3.1. Effects of Spent Engine Oil on Soil Characteristics
3.1.1. Soil pH
In the analysis of physico-chemical parameters of soil, the pH was found to be higher in soil sample D and it was lower in control soil sample E (Table 1). The soil became slightly alkaline in the sample D. The pH of the soil samples treated with 50, 100, 200 and 300 ml spent engine oil was 6.3, 6.5, 6.9 and 7.3 respectively (Table 1). This was agreed with the findings of Odjegba V J and Atebe J O (2007). They found that, soil pH was increased, when amended with used engine oil. The control (unpolluted) soil had a pH value of 7.1 and oil polluted soil had pH values ranging from 7.1 - 7.5.
Also Nwite J N and Alu M O (2015) found that, pH was generally higher than the control and increased with increase in spent engine oil application across treatments. These findings may contradictory with the results of Oyem and Isama Lawrence (2013). That is spent engine oil pollution decrease the soil pH. Some other researchers found no variations on the soil pH by the addition of spent engine oil. There was no significant difference (P>0.05) in the soil pH between the control and the contaminated soil in the study by Okonokhua B O et al., (2007).The optimum soil pH for the growth of plants was between 6 and 7. At this pH, the seeds need good fertile soil and adequate moisture to germinate. High or low pH cause deficiencies in essential nutrients that plants need to grow and causes stunted growth and yellowing of leaves.
3.1.2. Soil Electrical conductivity
Electrical Conductivity was higher in the control soil sample E (0.41 mhos/cm) and it was lower in soil sample D (0.21 mhos/cm) (Table 1). Electrical Conductivity of spent engine oil treated soil samples A, B, C, and D were found to be 0.26, 0.26, 0.24, and 0.21 respectively. The application of spent engine oil reduced the availability of exchangeable cations. This could be due to the presence of enhanced nutrients such as available Phosphorus. Kayodeet al., in 2009 and Uhegbuet al., in 2012 reported that spent engine oil pollution in the soils reduced the concentration of soil exchangeable cations.
3.1.3. Soil Organic Carbon
Organic carbon was higher in the soil sample D (2.07%) and it was relatively lower in control soil sample E (1.67%) (Table1). Amount of organic carbon in spent engine oil treated soil samples A, B, C and D were 1.71%, 1.95%, 1.98%, and 2.07% respectively (Table 1). According to Ekundayoet al. (1989), organic carbon content in the soil can be altered by spent engine oil pollution. Similarly, Okonokhua B O, et al., (2007) found that, the organic C (3.68) of the contaminated soils increased compared to the control (2.07). The organic carbon content was highest at the point of discharge of the spent engine oil (2–2.9%) having a mean value of 2.5%, the surrounding soil and control had the same mean OC value of 1.2% was observed in the study conducted by Otobong B. Iren and Victoria F. Ediene (2017). Nitrogen and organic carbon increased markedly with an increase in the oil treatment concentration for all the crops were found in a study conducted by Uquetan U I et al., in 2017. Similar results are also obtained in the study of Nwite J N and Alu M O (2015). The percentage of organic carbon (%OC) across the treatments of spent engine oil was significantly (P<0.05) higher than control.
3.1.4. Soil Nitrogen
Nitrogen was found to be higher in the control soil sample E (Table 1). As the treatment concentration increased, the nitrogen content in the soil samples was decreased. The amount of nitrogen in the soil samples A, B, C, and D were 191, 184, 176 and 165 kg/ha respectively. The nitrogen content in the control soil sample was 203 kg/ha (Table 1). Similar results are found in the results of Kayodeet al. (2009). They observed that, there was reduced nitrogen in soil treated with spent lubricant oil. Many authors prove that pollution by petroleum products may cause a reduction in nitrogen levels in plants (Wyszkowski and Wyszkowska2005). Soils contaminated with petroleum substances causes changed nitrogen–carbon ratio mainly due to the presence of hydrocarbons. This contributes to the inhibition of many nitrogen involving reactions in the soil (regarding both mineral and organic forms of nitrogen), as well as to a reduction in the intensity of ammonification and nitrification (Adam and Duncan 2003; Kucharski and Jastrzębska2005) which may explain the reduced levels of nitrogen in the soil.
