The influence of compost amendments on bioaccumulation of potentially toxic elements by pea plant cultivated in mine degraded soils

Potential toxic metal (PTE) accumulation in soil and water is one of the major sources of food crop contamination. PTE remediation from soil can be enhanced by the addition of organic matter to the growing media. An experiment was carried out to investigate the effect of different organic amendments on the accumulation of PTEs in pea plant grown on mine degraded soils. Mining soils from chromite mine (CM), soap stone mine (SSM), manganese mine (MM), and quartz mine (QM) were mixed with vermicompost (VC), leaf mould (LC), and spent mushroom compost (SMC) along with garden soil at 1:1:1 ratio. Various growth and yield-related attributes of pea plant as well as PTE concentrations in soil and plants were studied. The highest Cd (2.62 mg kg−1) and Cr (13.6 mg kg−1) concentration was reported in CM soil, while Pb (23.3 mg kg−1) and Mn (59.2 mg kg−1) concentration in SSM and MM soil, respectively. Mining soils significantly reduced the plant growth and yield, while organic amendments reduced the PTE availability and increased pea plant growth. Comparing the various organic fertilizers used, it was observed that VC efficiently reduced Cd, Cr, Pb, and Mn uptake by pea plant, subsequently, improved pea plant growth and productivity. In order to assess the effects of various amendments on PTE health risk reduction, various risk indices including plant tranfser factor, average daily intake, health risk, target hazard quotient, and target cancer risk were also calculated and the results revealed that application of compost particularly VC significantly reduced the dietary intake and health risks of PTEs.


