Effects of Bio-Slurry and Chemical Fertilizer Application on Soil Properties and Food Safety of Tomato ( Solanum lycopersicum Mill.)

. Tis study evaluated the efects of bio-slurry (BS) and chemical fertilizer (CF) application on soil properties and food safety of tomato ( Solanum lycopersicum Mill.). A feld experiment consisting of 100% BS (5 ton BS ha − 1 ), 100% CF (90kg N · ha − 1 +30kg P · ha − 1 +13 kg S · ha − 1 ), and control was conducted. Soil samples from all the treatments were collected for their physico-chemical characteristics. Te level of ten heavy metals in experimental soil and tomato fruit samples was also determined. Compared to CF and control, the application of BS improved soil physico-chemical characteristics. Te BC signifcantly reduced the mean concentrations of Cd and Mn in the tomato fruit samples. Te mean concentration of Ni (18.24 ± 0.61, 23.9 ± 0.3, and 9.66 ± 1.2mg kg − 1 ) and Mn (15.4 ± 2.4, 38 ± 3.3 and 21.8 ± 0.99mg kg − 1 ) in tomato fruit samples of BS-treated, CF-treated, and control soil, respectively, was above the safety limit set by the Food and Agriculture Organization/World Health Organization for human consumption. Similarly, the mean concentration of Cd (7.98 ± 0.72 and 3.29 ± 0.37mg kg − 1 ) in tomato fruit samples of CF-treated and control soil was above the safety limit. From this perspective, the consumption of these tomato fruits could be unsafe for human health with respect to Ni, Mn, and Cd toxicities. Te application of BS could remediate the Cd toxicities, yet other scenarios of phytoremediation would be praiseworthy to address Ni, Cd, and Ni toxicities.


Introduction
Vegetables constitute a major source of vitamins, crude fber, protein, antioxidant, and minerals [1]. Specifcally, tomato (Solanum lycopersicum Mill.) is one of the most important vegetable crops for its special nutritive value and widespread production [2]. It is the world's largest vegetable crop after potato and sweet potato [3]. In accordance, the Central Statistical Agency (CSA) of Ethiopia pointed out that Ethiopia is the world's 84 th largest producer of tomato where its national mean yield is 6.2 ton ha −1 [4].
Injudicious application of chemical fertilizer causes environmental pollution, damages soil physico-chemical characteristics, and causes various problems to human health [5][6][7][8][9]. Tis has initiated researchers around the globe to search for eco-friendly alternative fertilizers that would ensure agricultural biosafety and environmental and human health [6]. Bio-slurry (BS) is one of such alternatives, which is an aerobically digested organic material obtained from biogas plants. It is environmentally friendly with only few harmful efects when compared with chemical fertilizers. It contains appreciable amounts of organic matter (20 to 30%) and is much needed for poor soils [10]. Its use can reduce the quantity of chemical fertilizer up to 50% [11]. Specifcally, it is most suitable for farming horticultural crops [12].
Te quality of soil determines environmental health [13] and food safety and quality [14], which could be expressed in terms of physico-chemical indicators [15,16]. Soil organic matter is considered as one of the most important indicators of soil quality as it controls many soil properties, such as nutrient cycling, soil structure maintenance, and pH bufering [9,17]. However, soil contamination by heavy metals is one of the factors that cause soil health deterioration and plant health problems [18]. Among the main sources of heavy metals in the soil are agrochemicals (fertilizers and pesticides) [19]. Te uptake of heavy metals by plants depends on diferent factors, including solubility of heavy metals, soil pH, soil type, and plant species [20]. Vegetables can take up heavy metals (Cu, Zn, Fe, Pb, Cd, Mn, and Cr) from the soil on which they grow [21][22][23], which upon consumption could cause human health problems [24].
Increasing crop productivity by applying chemical fertilizers has been the main objective in most agricultural production systems, whereas product quality and environmentally friendly approaches are given little attention [25]. Increased productivity of cultivated land and higher input use efciency with no harm to the soil and product quality are among the development strategies in vegetable production [25,26]. Particularly, the use of chemical fertilizer is not the appropriate solution to overcome these restraints, exclusively for vegetables that have steadily short time and are consumed fresh. Moreover, the use of chemical fertilizer is not only a threat to human health [27] but also its continuous use may lead to the accumulation of heavy metals in plant tissues, which compromises the nutritional value and food safety [5,8,28]. Terefore, considering environmental and human health risks, sole dependency on chemical fertilizer is not recommended at least for small-scale farmers who can have options of using organic sources of fertilizer [29]. Replacing chemical fertilizer with BS reduces soil pollution and food safety problems [11,30].
In most areas of Ethiopia, the use of BS as a source of fertilizer is uncommon, even among farmers who have access to bio-slurry. In this regard, although a considerable number of farmers around the study area own biogas plants and have access to BS, the tradition of using it as organic fertilizer for tomato production is missing primarily due to lack of awareness and proper agricultural extension services. In spite of this, tomato is widely grown, primarily with the use of chemical fertilizers. In the study area, previous studies exploring how soil qualities and tomato food safety are afected by the application of BS and chemical fertilizer are lacking. Terefore, determining the efects of BS and chemical fertilizer application on (1) soil physico-chemical properties, (2) heavy metal concentration levels in soil and tomato fruit samples, and (3) the food safety of tomato fruits for human consumption were the objectives of the current study.