3.1.5. Soil Phosphorus
Phosphorus content was highest in the control soil sample E (Table 1). Phosphorus content in the spent engine oil treated soil samples A, B, C and D were 86, 74, 70 and 64 kg/ha respectively. Phosphorus was reduced in the oil contaminated soils compared to the control and this agrees with the findings of Ogboghodoet al., (2004) and Okonokhua B O et al., (2007). In the study of Okonokhua B O et al., (2007), the phosphorus in the control was 5.76 ppm and it progressively decreased into 4.85 ppm in the treated soil samples. In a study of Uquetan U I et al., (2017) observed that the available phosphorus content decreased from 93.40mg/kg in the control to 50.24mg/kg in treatment concentration of 800ml/20kg and further decreased to 41.0mg/kg as the treatment concentration increased. Similarly, Nwite J N and Alu M O (2015) found that, available phosphorus of control was significantly (P<0.05) higher than those of spent engine oil application across the treatments. Available phosphorus decreased with increase in spent engine oil application with the one applied at 1.0 l/poly bag giving the least value of 10.20 mg kg-1. The available phosphorus of control was 83% higher than the one applied at 1.0 l/poly bag.
3.1.6. Soil Potassium
Potassium was higher in the soil sample E (220 kg/ha) and it was lower in the soil sample D (103 kg/ha) (Table 1). Potassium content in spent engine oil treated soil samples A, B, C and D are 152, 143, 121 and 103 kg/ha respectively. Similar observations are also found in the study of Nwite J N and Alu M O in 2015. Exchangeable cations of potassium were generally higher in control than in different levels of spent engine oil application. The exchangeable cations of K and Mg were decreased with increasing spent engine oil pollution in the soil.
3.1.7. Soil Water Holding Capacity
Water holding capacity of the soil was higher in the soil sample E (43.84%) and it is lower in the soil sample D (31.46%) (Table1). The water holding capacity of soil samples A, B, C and D are 41.10%, 38.70%, 31.53%, and 31.46% respectively. This observation was agreed with the study of Rasaiahet al., (1990). They observed a decreased soil water holding capacity in soils polluted with spent hydrocarbon oil. Spent lubricant oil pollution in the soil causes an increased bulk density, decreased water holding capacity and aeration propensity (Kayodeet al., 2009). Agbogidi and Enujeke (2012) reported that plots with spent oil contamination had reduced water infiltration and percolation. Reduced hydraulic conductivity results in a low “soil water transmission”. It also leads to less water availability for plant roots to access for photosynthetic processes.
3.1.8. Soil Bulk Density
Bulk density was higher in the soil sample D (1.40g/cc) (Table 1) compared to the control soil sample. Bulk density of the soil samples A, B, C and D, treated with spent engine oil are 0.98, 1.17, 1.24, and 1.40g/cc respectively. The bulk density of control soil sample was 1.00g/cc. This result was agreed with the study done by Ewetola E Abosede (2013) observed that crude oil pollution increased bulk density and reduced total porosity of the soil. When comparing the control soil with polluted soil, the bulk density was increased by 7.1% and also the total porosity was reduced by 8.5%. This may result as blockage of pores spaces with the pollutant. Similar observations are also found in the study of Oyem and Isama Lawrence (2013). In this, values of the average bulk density obtained from Orgonoko and Arunton areas were1.31g/cm3 and 1.27g/cm3 respectively. But the oil impacted sample has the value of 4.16 g/cm3. Similarly Kayodeet al., in 2009, observed an increased bulk density in soils polluted with spent lubricant oil. Effect of bulk density of the soil by spent engine oil pollution is evident in the study of Nwite J N and Alu M O (2015) that, bulk density of spent engine oil treated soil was significantly (P<0.05) higher than the control. The significant increase in bulk density of spent engine oil treated soil could be attributed to compaction resulting from oil contamination as well as reduced porosity.
3.1.9. Soil Particle Density
Particle density was higher in the soil sample D (Table 1). The particle density of the soil samples A, B, C and D are 1.79, 1.96, 2.01 and 2.03 g/cc respectively. Particle density of the control was 1.62 g/cc, which was less than the particle densities of soil samples A, B, C and D. Increased bulk density of spent engine oil treated soil also affect the particle density of the soil. Compaction of soil results in increased particle density and also caused reduction in total porosity as the pore spaces could have been clogged by dispersed soil particles (Nwite J N and Alu M O, 2015).
3.1.10. Soil Porosity
The porosity of soil sample was higher in the control soil sample E (44.13%). The porosity of spent engine oil treated soil samples A, B, C and D are 39.63%, 37.07%, 32.36%, and 17.09% respectively. Porosity was very less in the soil sample D (Table 1). This result was agreed with the study of Nwite J N and Alu M O (2015). There was an inverse relationship between bulk density and total porosity of the soil. Spent engine oil pollution increased the total porosity of the soils. This was mainly because of the compaction of the soils. In the polluted soils the pore spaces have been clogged by dispersed soil particles.
