The results of the D. carota yield at harvest, as depicted in Fig. 1, indicated that the highest yield (451.3g) was produced when D. carota was cultivated in soil amended with 400g sewage sludge. This was followed by yields of 451.97g, 451.13g, 337.97g and 70.94g for soil amended with 200g, 300g, 100g and control, respectively. As seen in Fig. 2, the highest yield of S. oleracea (555.72g) was observed in the soil amended with 300g sewage sludge, with yields of 534.00g, 369.27g, 209.14g and 55.74g for 400g, 200g, 100g and control, respectively. Figure 3 further portrays the growth performance of S. oleracea, with the greatest leaf length (30.23 ± 2.483 cm) observed in S oleracea cultivated in soil amended with 300g sewage sludge from week 1 and the order for growth remained the same throughout the experiment. The growth of S. oleracea from soil amended with 200g sewage sludge showed increase in growth rate until harvest in week 9. The growth and yield of S.oleracea and D.carota harvested from the soil amended with sewage sludge was significantly better than the control plants. This could be attributed to the elevated levels of essential plant nutrients, including nitrogen (N), phosphorus (P) and potassium (K) found in the sewage sludge, as indicated in Tables 1 and 2. These findings agree with those reported by Aina et al. [23].
Variations in the properties of sludge-amended soils are contingent on the properties of the sludge and soil [24]. Applying excessive amounts of sewage sludge can cause plant toxicity due to the buildup of heavy metals in soils [25] as well as the rising salt content [26]. In the present study, Tables 1–5 below shows the levels of heavy metals in the leaves, roots and stalks of S. oleracea and D. carota harvested from soil treated with different quantities of sewage sludge. Of all the heavy metals measured, iron (Fe) showed the highest mean concentration of heavy metals in both harvested S. oleracea and D. carota. The highest mean concentration (555.30 ± 45.30 mg/kg) of Fe in D. carota was recorded in the leaves harvested from soil treated with 300g of sewage sludge and the lowest concentration (305.67 ± 64.82 mg/kg) was recorded in the roots of D. Carota harvested from soil treated with 200g of sewage sludge (Tables 1 and 3). For S. oleracea, the highest concentration (1503. 00 ± 82.31 mg/kg) of Fe was recorded in the leaves harvested from soil treated with 200g of sewage sludge whereas the lowest concentration (68.67 ± 6.97 mg/kg) was recorded in the roots of S. oleracea harvested from soil treated with 300g sewage sludge (Tables 2 and 4). The plant harvested from the amended soil showed better growth and yield compared with the plant harvested from untreated soil. The differences obtained in the concentrations of Fe as a result of the treatment in the leaves and roots of all the plants was significant (P < 0.05). In plants, Fe plays an important role of chlorophyll synthesis and contributes to the green colour in plants [27]. According to Miano,[28] and Olowoyo et al. [29], S. oleracea leaves are a rich source of Fe that protects from osteoporosis while the deficiency of Fe results in anaemia in human beings. Fe is also crucial for the functioning of the female and male reproductive systems [30]. The deficiency of Fe in plants results in chlorosis of immature leaves which can also be transferred to the older leaves as the severity of the shortage of Fe increases [30]. Even though Fe is present in trace quantities, it is required by the body and is essential for biological systems and also for sustaining aerobic life [31]. Fe consumption in excess has been associated with accidental death in young children aged under six years [31]. In the current study, the levels of Fe in both S. oleracea and D. carota were above WHO permissible limit (450 mg/kg) [32]. The results of the current study are in agreement with the results in Swain et al. [7] and Kumar et al. [33] where the leaves of S. oleracea harvested from soil treated with sewage sludge showed the highest uptake of heavy metals like Fe and other micronutrients as a result of the increase in the application quantities of sewage sludge.
