**6. Heavy metals in organic soil amendments and potential bioaccumulation in edible plants**

In consideration of the enormous worldwide consumption of fruit and vegetables, that is, various *Capsicum* spp., the use of capsaicin as an additive in food for medicinal and other beneficial purposes requires continued monitoring of the potential harmful effects of heavy metals (also known as trace elements) accumulation in pepper fruits and other edible plants when SS or any other soil amendment is used in growing plants. The storage of great variety of molecules in animal and plant cells and the mechanism of this storage or bioaccumulation allow living organisms to accumulate nutrients and essential minerals. During this process, plants and animal cells can also absorb and accumulate harmful substances from the soil solution such as heavy metals. There is limited information on heavy metal (such as Cd, Ni, and Pb) in soil amendments and absorption by edible plants. Presently, heavy metals in the food chain are one of the pollutants of most concern around the world [89]. However, reducing the accumulation of heavy metals in edible plants can be achieved using biochar. Survey of current adsorbents indicated that the large surface area of activated carbon (ranging from 500 to 2000 m<sup>2</sup>  g-1) makes it a perfect candidate for heavy metals adsorption. Results indicated that biochar effectively reduced the total amount of nitrate, ammonium, and phosphate in leachates by 34, 35, and 21%, respectively, relative to native soil alone [90]. Biochar adsorption of ammonia decreases NH3 and NO3 losses during composting and after manure applications and offers a mechanism for developing slow release fertilizers [91]. Antonious et al. [14] revealed that total metal content of soil and/or soil amendments is not a good predictor/ indicator of the metal concentrations in plants due to the fact that only a fraction of the total metals is available for plant uptake.

**Figure 4.** Yard waste made from yard and lawn trimmings, and vegetable remains obtained from Con Robinson Con‐ tracting Company, Inc. Lexington, Kentucky, USA.

Municipal SS, CM, horse manure (HM), and yard waste (YW) compost (**Figures 1**, **2**, **3**, and **4**) provide amendments useful for improving soil structure and nutrient status. However, soil amendments contain heavy metals that may potentially affect soil microbes and the enzymes they produce. Accordingly, it is important to monitor NH3 + , NO3 - , phosphate, and trace elements (Ni, Pb, Cd) concentrations in native soil amended with SS, CM, and their transport into runoff, seepage water, and potential bioaccumulation in edible plants at harvest [92]. According to the USEPA Part 503 Biosolids rule, 10 elements (As, Cd, Cr, Cu, Hg, Mo, Ni, Pb, Se, and Zn) in SS applied to soil are regulated [93]. The USEPA has defined limits for clean sludge in terms of its trace elements content (Zn 1400, Cu 1500, Ni 420, Cd 39, Pb 300, Cr 1200, Mo 75 mg kg-1) and reported that sludge could be added to agricultural soil if these elements are below the standard disposal limits [94]. However, total metal concentrations in SS or even native soil do not furnish sufficient information regarding the potential availability of elements for plant uptake. Antonious et al. [14] found that the total concentration of each metal in the soil was significantly greater than concentration of metal ions available to plants. Accumula‐ tion of trace elements in plants grown in biosolids (sewage sludge) varied among plant species [92] and even among accessions of the same species [95]. Intake of trace elements‐contaminated vegetables and fruits may pose risk to human health. Application of SS and CM to agricultural soils is based on crop requirements of NPK that are usually applied with a maximum appli‐ cation rate of 15 t year-1 [14]. In fact, there is no specific rate for animal manure application to soil, but the rate of application is proposed through good agricultural practices of fertilizer use. **Table 2** shows the bioaccumulation factor (BAF) of seven heavy metals from soil into pepper fruits grown under four soil management practices at Kentucky State University. BAF is defined as the concentration of metal in plant tissue divided by the concentration of the same metal in soil, expressed on dry weight basis. According to Antonious et al. [95], BAF values less than one are desirable and represent levels that do not pose human health hazards. Results revealed that BAF values in pepper fruits ranged 0–0–0.01 for Cr; 0.1–0.4 for Ni; 0.7–2.0 for Cu; 1.0–1.4 for Zn; 0.9–1.2 for Mo; 0.7–1.0 for Cd; and 0.2–0.8 for Pb.

indicator of the metal concentrations in plants due to the fact that only a fraction of the total

**Figure 4.** Yard waste made from yard and lawn trimmings, and vegetable remains obtained from Con Robinson Con‐

Municipal SS, CM, horse manure (HM), and yard waste (YW) compost (**Figures 1**, **2**, **3**, and **4**) provide amendments useful for improving soil structure and nutrient status. However, soil amendments contain heavy metals that may potentially affect soil microbes and the enzymes

elements (Ni, Pb, Cd) concentrations in native soil amended with SS, CM, and their transport into runoff, seepage water, and potential bioaccumulation in edible plants at harvest [92]. According to the USEPA Part 503 Biosolids rule, 10 elements (As, Cd, Cr, Cu, Hg, Mo, Ni, Pb, Se, and Zn) in SS applied to soil are regulated [93]. The USEPA has defined limits for clean sludge in terms of its trace elements content (Zn 1400, Cu 1500, Ni 420, Cd 39, Pb 300, Cr 1200, Mo 75 mg kg-1) and reported that sludge could be added to agricultural soil if these elements are below the standard disposal limits [94]. However, total metal concentrations in SS or even native soil do not furnish sufficient information regarding the potential availability of elements for plant uptake. Antonious et al. [14] found that the total concentration of each metal in the

+ , NO3 -

, phosphate, and trace

metals is available for plant uptake.

168 Organic Fertilizers - From Basic Concepts to Applied Outcomes

tracting Company, Inc. Lexington, Kentucky, USA.

they produce. Accordingly, it is important to monitor NH3



Each concentration in the table is an average of three replicates ± standard error.

YW = yard waste; SS = sewage sludge; CM = chicken manure; and NM = no‐mulch bare soil.

**Table 2.** Concentrations and bioaccumulation factor (BAF) of seven heavy metals in pepper (*Capsicum annuum* L.) fruits of plants grown under four soil management practices.

#### **6.1. Quantification of trace elements (Ni, Pb, Cd) in soil**

To monitor the concentration of trace elements in soil following the incorporation of soil amendments, samples collected from field plots should be oven‐dried at 105°C and ground manually with a ceramic mortar and pestle to pass a 1 mm sieve. Ten ml of concentrated nitric acid (HNO3) is added to each 1‐g of sieved dry powder, and the mixture is allowed to stand overnight and then heated for 4 h at 125°C on a hot plate. This mixture should be diluted with double‐distilled water (50 mL) and filtered through filter paper No.1 before quantification of heavy metals. Mehlich‐3 extractable Cd, Ni, and Pb can be determined in soil extracts using inductively coupled plasma (ICP) spectrometer [7, 14, 96, 97].

#### **6.2. Quantification of trace elements (Ni, Pb, Cd) in edible plants at harvest**

For the determination of trace elements in plant tissues, fruit samples of the growing plants of comparable size are collected at random, washed with deionized water, and dried in an oven at 65°C for 48 h. The dried samples should be grounded manually with ceramic mortar and pestle to pass through 1 mm sieve. Bioavailability of heavy metals can be defined as the total metals available in the soil, whereas the bioaccumulation factor (BAF) is defined as the ratio of the metal in the plant divided by total metal in the soil [98]. As described earlier, BAF values below 1 are desirable and present levels that do not pose human health hazards, while BAF values >1 would be less favorable. Assessing the bioavailability and speciation of trace elements in native soil and soil mixed with organic amendments is crucial to determining the environmental impact of contaminated soils.