Description of the Study Area.
Hawassa University main campus agricultural research farm was used for the experimentation. Hawassa University is located at 275 km south of Addis Ababa, the capital city of Ethiopia. It is situated at an elevation of 1768 meter above sea level ( Figure 1).

Agro-Climatic Conditions.
Hawassa receives a bimodal rainfall, where March to June is the main cropping season for planting late and mid-maturing maize varieties. Te months of June to October are used for growing early maturing crops, such as maize and pulses [31]. Te average mean monthly rainfall during the cropping period was 146.78 mm, while the maximum and minimum temperature during the same period was 30.8°C and 14.3°C, respectively. In addition, the monthly rainfall and mean monthly maximum and minimum temperature recorded during 1990-2019 are presented in Figure 2.

Agriculture and Soils.
Agriculture is the dominant means of livelihood for the majority of people living around the study areas. Vegetables (head cabbage, tomato, onion, and carrot), perennial crops such as ensete (Ensete ventricosum), chat (Catha edulis L.), cofee, and avocado, and annual crops such as maize, sweet potato, and haricot bean are widely grown [31]. Te soils are tropical Andosols with textural class ranging mostly from sandy loam to silty loam [29].

Experimental
Design, Treatments, and Procedure. Te experimental feld was ploughed, and plots were leveled manually. Tomato variety Venes was used as a test crop. Te tomato seeds were planted in a germination box in a mesh house. After 42 days, healthy and vigorous seedlings with four true leaves were transplanted to the experimental plots. Seedlings failed to establish were replaced within a week of transplanting to maintain the appropriate plant population.
Te experiment was laid out in a randomized complete block design with three replications. CF and BS were used as fertilizers. Tere were three treatments, including 100% BS (5 ton ha −1 ), 100% CF (92 kg N·ha −1 + 30 kg P·ha −1 + 13 kg S·ha −1 ), and control (Table 1). Te full dose of CF was applied at transplanting following Ethiopian Agricultural Research Organization (EARO) recommendation [33]. Te bio-slurry was obtained from a model farmer who resided close to the study area and had a biogas plant. Cow dung was the main feedstock.
Tere were 9 plots (Table 2), each measuring 3 m * 6 m (18 m 2 ). Te spacing between each block and plot was 1.5 and 1 m, respectively. Te spacing between each row and plant was 0.5 and 0.3 m, respectively. Tere were six rows, each with six plants, comprising 36 plants in each plot. All agronomic practices (weeding, cultivation, supplementary irrigation, etc.) were employed [34]. Weeding was done manually with a hand hoe four times between transplanting and harvesting. Tomato fruit samples were harvested at fourday interval, the frst harvest was done on the 85 th day after transplanting, and the fnal harvest was done on the 100 th day after transplanting.

Plant Protection Measure.
During the experimental period, the treatments were regularly observed for the occurrence of disease, pest, or any other kinds of disorders, and data were recorded. Ridomil gold MZ 68 WP 400 g/200 L fungicide was applied at a ten-day interval to control late blight and leaf blight.  Applied and Environmental Soil Science

Soil Sampling and Analysis for Physico-Chemical
Properties. Soil samples were collected from the top 20 cm depth in a zigzag manner using soil auger prior to transplanting. Te samples were thoroughly mixed to form one composite sample and were analyzed for physico-chemical properties. At harvest, a composite sample was taken from each of these plots at the same soil depth for physicochemical analysis.
Organic carbon (OC), pH, cation exchange capacity (CEC), total nitrogen (TN), available phosphorus (P), and exchangeable bases (Ca, Na, and K) were analyzed following the established procedures and methods.
Pre-and post-experiment soil bulk density, porosity, and moisture content were determined following established and standard procedures and methods.