Introduction
Mining activities disturb the surrounding environment badly and the toxic metals secretion from them negatively affect soil and water (Nouri et al. 2009). Toxic metals from mining can reach the soil mainly through water and subsequently contaminate edible crops. They can affect soil environment for a longer period of time due to their non-degradable properties and longer half-lives. Plants can absorb many different elements from soil, while some of them are essential for plants; others are non-essential and accumulate in various plant parts. PTEs may include plants essential (copper (Cu), manganese (Mn), and zinc (Zn)) and non-essential (cadmium (Cd), manganese (Mn), lead (Pb), chromium (Cr), etc.) elements. Growing plants on toxic metal polluted soils may affect the plant's metabolism Responsible Editor: Amjad Kallel Muhammad Irfan and Muhammad Azhar Shah equally contributed in this work. and biochemical processes, which results in restricted growth, poorer biomass production, and metal accumulation (Iftikhar et al. 2021;Khan et al. 2015a,b;Alam et al. 2020a). One of the common consequences of PTE toxicity is the formation of metal complex within the plant cell, after being accumulated (Kalaivanan and Ganeshamurthy 2016). Similarly, toxic metal accumulation can lead to the formation of reactive oxygen species (ROS), which can cause inactivation of enzymes, DNA damage, and its interaction with other vital plant cell constituents (Pourrut et al. 2013). Likewise, a large number of physiological and biochemical processes in plants are also negatively affected by metal toxicities. Toxic metals once absorbed by the plants can interfere with physiological activities including but not limited to photosynthesis, gaseous exchange, and nutrient absorption, consequently causes reductions in plant growth and yield (Tauqeer et al. 2021a;Khan et al. 2016). PTEs have detrimental effects on soil health, plant growth, biomass, and chlorophyll contents (Turan 2022(Turan , 2021. Furthermore, these PTEs interfere with uptake of essential nutrients and alter the vital functions of plants resulting in low water flow, nutrient deficiency, and affect photosynthesis. These toxic metals also have detrimental effects on biochemistry, physiology, and morphology of plants (Tauqeer et al. 2021a). They may also interfere with the levels of antioxidants, and reduce the nutritive value of the plants (Khan et al. 2015b;Sharma and Agrawal 2005).
Human, upon the consumption of toxic metal contaminated crops, can acquire chronic diseases including cancer, kidneys damage, cardiovascular diseases, gastrointestinal infections, and neurological and psychological issues, such as tremor, restlessness, anxiety, sleep disturbance, and depression (Mebrahtu and Zerabruk 2011;Muller and Anke 1994). Similarly, consumption of food containing toxic metals can significantly reduce the essential nutrients in the body and causes gastrointestinal cancer (Oliver 1997).
Normally, toxic metals are persistent in nature and accumulate in soil, water, and plants for longer periods (Tauqeer et al. 2021b;Paz-Ferreiro et al. 2014;Chehregani et al. 2005). Therefore, their removal through eco-friendly techniques is of utmost important (Tauqeer et al. 2021b). In this context, mixing organic matter to the contaminated soils can play a vital role (Turan 2022). Essentially, the goal of any remediation experiment based on soil amendments should be done to achieve maximum reduction in the bioavailability of PTEs by immobilization in soil. Various organic and inorganic amendments have been practiced in order to remediate or immobilize toxic metals from contaminated soils including municipal and biosolid composts, farmyard manures, sewage sludge, bark chips, woodchips, vermicompost, lime, charcoal, fly ash, and biochar (Turan 2022; Ahmad et al. 2014;Clemente et al. 2007;Somerville et al. 2018;Buema et al. 2020;Khan et al. 2020;Rozek et al. 2021). Soil amendments reduce metal toxicity through metal immobilization that may ultimately affect their bioaccumulation in cultivated plants. The immobilization of toxic metals is done through adsorption, precipitation, and formation of iron plaque and other complexes that transform toxic metals from solution to solid form reducing their movement in the soil and plants bioavailability (Iftikhar et al. 2021). Application of organic amendments help to enhance biomass production (Tauqeer et al. 2021a), by increasing soil fertility, improving soil enzymatic activities, enhancing soil pH, and reducing the available fraction of toxic metals (Turan 2022). Organic amendment enhances the binding capacity of toxic metals, improves the availability of soil organic material, and reduces their mobility in the soil (Turan 2021; Liu et al. 2007). Similarly, organic fertilizers have the ability to reduce phytotoxicity and bioavailability of PTEs in soil. Studies have shown that these organic fertilizers can significantly reduce the concentrations of toxic PTEs in soil (Alam et al. 2020b;Park et al. 2011). Soil amended with organic fertilizers can substantially improve plant biochemistry, antioxidant enzymes, and nutrition (Turan et al. 2018a,b); it was also reported that application of organic amendments have positive impacts on alkaline phosphatase, acid phosphatase, dehydrogenase, catalase, phosphomonoesterase, B-glucosidase, protease, and urease activities in the amended soil (Turan 2020). Similarly, organic amendments significantly reduced PTE translocation to the sub soil (root) and aerial tissues and efficiently developed soil moisture content, improved photosynthesis, and enhanced micronutrient and macronutrient contents resulting in improved nutritional quality of the cultivated crops (Turan 2019).
The present study was conducted to compare the efficiency of various organic fertilizers for the recovery of mine degraded soils and the uptake, availability, and translocation of PTEs in pea plant.

Soil sample collection
Soil samples were collected near mining sites including soap stone mines (SSM), manganese mines (MM), quartz mines (QM), and chromite mines (CM) from 0-to 25-cm depth at 0-20-m interval. Soil samples were collected from the mining sites of Prang Ghar, Mohmand Agency, having 34.43° longitude, 71.59° latitude (Fig. 1). Six subsamples were collected from each mining site and thoroughly mixed to make a composite sample. Further, the homogenized samples were air-dried and passes through 2-mm sieve. Physiochemical characteristics comprising pH, electric conductivity (EC), and organic matter contents (OM) were determined for each sample separately. The soil was then analyzed for total metal concentrations using standard method (Nezhad et al. 2014). For PTE analysis, 0.5 g of soil samples were digested with aqua regia; after digestion, the soil samples were cooled to room temperature, filtered, and diluted with distilled water, and concentration of metals were determine by using atomic absorption spectroscopy (AAS-Perkin-Elmer model 2380).