The concentration of manganese (Mn) in the leaves and roots of harvested D. carota ranged from 30.00 ± 1.95 mg/kg to169.80 ± 0.50 mg/kg (Tables 1 and 3). The leaves of D.carota harvested from the amended soil with 300g showed the highest concentration (226.10 ± 2.17 mg/kg) (Table 1) while the least value of Mn was recorded in the roots harvested from the soil amended with 200g (30.00 ± 1.95 mg/kg) (Table 3). With S.oleracea, the leaves harvested from 100g treated soil recorded the highest concentration of 90.57 ± 3.36 (Table 2) while the lowest concentration of Mn for S. oleracea (21.70 ± 0.56 mg/kg) was recorded in the roots of 300g amended soil. The differences obtained in the concentrations of Mn in the leaves and roots of the plants were all significant (p < 0.05). All the plants of D. carota and S. oleracea showed lower levels of Mn than the permissible limit for Mn stipulated by WHO (500 mg/kg)[34]. Mn is a crucial trace element for plant growth and is a component of photosynthesis (Alejandro et al., 2020) However, an excess of Mn in plants can interfere with their ability to absorb and use other minerals, affect their energy metabolism, lower their photosynthetic rates, and cause oxidative stress (Fernando & Lynch, 2015; Tavanti et al., 2020).
The concentration of zinc (Zn) in the roots and leaves of S. oleracea ranged from 62.23 ± 1.60 mg/kg − 120.60 ± 0.69 mg/kg. Generally, with Zn, in S. oleracea, the concentrations obtained for the leaves were higher than those recorded in the roots. Zn concentration harvested from plants treated with 400g of amended sewage sludge had the highest concentrations for Zn. The differences obtained in the concentrations of Zn as a result of the amendment used were all significant (p < 0.05). In the present study, all the samples recorded higher Zn concentrations compared to the concentration of 0.02 mg/kg as recommended by WHO [34]. Zn is the transition metal that is present in the greatest abundance in living things [35], because of its structural, catalytic, and activating properties. Zn is essential for plant development, reproduction, and signaling [36]. Zn is a necessary element for plants, but too much of it can seriously harm them [37]. Zn is one of the main pollutants that are discharged into the environment as a result of industrial activity, mining, smelting, and sewage sludge, as well as the ongoing use of Zn fertilizers [38].
Copper (Cu) concentration in the leaves were generally higher than the concentrations obtained for the roots with the exception of roots harvested from soil treated with 400g of the sewage sludge. The concentrations of Cu in S. oleracea varied between 5.68 ± 0.42 mg/kg – 27.57 ± 3.64 mg/kg. The highest value for Cu from S. oleracea was recorded from the roots of the soil amended with 400 g of the sewage sludge. The differences obtained in the values recorded for Cu from the different treatments were not significant either in the leaves or the roots (Tables 2 and 4). For D. carota, the range of the Cu concentration was from 5.76 ± 0.13 mg/kg to 11.80 ± 0.46 mg/kg. Unlike the S. oleracea, the concentrations obtained in the roots were more than those recorded for the leaves and the differences obtained in the concentrations of Cu as a result of the treatment either from the values recorded in the roots or leaves were also significant (p < 0.05). An increase in the concentration of sewage sludge increases the concentration of Cu both for the leaves and roots of D. carota (Tables 1 and 3). Cu is essential for the proper growth and development of plants, as it is involved in numerous morphological, physiological, and biochemical roles [39]. However, an excessive amount of Cu in the soil can have a detrimental effect on the production and growth of plants [40]. To counter this, plants are able to tolerate Cu toxicity by immersing the excess amounts into parts which can be harvested, such as leaves, cell walls, and the vacuolar membrane of the root cortex (Zandi et al., 2020).
The concentrations of Cr ranged from 0.60 ± 0.04 mg/kg − 3.80 ± 0.06 mg/kg in D.carota (Tables 1 and 3) while it ranged from 1.37 ± 0.08 mg/kg – 4.57 ± 0.14 mg/kg in S. oleracea. Values obtained in the leaves of both plants were more than values obtained in the roots of both plants respectively (Tables 1–4). With S. oleracea the differences obtained for Cr was only significant in plants harvested from soil treated with 100g of the sewage sludge when compared with other treatments (p < 0.05). The WHO recommended limit for Cr was set to be 1.30 mg/kg. In the present study, the highest concentration of Cr (4.57 ± 0.14 mg/kg) was recorded in the roots of S.oleracea harvested from the soils treated with 200g of sewage sludge whereas the least concentration (0.60 ± 0.04 mg/kg) was found in the roots of carrot harvested from the soils treated with 100g of sewage sludge. It has been shown that there is no known biological function of Cr in plant physiology [41]. However, it is generally accepted that excessive levels of Cr in plant tissues can cause a range of morphological, physiological and biochemical changes in plants [42, 43]. This is because heavy metal toxicity is usually the result of complicated interactions between the heavy metals, genetic processes, signal transduction pathways, and cellular macromolecules [44].