Bio-Slurry Sample for Physico-Chemical Analysis.
A sample of BS of 0.5 kg was analyzed for pH, OC, TN, CEC, and Av. P using standard methods.

Soil Sample Collection, Preparation, and Analysis for
Heavy Metals. A 0.5 kg composite soil sample from each treatment was prepared and stored in plastic bags. Te sample was properly labeled and placed inside plastic bags and transported to HortiCoop Laboratory, Bishoftu, Ethiopia, for heavy metal analysis. Te soil sample was oven dried at 25°C for 2 days until constant weights were reached. Te sample was then crushed into powder using mortar and pestle, sieved through 2 mm sieve, and then stored in plastic bags until analysis. Te dried 0.5 g soil sample was transferred into digestion vessel containing HCl, HNO 3 , and H 2 O 2 mixture and digested at 100°C for 2 hours. Ten, the sample was removed from block digesters and allowed to cool down, and 40 mL of distilled water was added and mixed well. After cooling to room temperature, the digested sample was fltered using Whatman No. 42 flter paper. Finally, the concentrations of heavy metals (As, Zn, Cd, Pb, Fe, Cu, Mn, Ni, Cr, and Co) were determined using inductively coupled plasma optical emission spectrometry (ICP-OES).
2.9. Bio-Slurry Samples for Heavy Metal Analysis. A 0.5 kg BS sample was analyzed for heavy metals (As, Zn, Cd, Pb, Fe, Cu, Mn, Ni, Cr, and Co) using ICP-OES.

Tomato Fruit Samples for Heavy Metal Analysis.
Six ripe and healthy tomato fruit samples were collected randomly from each treatment and washed with distilled water to remove any adhered contaminants. Te samples were then cut into small pieces using clean sterilized plastic knife and dried in oven at 60°C for 24 hours to remove moisture and maintain constant mass. Te samples were then properly labeled and placed in clean plastic bags and transported to HortiCoop Laboratory, Bishoftu, Ethiopia, for further processing and analysis. Te dried samples were crushed into powder using mortar and pestle and then screened to pass through a sieve of 2 mm mesh size. Te sieved samples were carefully labeled and stored in polyethylene bags and kept in desiccators.
A 0.5 g of homogenized powdered sample was added into a digestion fask and then 10 mL aqua regia (with a 3 : 1 ratio of HCL to HNO 3 ) and 3 mL H 2 O 2 were added. Te mixture was heated at 300°C for 1 h on block digester. After digestion has been completed, the fnal mixture was fltered out using Whatman No. 42 flter paper and then the clear and colorless solution was transferred to a 50 mL volumetric  fask. Te samples were diluted with distilled water, and then the concentrations of heavy metals (As, Zn, Cd, Pb, Fe, Cu, Mn, Ni, Cr, and Co) were determined using ICP-OES. After analysis, the concentration levels of heavy metals in the soil, BS, and tomato fruit samples were compared with the maximum permissible limit (MPL) for the respective parameters according to FAO/WHO [35].

Assessment of Ecological Risk of Heavy Metals in the Soils.
Contamination factor (CF) and pollution load index (PLI) were used to ascertain the potential ecological risk of heavy metals in the soil.

Contamination Factor (CF).
Te CF was used to express the level of contamination of soil by heavy metals [36]. Tis method has been developed further in soil analysis [37,38]. It was calculated as a ratio between the measured concentration of the heavy metal in soil and the preindustrial reference value of the same metal as follows: where CF is the contamination factor, Cs is the concentration of metal in the soil (mg kg −1 ), and Cb is the baseline concentration.