Growing media preparation for pea plants
Mine soils from CM, SSM, QM, and MM were mixed separately with garden soil. These mixtures were then amended separately with three different composts including vermicompost (VC), leaf compost (LC), and spent mushroom compost (SMC) at 1:1:1 ratio. 5 kg of each separate growing media were then filled in plastic pots. A composite sample from each mining soil and composts were sampled for analyzing its various characteristics (Table 1).
Pea (Pisum sativum cv. Peshawar local) was sown in pots containing growing media inside the green house. Two seeds were sown in each pot. After sowing, appropriate agronomic practices were performed throughout the experiment. All pots were regularly irrigated with distilled water to maintain the minimum required water level. Some plant growth and yield attributes including days to seed emergence, plant height, stem diameter, chlorophyll content, fresh weight, dry weight, pod length, no of pods plant −1 , no of seeds pod −1 , and 100 seed weight were acquired during the course of study.

Preparation of pea seeds for PTE analysis
Once the pea pods were ready for harvesting, pods were randomly selected and collected separately from each treatment in replicates in sterile bags and brought to the laboratory. Seeds from pods were carefully removed and kept in sterile bags. They were then washed with distilled water and then dried in oven at 70 °C. The dried seeds were grounded to powder. For further analysis of PTEs in the samples, 0.5 g of powdered sample was taken in digestion tubes, with 10 ml of concentrated nitric acid (HNO 3 ) and kept overnight at 24 ± 2 °C. Then, 5 ml perchloric acid (HClO 4 ) was added and kept in digestion chamber, until complete digestion happened with the appearance of white fumes. The digested samples were cooled down and filtered. Then, the volume was raised to 50 ml by adding distilled water (Alam et al. 2020b;Khan et al. 2010). Finally, PTE analysis was performed with the help of atomic absorption spectroscopy (AAS-Perkin-Elmer model 2380).

Plant transfer factor
The soil to plant transfer of PTEs was calculated using following formula (Cui et al. 2005;Khan et al. 2010)

Average daily metal intake
To know the average daily intake (ADI) of toxic metals through the consumption of peas that were cultivated on mixed soil, the following formula was used (Alam et al. 2018;Khan et al. 2016) In the above formula, ADI is the average daily metal intake, C metal indicates the amount of metals in peas (mg kg −1 ), C factor is conversion factor (fresh plant into dry weight) which is 0.085, D food intake is intake of peas on daily basis when it is available (0.345 kg person −1 day −1 ) and BW average weight is the consumer average body weight 75 kg (Khan et al. 2010).

Health risk assessment
Health risk index (HRI) is normally computed to determine the risk of toxic metals associated with the consumption PTF = Cplant Csoil ADI = C metal × C factor × D food intake BW average weight of contaminated vegetables in daily diet. Actually, these are not accurate values but rather an estimation of risk associated with consumption of contaminated vegetables. HRI was calculated by using the following formula.
where ADI is average daily dietary intake of metal through contaminated vegetables and RfDo is oral reference dose value (mg kg −1 day −1 ), which is a safe level of human exposure for life time. RfDo values were taken from USEPA (2006). The cancer risk assessment (TCR) and noncancer risk (target hazard quotient (THQ) were assessed for the potential consumers of the vegetable grown in contaminated soil amended with different organic material using the following formula.
In the equations, MC is metal concentration, I is ingestion rate (255 g person −1 day −1 ), EFr represents exposure frequency (350 days year −1 ), ED is the total exposure duration (70 years), BW represent average body weight, CPSo is carcinogenic potency slope (μg g −1 day −1 ), and AT is average time (ED × 365 days year −1 ).

Statistical analysis
The experiment was carried out in completely randomized design with two factors replicated three times. All the data were statistically analyzed using statistical package, Statistix version 8.1, to determine any significant effect of different treatments on metal concentration and bioaccumulation. The significance levels P < 0.01 was used to show the significant differences among treatments used. When data were found significantly different, then least significant difference at 1% level of significance was performed by using LSD test. Graphs were prepared using sigma plot 10.0.