Nickel (Ni) concentration in the leaves and roots of D.carota varied between 1.27 ± 0.31 mg/kg − 4.61 ± 0.48 mg/kg. The roots of D.carota harvested from 300g sewage sludge amended soil showed the highest concentration (4.61 ± 0.48 mg/kg) while the least concentration (1.27 ± 0.31 mg/kg) was found in the roots of the control soil. For S.oleracea, the highest concentration (19.17 ± 0.37mg/kg) of Ni was found in the roots of plants harvested from the 400g sewage sludge amended soils and the least concentration (2.34 ± 0.20 mg/kg) was found in the leaves of the soils treated with 200g of sewage sludge. Comparing this study to the WHO accepted limit of (10.00 mg/kg), the study showed that the plants were within the acceptable limit. Nickel (Ni) is an essential trace element for plant growth, without which the plant cannot complete its life cycle [45]. However, excessive amounts of this heavy metal can have a devastating effect on the plant's physiology, disturbing processes such as photosynthesis, root growth, enzyme activity, and mineral nutrition [46]. The differences obtained for the values of Ni from the leaves of S.oleracea was significant with plants harvested from 400g of soil (p < 0.05). However, with carrot, the differences obtained as a result of the treatment were not significant (p > 0.05).
The concentration of Cd in the leaves and roots of D.carota ranged from 0.23 ± 0.002 mg/kg– 0.68 ± 0.03 mg/kg (Table 1). The highest concentration (0.68 ± 003 mg/kg) of Cd was recorded from the leaves harvested from the soils amended with 300g of sewage sludge while the least concentration (0.23 ± 0.00 mg/kg) was recorded from the roots and leaves harvested from soils treated with of 200g and 300g of sewage sludge amended soils respectively. The concentration of Cd in S.oleracea harvested from soil amended with 100g of sewage sludge showed the highest concentration of 0.36 ± 0.01 mg/kg (Table 2) whereas the lowest concentration (0.13 ± 0.01 mg/kg) was recorded in the roots of S. oleracea harvested from the 100g soils (Table 4). Differences obtained in the concentrations of Cd in both plants either in the roots or leaves were not significant (p > 0.05). The combustion of zinc mines, non-ferrous smelting, phosphate fertilizers, fossil fuels and sewage sludge application are the main sources that are linked to the accumulation of Cd in soil and plants [47]. Exposure to high levels of Cd can cause a decrease in growth rate, weak mineralization of bones, hypertension, anemia, and damage to the renal tubules [47], while chronic exposure to Cd at lower levels can lead to problems in the skeletal system, kidneys, and respiratory system [44]. The values recorded for both S. oleracea and D. carota in the current study in some of the plant parts were slightly above the [32] permissible limit 0.3 mg/kg.
The concentration of Pb in the leaves and roots of D.carota ranged from 0.25 ± 0.00 mg/kg– 1.05 ± 0.03 mg/kg. The highest concentration of Pb with a value of 1.05 ± 0.03 mg/kg was found in the leaves of soil amended with 300g of sewage sludge and the lowest concentration value of 0.025 ± 0.001 mg/kg in the roots of the plant harvested from soils treated with of 200g of sewage sludge (Tables 1 and 3). For S.oleracea the highest concentration of 1.47 ± 0.00 mg/kg was recorded in the roots of soils treated 200g sewage sludge compared to the lowest value of 0.31 ± 0.00 mg/kg in the leaves of the vegetable harvested from the soils treated with 300g of sewage sludge. All the values obtained for Pb in the roots of S. oleracea were above the recommended limit by WHO. Excess Pb may cause acute and chronic poisoning. It also has an effect on the functioning of the kidney and liver [48].
Table 1
Heavy metals (mg/kg) in the leaves of Daucus carota harvested from soil treated with different amounts of sewage sludge.