Pollution Load Index (PLI).
Te level of soil contamination by heavy metals was evaluated using PLI that provides a simple and comparative means of assessing the soil quality. Tis parameter allows assessing the level of environmental contamination in order to undertake monitoring or repair activities aimed at improving soil quality [40]. Each sampling site can be evaluated for the extent of heavy metal pollution, employing the PLI method developed by Tomlinson et al. [41] and later applied by Bhutiani et al. [42] as below: where CF is the contamination factor, n is the number of metals studied, and CF n is contamination factor for n th element as mentioned above. According to Tomlinson et al. [41], PLI < 1 denotes no heavy pollution and PLI > 1 denotes heavy metal pollution.
2.14. Statistical Analysis. Statistical Analysis System (SAS) software version 9.4 [43] was used for data analysis. Wherever there was signifcant diference, mean separation was carried out using the least signifcant diference (LSD). Signifcant diference between means of treatments was determined at the 5% signifcance level (p < 0.05).  (Table 3). Te soil is moderately acidic. Moderately acidic soils have a pH ranging from 5.6 to 6.0 [44]. Te OC is in a medium range [45] where soil OC content ranging from 1-2%, 2-4%, and 4-6% is rated as low, medium, and high, respectively. Te TN is within the range of medium [46] where the TN content rated as <0.1, 0.1-0.15, 0.15-0.25, and >0.25% is categorized as very low, low, medium, and high, respectively. Te available P is categorized in the high range (>10 mg·kg −1 ) [47]. Te CEC is in the medium range where CEC value ranging from 5-15, 15-25, and 25-40 cmol·kg −1 is rated as low, medium, and high, respectively [45]. Te concentration of exchangeable bases of Ca, Na, and K is high, low, and very high, respectively [48]. Most of the soil properties appear to be favorable for the growth of vegetables.

Results and Discussion
Te mean concentrations of heavy metals (mg kg −1 ) in preexperiment soil were As (16.5), Pb (27.2), Zn (15.75), Cd (8.62), Cu (0.88), Ni (2.45), Co (0.3), Fe (120.9), Mn (142.1), and Cr (4.53). Except for Cd, these concentrations were below the maximum tolerable limit of FAO/WHO [35] of heavy metal concentrations in the soil. Tis shows that the soil is slightly contaminated with Cd (Table 4). Pesticides and phosphate chemical fertilizers that had previously been used for conducting numerous tests on the same experimental feld could be the most likely sources of the Cd contamination. Cd can enter agricultural soil through various pathways which include application of agrochemicals (synthetic phosphate fertilizers and pesticides) [49,50] and contaminated animal manure [50].
improves the quality of agricultural soil by neutralizing acid condition [52]. Te high organic carbon in BS could be important to maintain nutrient balance by suppressing the mobility of heavy metals and facilitating the decomposition of organic matter [53]. Te high OC in BS could also imply that it can be a good source of plant nutrients. Tis is because increasing OC levels increases overall soil CEC and increases the ability of soils to store NH 4 + , Ca +2 , Mg +2 , and K + . Such soil conditions would in turn make fertilization more efcient [52,54,55]. Organic materials are slightly alkaline and could improve soil suitability for plant growth [48]. Besides, the higher TN and available P in BS implies that its application can supply the soil with high amounts of TN and P. BS sourcing from livestock contains high concentrations of available nutrients, especially N and P [56,57].
Apart from that, the mean concentrations of heavy metals in BS were within the acceptable range of FAO/WHO [35] except for Cd (Table 4). Te source for higher concentration of Cd in the BS might attribute to the cow dung used as feedstock. Te cow feed was prepared from diverse sources including plant residues and additives. Signifcant amount of heavy metals including Cd can be found in animal manures based on the types of animal feed and the additive used [58,59].

Effects of BS and CF Application on Soil
Physico-Chemical Properties

Soil Physical Properties.
Te application of BS slightly reduced bulk density (0.83 g/cm −3 ) and increased porosity (70%) compared with the application of CF (Table 5). It reduced bulk density by 17% and increased porosity by 11% compared with the CF, featuring positive improvements in soil physical properties (Table 5). Te efect of the application of CF on porosity of soils is minimal. Tis exhibits the advantage of applying organic fertilizer over CF in managing the physical properties of soils over short term. Te decrease in bulk density after the application of BS may be related to the increase in OC, which modifes the porosity and bulk density of soils. White [60] stated that the value of bulk density for soils having high OC ranges from <1 g/cm 3 , that for well-aggregated soils ranges from 1 to 1.4 g/cm 3 , and that for sandy soils ranges from 1.4 to 1.8 g/cm 3 . In accordance, the application of BS yielded higher OC, which might be the reason for the increased porosity and decreased bulk density of the respective soils (Table 5). Apart from that, the application of BS increased the moisture content of the soil compared with the application of CF. It increased the moisture content by 58.3% and 58.1% over CF-treated soils and control soils, respectively (Table 5). Findings from previous studies reported that the application of farmyard manure, BS, and other organic fertilizers increases porosity and decreases bulk density, which are associated with increasing water-holding capacity of soils [61][62][63][64].