Soil and organic amendment physiochemical characteristics
The soil collected for pot experiment was neutral to slightly alkaline in nature, and pH was different for different mine soils. an increase in soil pH was observed for VC (8.23), LC (8.18), and SMC (8.12). Organic amendments increased soil pH because of their alkaline nature, dissolution of their carbonates, and oxides in amended soil. Increase in pH reduces bioavailability and mobility of PTEs in soil (Hargreaves et al. 2008). Khan et al. (2013a,b) reported that with the addition of compost to contaminated soil, an increase in the pH (4.0 to 5.4) was observed and significantly increase in EC (38.7 to 992 µS/cm) was reported. The soil pH plays an important role in the bioavailability and plant bioaccumulation of PTEs. In another study, it was reported that biochar application increased the soil pH, which in turn immobilized PTEs (Beesley et al. 2010;Khan et al. 2020).
Similarly, the addition of organic amendments can increase the content of essential nutrients such as phosphorous (P), potassium (K), boron (B), magnesium (Mg), calcium (Ca), and sodium (Na), as well as reduce the solubility, bioavailability, and accumulation of metals in plants (Walker and Schüßler 2004). The higher postharvest PTE concentration in amended soil compared to contaminated soil indicated that compost application has significant effect on PTE mobility and bioavailability (Fig. 2). The compost resulted in immobilization of PTEs may be attributed to increase in pH and formation of metal complexes. Previous studies revealed that application of compost to contaminated soil significantly increased soil pH and resulted in metal immobilization (Soares et al. 2015), which is attributed to the presence of higher concentration of organic carbon in the compost (Beesley et al. 2014). Iftikhar et al. (2021) observed that in amended soil, the toxic metals were immobilized by precipitation and metal ion complex formation. Thus, the application of organic amendments is an important approach for the immobilization and remediation of toxic metals in contaminated soil (Angelova et al. 2013).