Plant | Sewage sludge (g) | Heavy Metals (mg/kg) |
Fe* | Mn* | Zn* | Cu** | Cr * | Ni** | Sr | Cd** | Pb** |
Daucus carota | Control | 306.00 ± 11.14 | 77.97 ± 2.97 | 29.23 ± 1.95 | 7.14 ± 0.24 | 1.28 ± 0.67 | 1.27 ± 0.31 | 31.84 ± 0.28 | 0.23 ± 0.02 | 0.51 ± 0.03 |
100 | 535.70 ± 36.25 | 137.50 ± 6.50 | 53.70 ± 3.97 | 7.51 ± 0.51 | 3.59 ± 0.14 | 2.14 ± 0.74 | 46.53 ± 0.71 | 0.25 ± 0.01 | 0.51 ± 0.01 |
200 | 364.30 ± 14.01 | 68.07 ± 5.25 | 62.33 ± 1.68 | 8.43 ± 0.55 | 2.73 ± 0.04 | 1.79 ± 0.36 | 44.95 ± .0.48 | 0.27 ± 0.02 | 0.55 ± 0.02 |
300 | 555.30 ± 45.36 | 226.10 ± 2.17 | 106.50 ± 2.74 | 10.53 ± 0.50 | 3.53 ± 0.10 | 3.76 ± 0.36 | 55.47 ± 0.46 | 0.68 ± 0.03 | 1.05 ± 0.03 |
400 | 434.00 ± 7.94 | 169.80 ± 0.50 | 103.90 ± 2.16 | 9.43 ± 0.48 | 3.80 ± 0.06 | 3.49 ± 0.15 | 47.34 ± 0.27 | 0.43 ± 0.02 | 0.36 ± 0.00 |
*: Significant differences in the values obtained for this element at different rates of sewage sludge application (P < 0.05).
**: No significant difference in the values obtained for this element at different rates of sewage sludge application (P > 0.05).
Table 2
Heavy metals (mg/kg) in the leaves of Spinacia oleracea harvested from soil treated with different amounts of sewage sludge.
Plant | Sewage sludge (g) | Heavy Metals (mg/kg) |
Fe* | Mn** | Zn* | Cu* | Cr** | Ni ** | Sr** | Cd** | Pb** | |
Spinacia oleracea | Control | 1294.00 ± 72.51 | 86.87 ± 5.19 | 118.60 ± 4.84 | 10.90 ± 0.98 | 2.23 ± 0.07 | 3.67 ± 0.56 | 39.58 ± 0.61 | 0.17 ± 0.01 | 0.55 ± 0.01 | |
100 | 1431.00 ± 89.06 | 90.57 ± 3.36 | 78.47 ± 0.85 | 8.97 ± 0.47 | 1.64 ± 0.03 | 2.97 ± 0.53 | 33.46 ± 0.42 | 0.36 ± 0.01 | 0.37 ± 0.02 | |
200 | 1503.00 ± 82.31 | 82.70 ± 2.25 | 109.10 ± 3.82 | 14.90 ± 0.36 | 1.37 ± 0.08 | 2.34 ± 0.20 | 35.48 ± 0.35 | 0.19 ± 0.02 | 0.35 ± 0.00 | |
300 | 1252.00 ± 56.16 | 79.70 ± 1.73 | 111.3 ± 1.81 | 16.07 ± 0.38 | 1.86 ± 0.18 | 4.94 ± 0.05 | 23.15 ± 0.32 | 0.17 ± 0.01 | 0.31 ± 0.00 | |
400 | 616.00 ± 20.00 | 89.03 ± 0.64 | 120.60 ± 0.69 | 14.03 ± 0.65 | 1.58 ± 0.07 | 6.71 ± 0.33 | 25.05 ± 0.19 | 0.30 ± 0.02 | 0.39 ± 0.02 | |
*: Significant differences in the values obtained for this element at different rates of sewage sludge application (P < 0.05).
**: No significant difference in the values obtained for this element at different rates of sewage sludge application (P > 0.05).
Table 3
Heavy metals (mg/kg) in the roots of Daucus carota harvested from soil treated with different amounts of sewage sludge.