Soil Chemical Properties.
Te application of BS and CF infuenced the status of soil pH, OC, TN, available P, CEC, and exchangeable bases (Ca, Na, and K) ( Table 5). Te application of BS increased soil pH from moderately acidic (5.6) to faintly alkaline (7.4). In contrast, the application of CF slightly reduced soil pH. It is supported by fndings from previous studies: increment in pH after BS amendment could be explained by decomposition of organic materials that release basic cations K+, Ca 2 +, Mg 2 +, and OH − to the soil and substitute acid cations (H + , Al 3+ , and Fe 3+ ), which would in turn result in a slight increase in soil pH [65]. Application of compost releases alkaline substances and cations such as Ca 2+ , Mg 2+ , and K + , which increase CEC and pH level and counteract soil acidifcation [66]. Te application of organic fertilizers increases soil pH and reduces exchangeable acidity while the application of chemical fertilizer slightly infuences soil pH [62,67].
Te application of BS and CF yielded the highest (2.98%) and lowest (2.5%) soil OC, respectively (Table 5). Te application of BS increased the OC content by 8.4% over the control soil as well as 16.1% over the CF-treated soil. Nevertheless, the application of CF reduced the OC content of the control soil by 8.42%. Tis shows that the application of BS increases OC relative to the application of CF. Tis might be attributed by high amount of organic matter in BS, which enhances the OC content of the soil; organic materials have a major impact on mineralization rates by increasing soil organic carbon directly [68]. Other previous studies also demonstrated signifcantly increasing OC of the soil by the application of organic fertilizers compared to CF [69][70][71][72].
Te application of BS increased the TN (0.26%) contents compared to the control soil (0.22%) and CF-treated soil (0.24%). Te application of BS and CF increased TN by 15.38% and 8.3% over the control soil, respectively (Table 5). BS contains a high concentration of organic nitrogen and contributes to the direct addition of nitrogen from nitrogen fertilizers [56]. Organic fertilizers increase the TN content of soils compared to pre-application [73].
Te available P in the soil treated with BS (59.6 mg·kg −1 ) and CF (42.5 mg·kg −1 ) was higher compared to the same in the control soil (42.3 mg·kg −1 ) ( Table 5). Te soil with available P (mg·kg −1 ) content <3 is very low, 4-7 is low, 8-11 is medium, and >11 is high [47]. Accordingly, the available P contents of all treatments are higher except the soil treated with BS, which is higher than that of the soil treated with CF and control (Table 5). Organic amendments can increase the P recovery in the soil by increasing the P mobility in the soil [74]. It could also be due to the fact that organic fertilizers, on decomposition, solubilize insoluble organic P fractions through the release of various organic acids, thus resulting in a signifcant improvement in soil available P content [57,75].
Te CEC value of the control soil (24.9 cmol·kg −1 ) is lower than the CEC value of BS-treated soil (29 cmol·kg −1 ). Tis was, however, higher than the CEC value of CF treated soil (23.6 cmol·kg −1 ) ( Table 5). Ranges in CEC of 5-15, 15-25, and 25-40 cmol·kg −1 are rated as low, medium, and high, respectively [45]. Accordingly, the CEC of the soil is rated as high. Te application of BS increased the CEC from the medium in control soil to high; however, the application of CF decreased the CEC of the soil, which could attribute to the decrease in soil pH and exchangeable bases.

Applied and Environmental Soil Science 7
Te application of CF decreased the OC content of the soil and consequently lowered the CEC value. Tis result agrees with fndings from previous studies: application of organic fertilizers increases CEC while chemical fertilizers decrease CEC [64,76]. Tere is a direct association between OC and CEC; soils with low CEC are often low in OC [77,78]. An increase in soil OC and CEC content increases the bufering capacity of soil and soil fertility through retaining nutrients against leaching and enhancing their availability.
Unlike CF (22.2 cmol·kg −1 ), the application of BS increased the amount of exchangeable Ca (28 cmol·kg −1 ) compared to the control soil (24.6 cmol·kg −1 ) ( Table 5). Tis indicates that the application of BS yields at higher amount of Ca over the application of CF. Similarly, the application of BS increased the amount of exchangeable Na (0.23 cmol·kg −1 ) over the control soil (0.21 cmol·kg −1 ) and CF (0.2 cmol·kg −1 ) ( Table 5). In addition, the application of BS increased the amount of exchangeable K (2.7 cmol·kg −1 ) over the control (2.29 cmol·kg −1 ) and the application of CF (2.18 cmol·kg −1 ). Tis indicates that in contrast to CF, the application of BS increases the amount of soil K (Table 5). In general, the application of BS increases exchangeable bases and CEC and improves other soil physico-chemical characteristics. Tis could attribute to the higher CEC and organic matter content in BS and the nature of the organic matter to bufer change in pH. Previous studies reported increasing organic matter content and available nutrients with the application of organic fertilizers, which in turn increases the exchangeable bases and the cation exchange capacity of the soil [17,69,79]. Organic matter increases the bufering capacity of soils and prevents acidifcation through binding cations [80].

