cu, Zn, Ni, and chromium (Cr) are essential to human beings, and biofortification approaches to improve levels of some elements in plant edible parts are underway (Bouis et al., 2012; Ricachenevsky et al., 2015). However, these HMs may be toxic if accumulated, and may only be ingested in very small quantities (EPA-U.S. Environmental Protection Agency, 1995; FAO-Food Agriculture Organization of the United Nations, 1995; Tchounwou et al., 2012). On the other hand, Pb, Cd, As, and Br are not essential and can be toxic even at low concentrations (Tchounwou et al., 2012). The safe daily intake level for As, Cd, Cr, Cu Ni, Pb, and Zn is 20, 300, 1500, 4, 20, 40, 300 μg kg−1 of body weight per day, respectively (EPA-U.S. Environmental Protection Agency, 1993). These levels are based on the degree to which the element may cause disturbance, the capacity of the body to accumulate the element and the weight of the individual who is ingesting it (Abbasi et al., 2013). However, when HMs are ingested for long periods, even at doses considered safe, they can cause harmful effects, known as chronic intoxication (Jorge Mendoza et al., 2017; Li et al., 2017). The increase in total HM concentration and their chemical forms in soils can occur naturally due to atmospheric deposition, weathering of rocks, and anthropic activities such as mining, deposition of ash from coal burning, application of pesticides in plants, addition of mineral and organic fertilizers, among others (Guilherme et al., 2005). HM accumulation in the soil is typically assessed by indicators such as Geo-accumulation index (Igeo) (Equation 1) (Müller, 1979) and Enrichment Factor (EF) (Equation 2) (Abbasi et al., 2013) that allow the identification of the presence and the intensity of deposition of anthropogenic contaminants in topsoil.