Plant | Sewage sludge (g) | Heavy Metals (mg/kg) |
Fe* | Mn* | Zn* | Cu* | Cr* | Ni* | Sr** | Cd** | Pb** |
Daucus carota | Control | 505.00 ± 24.98 | 50.20 ± 2.62 | 31.53 ± 2.63 | 5.76 ± 0.13 | 1.39 ± 0.13 | 2.32 ± 0.42 | 11.83 ± 0.11 | 0.28 ± 0.01 | 0.51 ± 0.01 |
100 | 368.67 ± 36.74 | 55.57 ± 3.76 | 42.40 ± 2.63 | 8.08 ± 1.06 | 0.60 ± 0.04 | 1.38 ± 0.18 | 14.09 ± 0.13 | 0.33 ± 0.02 | 0.29 ± 0.01 |
200 | 305.67 ± 64.82 | 30.00 ± 1.95 | 51.00 ± 3.54 | 8.96 ± 0.12 | 1.08 ± 0.08 | 3.19 ± 0.20 | 13.16 ± 0.05 | 0.23 ± 0.02 | 0.25 ± 0.01 |
300 | 417.00 ± 7.81 | 48.23 ± 3.10 | 57.10 ± 5.33 | 10.37 ± 0.40 | 2.25 ± 0.05 | 4.61 ± 0.48 | 14.67 ± 0.05 | 0.29 ± 0.00 | 0.59 ± 0.02 |
400 | 513.00 ± 22.72 | 57.30 ± 3.83 | 67.67 ± 1.27 | 11.80 ± 0.46 | 1.67 ± 0.04 | 4.13 ± 0.11 | 13.88 ± 0.16 | 0.30 ± 0.01 | 0.25 ± 0.01 |
*: Significant differences in the values obtained for this element at different rates of sewage sludge application (P < 0.05).
**: No significant difference in the values obtained for this element at different rates of sewage sludge application (P > 0.05).
Table 4
Heavy metals (mg/kg) in the roots of Spinacia oleracea harvested from soil treated with different amounts of sewage sludge.
Plant | Sewage sludge (g) | Heavy Metals (mg/kg) |
Fe* | Mn* | Zn* | Cu* | Cr** | Ni* | Sr** | Cd** | Pb* | |
Spinacia oleracea | Control | 518.00 ± 40.85 | 31.20 ± 0.36 | 63.33 ± 0.68 | 6.04 ± 0.3 | 3.01 ± 0.10 | 3.14 ± 0.20 | 27.37 ± 0.30 | 0.29 ± 0.03 | 1.10 ± 0.02 | |
100 | 276.00 ± 9.85 | 27.40 ± 1.95 | 72.1 ± 4.23 | 6.11 ± 0.33 | 2.56 ± 0.07 | 6.74 ± 0.26 | 29.35 ± 0.32 | 0.13 ± 0.01 | 1.29 ± 0.24 | |
200 | 106.70 ± 2.08 | 21.90 ± 0.26 | 62.23 ± 1.6 | 5.68 ± 0.42 | 4.57 ± 0.14 | 6.42 ± 0.08 | 34.08 ± 0.35 | 0.18 ± 0.01 | 1.47 ± 0.00 | |
300 | 68.60 ± 2.82 | 21.70 ± 0.56 | 74.47 ± 2.61 | 6.45 ± 0.36 | 4.56 ± 0.01 | 5.85 ± 0.27 | 29.99 ± 0.27 | 0.18 ± 0.02 | 1.35 ± 0.01 | |
400 | 96.67 ± 6.97 | 35.50 ± 0.36 | 90.47 ± 1.12 | 27.57 ± 3.64 | 3.25 ± 0.13 | 19.17 ± 0.37 | 27.73 ± 0.08 | 0.24 ± 0.01 | 0.78 ± 0.00 | |
*: Significant differences in the values obtained for this element at different rates of sewage sludge application (P < 0.05).
**: No significant difference in the values obtained for this element at different rates of sewage sludge application (P > 0.05).
Table 5
Transfer factors in the leaves of S. oleracea and D. carota harvested from soil treated with different quantities of sewage sludge.