Level of Heavy Metals in Soil.
Compared with the application of CF and control, the application of BS resulted in the highest mean concentrations of Fe (213.1 ± 6.55 mg·kg −1 ), Mn (144.7 ± 3.5 mg·kg −1 ), Cu (1.88 ± 0.08 mg·kg −1 ), Ni (4.96 ± 1.4 mg·kg −1 ), Co (0.33 ± 0.011 mg·kg −1 ), and Zn (16.43 ± 1.22 mg·kg −1 ) (Table 6). Tis might attribute to the very nature of the applied BS, which was sourced from cow dung. Luo [81] reported higher concentrations of Cu, Ni, Zn, and Fe in soils from livestock manure application. Similarly, the application of BS resulted in higher concentrations of Fe, Zn, Mn, and Cu [82]. On the contrary, the application of CF yielded the highest concentration of As (16.6 ± 2.7 mg·kg −1 ), Pb (28.1 ± 1.2 mg·kg −1 ), Cd (12.2 ± 3.27 mg·kg −1 ), and Cr (6.14 ± 0.66 mg·kg −1 ) compared to the BS-treated and control soils. At the same time, the application of CF decreased soil pH, which might have contributed to the high concentration of toxic heavy metal (for instance, Mn, Cd, and Cr) in the soil. Low pH increases the solubility and availability of toxic metals like Mn, Cd, and Al in soils [83]. Te application of CF increased the mean concentration of Cd, As, Cr, and Pb in soil [84][85][86].
Te application of BS signifcantly (P < 0.05) increased the mean concentration of Zn, Cu, Fe, and Ni over the application of CF and control soil. In addition, the application of CF in turn signifcantly (P < 0.05) decreased the mean concentration of Zn, Cu, Fe, and Mn over the control. Regardless of treatments, only the mean concentration of Cd was above the maximum permissible limits for agricultural soils recommended by FAO/WHO [35]. Te application of the BS reduced the Cd concentration of the control soil; however, this did not bring the level of the metal to the permissible range. In this sense, the application of BS and CF as well as the control soil itself may not be suitable for the tomato cultivation with respect to Cd toxicity, which agrees with fndings from earlier studies [87].