Effect of organic amendments on plant physiological parameters
In the current study, the organic fertilizers has shown good results, when mixed with mine soils, on plant physiological parameters, i.e., days to seed emergence, plant height, stem diameter, chlorophyll content, fresh weight, dry weight, pod length plant −1 , number of pod plant −1 , number of seeds pod −1 , and 100 seed weight (Table 2). Results indicated that amendment of organic fertilizers has both synergistic and antagonistic effects on plant physiological properties depending upon the type of soil and amendment used. Decrease in days to seed emergence were observed with all the amendments in all contaminated soils. The highest decrease of 27.8% with the application of VC in MM was observed. The results were similar to Wang et al. (2003), who also reported a decrease in days to seed emergence. The maximum increase in plant height was observed in VC amended GS, while the maximum decrease, compared to control, in plant height was observed in QM with no amendment. The decrease in plant height might be due to the effect of PTEs present in the soil that it can limit the growth of plants and finally effect its productivity (Shafiq and Iqbal 2005;Shanker et al. 2005). PTEs present in soil severally affect the availability and accumulation of essential nutrients by plants, thus affecting their growth (Khan et al. 2016;2019). The highest increase of stem diameter was observed in GS amended with VC. These results were similar to the findings of Arancon et al. (2005) and Atiyeh et al. (2000), while the decrease in stem diameter was observed in soil contaminated with QM. Similarly, significant increase was observed in chlorophyll content of pea plant cultivated in VC amended GS. Our finding was in agreement with that of Arancon et al. (2005), who reported that vermicompost not only increases the plant growth but can also enhance the chlorophyll contents in most of the vegetables. A positive impact on plant photosynthetic activities was also observed by Turan (2019) when the contaminated soil was amended with organic fertilizers. Like other plant growth attributes, plant grown in QM contaminated soil resulted in decrease of chlorophyll content compared to control. Fresh weight and dry biomass showed similar effects to mine impacted and compost amended soils. The maximum weight was observed for plant grown in VC amended GS, while, unexpectedly, maximum decrease was observed in SMC amended QM soil. These results are in agreement with the findings of Edwards et al. (2007). In another study, Sönmez et al. (2016) reported that the application of organic amendments to soil significantly increases the essential macronutrient concentration responsible for plant growth and biomass production. In our study, the increase in plant height and biomass may be attributed to the contribution of essential macronutrients and micronutrients by organic amendments alongside metal immobilization. The proportion of grain to straw in the yield improves with the addition of compost and also increased the total dry biomass of the crops. A decrease in biomass after compost amendment was reported by Manivasagaperumal et al. (2011). The effect on pea pod is an important factor for agronomic purposes, because changes in pods parameters have a direct impact on productivity and economic of the formers. In this study, the impact of compost amendment was observed on pod length, numbers of pods, number of seed per pod, and seed weight; the results revealed that pod length showed maximum increase under VC amendments, while pod length was decreased in mine impacted soil, with maximum decrease in CM; similarly, the number of pods plant −1 also showed maximum variation with the application of VC. Maximum increase in number of seeds pod −1 and 100 seed weight was observed in plants grown in VC amended GS. The results revealed that VC was the most effective treatment to improve plant growth, survival, and biomass production. The results were more satisfactory when the VC was applied to soil with no or low contamination. This is because in contaminated free soil, compost improves the fertility thus enhancing plant growth, while in contaminated soil, most of the compost served to immobilized PTEs by forming metal complexes, along with improving soil basic physiochemical parameters (Table 2). Previously, it was reported that amendment of PTE contaminated soil with compost reduced PTE uptake and improve plant survival (Gadepalle et al. 2007). Soil organic matter has the potential to bind toxic metals and moderate soil toxicity (Datta et al. 2001). Duong et al. (2013) reported that application of compost can improve the soil quality and productivity as well as sustainability of agriculture production. Organic matters play important role in improving chemicals, physical properties, and biological properties of the soil. Soil organic matter and clay particles can bind each other to improve soil structure. Compost amendment improves soil structure by reducing bulk density and increases the soil porosity and water holding capacity of the soil (Ngo et al. 2011;Song et al. 2015). The increase in water holding capacity facilitates water mobility from soil to the leaves via xylem and enhanced the production of essential macronutrients and chlorophyll contents (Suliman et al. 2017;Hafeez et al. 2017). Similarly, application of organic amendments has positive impacts on soil fertility by increasing soil phosphorous (Sönmez et al. 2016) and nitrogen contents (Turan et al. 2019), which are the essential nutrients required for plant growth. PTEs in pea seeds and compost effects PTE concentration in pea seeds cultivated in contaminated and compost amended soil showed significant variations (p < 0.01). The highest concentration of Cd, Cr, and Pb was reported in CM contaminated soil, while Mn was reported in MM contaminated soil (Fig. 3). The application of VC, LC, and SMC showed significance (p < 0.01) reduction in the bioaccumulation of PTEs in pea seeds. The organic fertilizer amendments had varying effects on PTE uptake by pea plant. Toxic metal bioaccumulation was efficiently reduced in all the treatments as compared to contaminated soil. The result indicated that the amendment had good effects on restricting PTE uptake by pea plant depending upon amendment used.