Plant | | Fe | Mn | Zn | Cu | Cr | Ni | Sr | Cd | Pb |
S. oleracea | Control | 0.61 | 1.55 | 0.93 | 1.24 | 0.92 | 0.55 | 2.69 | 0.82 | 1.00 |
100 | 1.45 | 2.47 | 1.27 | 0.93 | 5.98 | 1.55 | 3.30 | 0.76 | 1.76 |
200 | 1.19 | 2.27 | 1.22 | 0.94 | 2.53 | 0.56 | 3.42 | 1.17 | 2.20 |
300 | 1.33 | 4.69 | 1.87 | 1.02 | 1.57 | 0.82 | 3.78 | 2.34 | 1.78 |
400 | 0.85 | 2.96 | 1.54 | 0.80 | 2.28 | 0.85 | 3.41 | 1.43 | 1.44 |
D. carota | Control | 2.50 | 2.78 | 1.87 | 1.80 | 0.74 | 1.17 | 1.45 | 0.59 | 0.50 |
100 | 5.18 | 3.31 | 1.09 | 1.47 | 0.64 | 0.44 | 1.14 | 2.77 | 0.29 |
200 | 14.09 | 3.78 | 1.75 | 2.62 | 0.30 | 0.36 | 1.04 | 1.06 | 0.24 |
300 | 18.25 | 3.67 | 1.49 | 2.49 | 0.41 | 0.84 | 0.77 | 0.94 | 0.23 |
400 | 6.37 | 2.51 | 1.33 | 0.51 | 0.49 | 0.35 | 0.90 | 1.25 | 0.50 |
Table 5 showed the transfer factor which is an indication of bioaccumulation potentials of the plants. From Table 6, with the exception of some values reported for Cr, most of the heavy metals determined in the study were bioaccumulated from the soil ((TF > 1). In the present study, from Table 1, 2 and 3, an increase in the concentrations of the sewage sludge especially when increased between 300g and 400g might have resulted in an increase in the concentrations of heavy metals in the plant tissues. This is similar to the findings of Koutroubas et al. [49] where an increase in soil amendments led to an increase in the concentrations of the heavy metals in the plant tissue. Application of sewage sludge on agricultural land can boost crop output by enhancing soil fertility and physical characteristics [49]. Many factors have been attributed to the cause of uptake of heavy metals by the planted plants such as soil organic matter, and the soil pH [29]. The pH less than 6 has been reported to enhance leaching of heavy metals, making them available for plants uptake [50].
Liu et al. (2013) and Bi et al. (2018) reported high metal uptake in vegetables because of higher levels in the background soil as a result of industrial activities. Other studies have reported similar findings of higher heavy metal uptake because of higher levels in sewage sludge [50–52], which are in accordance with the present result. Contrary to these findings, Ok et al. [53] recorded lower metal uptake as a result of higher organic matter content and soil conductivity which immobilized heavy metals. Demirezen and Ahmet,[54] and Sharma et al. [55] have attributed the uptake of heavy metals to the wastewater used for irrigation. One of the main ways that heavy metals from sewage sludge-amended soils enter the food chain is through plant uptake. The plants in the food chain might take in enough heavy metals to endanger consumers' health. The increase in yield brought about by the application of sewage sludge in the current study is consistent with previous studies showing that sewage sludge has the potential to increase the crop yield [56, 57]. Sewage sludge's high nutrient content, which favorably affects soil structure, improves soil aeration, and fosters the activities of living organisms within the soil, is responsible for the increase in yield as a result of its use[58] .
Transportation of heavy metals in soils are facilitated by different factors including the concentration of heavy metals in the soil, the nature of organic matter and the acidic nature of the soil used for planting. In this stud, the soil pH were slightly acidic which might have favored the transportation of the heavy metals in the soil. The characterization and profiling of the soil also showed that the soil was sandy clay loam soil which could have also facilitated the movement of the heavy metals in the soil. In addition, from the current study, looking at the control sites, the concentrations of heavy metals in the harvested plants differed from those that were harvested either from pots added with 300g or 40g of sewage sludge used in the study. This might have been the impact of raw sewage sludge used in the study. The report of Buta et al. [59] showed that raw sewage sludge has the capacity of introducing heavy metals in the soil owing to their sources. Generally, from the current study, the two crop plants have a great bioaccumulation potential hence the current levels of heavy metals in their tissues.