Level of Heavy Metals in Tomato Fruits.
Te mean concentration of heavy metals in tomato fruits sampled from BS-treated, CF-treated, and control plots followed the order of Fe > Ni > Zn > Mn > Co > Cu > Cr > Cd > Pb > As, Fe > Mn > Ni > Zn > Cd > Co > Cu > Pb > Cr > As and Fe > Mn > Zn > Ni > Co > Cd > Cu > Cr > Pb > As, respectively (Table 7).
Te mean concentration of Pb in tomato fruits sampled from CF-treated soil was higher (0.25 ± 0.2 mg kg −1 ) than those sampled from BS-treated soils (0.016 ± 0.011 mg kg −1 ), which was statistically insignifcant (P > 0.05). In fact, the mean concentration of Pb in tomato fruits sampled from all treatments was below the MPL for human diets [35]. Tese tomatoes could therefore be safe for human consumption with regard to Pb toxicities. In contrast, there were significant diferences (P < 0.05) among the mean concentrations of Cd in tomato samples collected from all the three treatments. In this regard, the application of CF yielded higher mean concentration of Cd (7.98 ± 0.72 mg kg −1 ) compared to the application of BS (0.0246 ± 0.23 mg kg −1 ). Tis may attribute to the presence of Cd in phosphate fertilizer and its high mobility and bioavailability at the low pH of the soil to plants. Sêkara et al. [88] suggested that Cd is found in CF and it is a mobile element, easily absorbed by the roots and transported to shoots where it is uniformly distributed in plants. Besides, the lowest concentration of Cd in tomato samples collected from BS-treated soil might be due to the ability of organic matter to immobilize the heavy metals like Cd, Pb, and As in the soil [89]. On top of that, the    Applied and Environmental Soil Science 9 mean concentration of Cd in tomato sampled from CFtreated soil and control soil was above the MPL standards for human diets [35]. Terefore, the application of CF and the control soil is unsafe for tomato cultivation for human consumption regarding Cd toxicities. Findings from previous studies showed that Cd is a highly mobile metal and is found to accumulate in plants in large amounts without showing phytotoxic symptoms [90,91]. Similarly, higher mean concentration of Cd due to the application of CF was reported in previous studies [87,92,93]. Te diferences in mean concentrations of Ni in tomato samples among all the treatments were statistically signifcant (P < 0.05), with CF yielding the highest Ni concentration (23.9 ± 0.33 mg·kg −1 ) and the control soil yielding the lowest Ni concentration (9.66 ± 1.2 mg·kg −1 ). Tere are similar reports from previous studies [94,95]. More importantly, the mean concentrations of Ni in tomato samples from all treatments were above the MPL for human diets [35]. Tis shows that tomatoes from the study site are unsafe for human consumption with respect to Ni toxicities.
Te highest mean concentration (10 ± 0.22 mg·kg −1 ) and the lowest (2.15 ± 0.34 mg·kg −1 ) of Co were recorded in tomato samples sampled from BS and CF-treated soils, respectively, which were statistically signifcant (P < 0.05). Tere are similar fndings from previous studies [96,97]. Contextually, the mean concentration of Co in tomato samples from all treatments, including the control, is below the MPL for human diets [35]. Terefore, BS and CF application as well as the sole agricultural soils can be used for tomato cultivation, which is safe for human consumption with regard to Co toxicities.
Tere were signifcant diferences (P < 0.05) between the mean concentrations of Zn in tomato fruits sampled from BS-treated (16.87 ± 2.26 mg·kg −1 ) and CF-treated soil (10 ± 0.74 mg·kg −1 ) and those from BS-treated and the control soil (9.77 ± 1.15 mg·kg −1 ). Te higher mean concentration of Zn in tomato fruit samples might have stemmed from the BS, which is in line with results from previous studies [92,98,99]. However, the mean concentrations of Zn in tomato fruit samples from all treatments, including the control, were below the MPL for human diets [35]. Tis shows that the application of BS and CF as well as the agricultural soils can be used for tomato cultivation, which is safe for human consumption with respect to Zn toxicities.
Tere were signifcant diferences (P < 0.05) among the mean concentrations of Fe in tomato fruit samples from all the treatments, with the highest mean concentrations of Fe (57.1 ± 1.853 mg kg −1 ) in BS-treated soil and the lowest (42.1 ± 0.34 mg kg −1 ) in the control soil. Te high Fe concentration in tomato fruits might be sourced from the BS, CF, and the agricultural soil itself. Tere are similar reports from previous studies [96,100,101]. Yet, the mean concentrations of Fe in tomato fruit samples were below the MPL for human diets [35]. Tis indicates that tomato can be grown with the application of BS and CF in the study site that is safe for human consumption with respect to toxicities of Fe.
Tere were signifcant diferences (P < 0.05) among the mean concentrations of Mn in tomato fruit samples from all treatments, with the highest mean concentrations of Mn (38 ± 3.3 mg·kg −1 ) in tomato fruit samples from CF-treated  Tere was no signifcant diference (P < 0.05) in the mean concentrations of As in tomato fruit samples from all treatments despite the fact that the CF yielded higher mean concentration of As (0.04 ± 0.01 mg·kg −1 ) than the BS application (0.013 ± 0.011 mg·kg −1 ). Tere are similar reports from previous studies [93,96]. Furthermore, the mean concentrations of As in all tomato fruit samples were below the MPL for human diets according to the FAO/WHO [35] standards. Tis shows that tomato grown in all the three soil treatments could be safe for human consumption with respect to As toxicity.
Tere was a signifcant diference (P < 0.05) in the mean concentrations of Cu between the tomato fruit samples from BS-treated and CF-treated soil as well as between bio-slurrytreated and control soil. Te Cu concentration might be sourced from the BS that contains high concentrations of Cu (Table 4). Tere are similar reports from previous studies [94,103]. Te mean concentration of Cu in the tomato grown in all treatments is below the MPL for human diets according to the FAO/WHO [35] standards. Tis shows that tomato can be grown in the study site with the application of BS and CF, which is safe for human consumption in relation to Cu toxicity.
Te highest mean concentration of Cr (0.173 ± 0.011 mg kg −1 ) and the lowest (0.03 ± 0.005 mg·kg −1 ) were recorded in tomato grown with CF-treated soil and the control soil, respectively. Tere were statistically signifcant diferences (P < 0.05) among the mean concentrations of Cr in the tomato fruit samples from all treatments. Tere are similar reports from previous studies [87,95,104,105]. Te mean concentrations of Cr in tomato fruit samples from all treatments were below the MPL for human diets according to the FAO/WHO [35] standards. Tis shows that the application of BS and CF as well as the agricultural soil of the study site could be used for cultivation of tomato, which is safe for human consumption in association with Cr toxicity.