The results showed that a higher reduction was observed in the accumulation of selected PTEs in VC amended soil. Among the various composts, CM amended with VC was the most efficient in decreasing PTE concentration in pea seeds. Cd concentration was reduced from 0.98 mg kg −1 in CM contaminated soil to 0.13 in VC amended soil (87% reduction), while Cr concentration was reduced from 2.98 mg kg −1 in CM contaminated soil to 1.94 mg kg −1 in VC amended soil (35% reduction). Similarly, Mn uptake was reduced with the application of VC from 19.67 in MM contaminated soil to 8 mg kg −1 in VC amended soil (59% reduction) and Pb concentration was reduced from 0.33 in CM contaminated soil to 0.18 mg kg −1 with VC application (45% reduction). The average highest reduction was shown by VC amended CM soil (Fig. 3). Among the selected PTEs, the maximum reduction for Cd and Pb was observed in VC amended GS, for Cr in VC amended SSM contaminated soil, and for Mn in VC amended CM contaminated soil. The maximum reduction in Cd concentration by VC, LC, and SMC was observed in MM contaminated soil, Pb concentration was significantly reduced by VC in MM contaminated soil, and LC and SMC in QM contaminated soil. Almost similar reduction in Cr and Mn bioaccumulation was observed in compost amended soil compared to mine impacted soil. Overall, our experiment showed that the accumulations of Cd, Cr, Mn, and Pb were reduced efficiently in pea seeds following VC, LC, and SMC amendment. Among these three amendments, VC application showed the best results in decreasing the accumulation of PTEs in pea seeds, as compared with other composts (LC and SMC). Our results are supported by the findings of Walker and Schüßler (2004) who reported that organic applications reduced the solubility and bioavailability of PTEs in contaminated soil, by forming metals complexes (Barancikova and Makovnikova 2003). It might be due to the increase in the soil pH with different amendments which ultimately reduces its concentration. Similarly, in another study conducted by Alam et al. (2020b), a reduction in heavy metal accumulation in reddish was observed with application of compost, particularly VC (Table 3). Composts prepared from different raw materials have varying effects on heavy metals mobility and their bioaccumulation in food crops. As shown in Table 3, some organic amendments have significantly reduced metal bioaccumulation, while others have no significant effects. Sewage sludge slightly reduced Cd and Cr bioaccumulation with no effects on Pb in rape, while in sunflower, the concentration of heavy metals were higher in amended soil compared to control (Kominko et al. 2022). Similarly, biochar application at different concentration significantly reduced heavy metal bioaccumulation in tomato (Alam et al. 2020a). These varying effects of organic amendments on heavy metal bioaccumulation may be attributed to the properties of the organic amendments used, as feed stock used for preparation of compost may also have traces of heavy metals which may result in higher heavy metal bioaccumulation (Table 3). Furthermore, the organic amendments result in precipitate and iron plaque formation that reduces the mobility and bioavailability of toxic metals (Iftikhar et al. 2021). Same observations were recorded by Bian et al. (2013), who reported a significant decrease (20-90%) of Cd in rice grains with the application of VC. Soil health and fertility are closely associated with vermicompost as it contains high nutrients (Suthar 2007). At higher pH (> 6), sulphur oxide charge increases with the chelation of organic matter and the precipitation of metal hydroxides that reduces the concentration of metal ions (Mouta et al. 2008). The addition of organic fertilizers maintains a positive nutritive balance and enhances soil quality (Medina et al. 2006). Due to compost, the free ion chelates make strong bonds with the PTEs and increase the soil pH and EC, thus reduced metal bioavailability and uptake by plants. The application of compost has the ability to bind metals, and the plants do not accumulate them, and thus, plants have better growth and yield. The application of organic compost led to the effective binding of phyto accessible form of different PTEs in soil and makes them unavailable to plants (Angelova et al. 2010). Organic matter decomposition results in a reduction in the mobility of metals in the soil and release salts of phosphates, and carbonate minerals, which help in the formation of insoluble metal complexes (Walker & Schüßler 2004). Roberts et al. (2007) and Singh and Sharma (2003) reported that the application of vermicompost increased the crop yield, soil nutritional status, and nutrient uptake, and reduced the effect of toxicity. Through adsorption reaction, PTEs immobilize in soil with the addition of organic fertilizers. This may be due to the bindings of metal compounds (Khan et al. 2017).