Potential Ecological Risks of Heavy Metals in the Soil.
Contamination factor (CF) and pollution load index (PLI) were used to assess the potential ecological risks of heavy metals in the soil.

Contamination Factor (CF).
Te CF in BS-treated, CFtreated, and control soil followed this decreasing order: As > Pb > Fe > Cd > Zn > Mn > Ni > Cr > Cu > Co, As > Cd > Pb > Zn > Mn > Cu > Cr > Ni > Fe > Co and As > Pb > Cd > Zn > Mn > Cr > Ni > Fe > Cu > Co, respectively (Table 8). It mostly appears that there is a similarity in the distribution of heavy metals regardless of their sources. Te CF values indicated contamination ranging from no contamination to moderate contamination of the soils. Accordingly, soil samples from the CF treatment and the control were moderately contaminated with Cd. Tis might be due to the fact that the agricultural soils used for the present study had been receiving various kinds of chemical fertilizers and agricultural pesticides in the production of various kinds of crops. Said et al. [106] also unveiled similar fndings from their previous studies. Te remaining heavy metals experienced CF values less than 1. Terefore, there is no contamination with reference to these metals (Table 8). Tere are similar fndings from previous studies [22,[107][108][109]. Table 8 presents the PLI and the combined pollution efect of diferent metals at diferent sampling locations. Te PLI values of heavy metals were 0.13, 0.28, and 0.25 in soil treated with BS, CF, and control, respectively (Table 8). Te highest PLI value was recorded in CF-treated soil, followed by the control soil, with the BStreated soil registering the lowest PLI value. Despite that, the PLI values of all soil samples were less than 1, indicating no detectable pollution with heavy metals. Te agricultural soil in the study site is not polluted (PLI < 1) with heavy metals and is therefore safe for tomato cultivation. Tere are similar reports from previous studies [22,[110][111][112]. Mean values with the diferent superscript letters in a row are signifcantly diferent from each other at α � 0.05. MPL � maximum permissible limit of agricultural soil [35].

Conclusions
Compared to chemical fertilizers, the application of BS improved the physico-chemical properties (soil bulk density, porosity, and moisture content) of the soil. Te agricultural soil of the study sites experienced Cd concentrations exceeding FAO/WHO standard for agricultural soils. Te application of bio-slurry signifcantly infuenced the mean concentrations of Cd, Cr, Cu, Ni, and Fe of the soil. Te application of chemical fertilizers signifcantly increased the Cd concentration level in the soil. Te application of bio-slurry signifcantly reduced the mean concentrations of Cd and Mn in the tomato fruit samples and optimized the Cd concentration to the range of safety limit for human consumption. Regardless of their sources, the concentrations of Ni and Mn in tomato fruit samples were above the safety limit for human consumption. When growing tomatoes, it is important to pay attention to the agricultural soil as well as the levels of heavy metals present in bio-slurry in relation to both food safety and soil health. Tus, monitoring the heavy metal concentrations in fertilizers, agricultural soil, and plant tissues is important to prevent excessive build-up of heavy metals in the human food chain and safeguarding food safety. Particularly, to reduce the accumulation of Mn and Ni in the tomato, other alternative heavy metal remediation methods such as phytoremediation are commendable.

Data Availability
Te data used to support the fndings of this study are included within the article.

Disclosure
A preprint has previously been published in Research Square in the following link: https://www.researchsquare.com/ article/rs-1649597/v1.

Conflicts of Interest
Te authors declare that they have no conficts of interest.