Plant transfer factor
The soil to plant transfer (PTF) of PTEs in contaminated and amended soil is given in Fig. 4. The PTF values showed great variation among different PTEs and amendments used. The soil to plant transfer of PTEs depends on soil physiochemical parameters including pH, EC, soil organic matter, soil structure and texture, geology, climatic factors and types, and concentration of amendments used (Khan et al. 2017 Other treatments and PTEs also showed similar variation in contaminated compared amended soil PTF values (Fig. 4). In order to assess the exposure and health risk of contaminated vegetables, PTF is an important components to be considered.

Health risk reduction
Human beings are exposed to PTE contamination through various exposure pathways, including ingestion of contaminated food and water. Plants grown in metal contaminated soil are consumed by the local residents will, certainly, ingest these PTEs resulting in various health disorders. The average dietary intake (ADI) values (mg kg −1 day −1 ) for individuals by consumption of pea plants grown in soils with different organic amendment are given in Table 4. The ADI values of different metals in contaminated and amended soils showed substantial variation, for both children and adults, depending upon amendments used (Table 4). Although substantial variation was observed in ADI values for different contaminated and amended soil cultivated pea plants, all the values were below the critical limit (ADI < 1). The highest ADI values for all the studied PTEs both children and adults were reported pea plant grown in CM contaminated soil except for Mn, while the lowest ADI values of PTEs through the consumption of pea seeds were observed in soil amended with VC. For instance, a reduction of 3-59%, 18-25%, 27-36%, and > 100%, for Cd, Cr, Pb, and Mn, respectively, was observed in plants grown in composted amended CM soil compared to contaminated soil, while in compost (VC, LC, SMC) amended SSM soil, the Cd, Cr, Pb, and Mn dietary intakes were reduced by 7-58%, 18-32%, 12-32%, and 3.8-7.3 folds, respectively.
Consumption of pea plants grown in QM soil amended with compost (VC, LC, SMC) showed a reduction of 3-42%, 13-21%, 21-49%, and 6-33% for Cd, Cr, Mn, and Pb, respectively. Similarly, the ADI was reduced by 1.65-56%, 16-23%, and 4.74-8 folds and 25-40% for Cd, Cr, Mn, and Pb, respectively, for pea plants cultivated in compost amended MM soil. HRI values of different PTEs amended with different composts are given in Table 5. HRI values less than 1 showed that the material is assumed to have minimal acceptable risk. HRI were found less than 1 for both children and adults in all contaminated and amended soils including control. Among the mine impacted soil, the highest HRI was observed for CM contaminated soil followed by MM. Similarly, in different amendments, VC was the most effective in reducing the potential health risk through consumption of contaminated vegetables in all contaminated vegetables. Among different age groups, children were more at risk compared to adults. The THQ is often used as a tool for assessment of potential health risks resulting from consumption of metal contaminated food. The THQ values calculated for pea seeds are presented in Table 6. Both increasing and decreasing patterns were observed in THQ values of different amendments as compared to contaminated and control soils. The THQ for Cr and Mn was > 1 for all treatments, while for Cd in all treatments except for VC amended GS. Similarly, for Pb, all the treatments have THQ < 1. Comparatively, the VC amendments have significant  Table 6. The TCR value was not calculated for Cd and Mn because we could not found any CPSo values for these two elements. The TCR values for both children and adults were below the critical limit of 1 indicated minimal exposure and risk. The highest TCR values for Cr were 1.01E − 02 and 6.75E − 03 in children and adults, respectively, while for Pb were 1.91E − 05 and 1.27E − 05, respectively. Like other risk indices, the maximum reduction was observed with application of VC. Food chain is one of the most important pathways to PTE exposure through the consumption of contaminated vegetables (Khan et al. 2008(Khan et al. , 2016. The consumption of toxic metals in vegetables grown on contaminated soil can be managed by the application of organic fertilizers, and thus, ADI, HRI, and THQ and TCR can be reduced. In the present study, among different amendments, VC was the most effective treatment to reduce the plant metal uptake and their subsequent health risks.

Conclusions
Organic fertilizers, such as VC, LC, and SMC, have substantial impact on contaminated soils and can play an important role in sustainable farming system by enhancing the yield and biomass of food crops; improving the soil structure, nutrient availability, and metal retention; and minimizing the human health risk. It was observed that the application of VC showed maximum reduction in Cd, Cr, Mn, and Pb availability in pea seeds among the three amendments, followed by LC and SMC. The results of the present study showed that the use of all the tested organic amendments decreased the bioavailability of PTEs in soil and their subsequent bioaccumulation in pea seeds. As a consequence, vegetable growth, chlorophyll content, and biomass production was enhanced. The ADI, HRI, THQ, and TCR (Cr and Pb only) of selected PTEs for different age groups were also significantly reduced by the application of organic fertilizers. The result of this study indicate that the use of organic amendments such as VC could be an effective management strategy for reducing PTE uptake in food plants and their dietary exposure.