Table 1
Analysis of soil parameters
Soil Sample | pH | EC mhos/cm | Org. C (%) | N (kg/h) | P (kg/ha) | K (kg/ha) | WHC (%) | BD (g/cc) | PD (g/cc) | Porosity (%) |
A (50 mL) | 6.3 | 0.26 | 1.71 | 191 | 86 | 152 | 41.10 | 0.98 | 1.79 | 39.63 |
B (100mL) | 6.5 | 0.26 | 1.95 | 184 | 74 | 143 | 38.70 | 1.17 | 1.96 | 37.07 |
C (200mL) | 6.9 | 0.24 | 1.98 | 176 | 70 | 121 | 31.53 | 1.24 | 2.01 | 32.36 |
D (300mL) | 7.3 | 0.21 | 2.07 | 165 | 64 | 103 | 31.46 | 1.40 | 2.03 | 17.09 |
E (control) | 6.2 | 0.41 | 1.67 | 203 | 92 | 220 | 43.84 | 1.00 | 1.62 | 44.13 |
3.2. Effect on selected phytochemicals of Amaranthushybridus
3.2.1. Estimation of Alkaloids
Alkaloid content in the leaf samples was high in the control E compared to the A, B, C, and D samples (Table 2). Alkaloids in the leaf samples A, B, C and D are 2.45, 2.18, 1.83, and 0.98 mg/100gm respectively. This observation was agreed with the results of OnyegemOkerentaet al., (2002). In their study, the alkaloid content in the plant material was significantly decreased due to the spent engine oil pollution. In a study conducted by Emurotu and Marvelous Olubunmi (2019), the bean plant grown in crude oil contaminated soil only alkaloid was present while the rest of phytochemicals (phenol, flavonoids) were absent.
3.2.2. Estimation of Flavonoids
Flavonoid content in the control sample was 0.82 mg/100gm and it was higher than that of the samples A, B, C and D. In a study conducted by Emurotu and Marvelous Olubunmi (2019), the bean plant grown in crude oil contaminated soil only alkaloid was present while the rest of phytochemicals were absent. Total phenol and flavonoid contents showed a decline of 57.6% and 41.3% respectively in the Amaranthuscruentus samples exposed to PAH pollution (Tandey R et al., 2020).
3.2.3. Estimation of Saponin
The content of saponin in the leaf sample was higher in the control E (Table 2). Saponin content in leaf samples of the plants grown in the soil samples A, B, C, and D were 1.21, 1.03, 0.87 and 0.73 mg/100gm respectively. This result was agreed with the findings of OnyegemOkerentaet al., (2002). The impact of the pollution results in a marked reduction in saponin concentration in A. esculentus as the level of pollution in the soil increased respectively. The responsive reduction in the saponin content could be attributed to the higher levels of phytotoxic damage to the plant that might have been caused by spent engine oil pollution.
3.2.4. Estimation of Tannin
The tannin concentration was higher in the plant grown in highly polluted soil sample D (0.91mg/100gm). The tannin content was lower in the control sample E (Table 4). Tannin in the leaf samples A, B, C, and D was 0.51, 0.58, 0.65, and 0.91mg/100gm respectively. In food legumes, tannin that occurs naturally reacts with complexes of tannin-protein that form with protein which result to inactivation of some enzymes and protein insolubility as reported by Reddy N.R. et al (1985).
3.2.5. Estimation of Total Phenol
The total phenol content in the leaf samples was higher in the control E (0.35mg/100gm) and it was lower in the leaf sample D (Table 2). The total phenol content in the leaf samples A, B, C, and D were 0.26, 0.19, 0.13, and 0.09 mg/100gm respectively. In a study conducted by Emurotu and Marvelous Olubunmi (2019), the bean plant grown in crude oil contaminated soil only alkaloid was present while the rest of phytochemicals were absent. Total phenol and flavonoid contents showed a decline of 57.6% and 41.3% respectively in the Amaranthuscruentus samples exposed to PAH pollution (Tandey R et al., 2020).
3.2.6. Estimation of Phytic Acid
The phytic acid concentration was highest in the control sample E (Table 2). The phytic acid concentration in the leaf samples A, B, C, and D are 1.05, 0.87, 0.73 and 0.63 mg/100 gm respectively. Phytic acid concentration represents 50–85 % of total phosphorous in plants (Reddy et al., 1985).
Table 2
Phytochemical Screening of Amaranthushybridus
Leaf Samples/ Phytochemicals | Alkaloid (mg/100gm) | Flavonoid (mg/100gm) | Saponin (mg/100gm) | Tannin (mg/100gm) | Total Phenol (mg/100gm) | Phytic acid (mg/100gm) |
A (50 mL) | 2.45 | 0.71 | 1.21 | 0.51 | 0.26 | 1.05 |
B (100mL) | 2.18 | 0.66 | 1.03 | 0.58 | 0.19 | 0.87 |
C (200mL) | 1.83 | 0.62 | 0.87 | 0.65 | 0.13 | 0.73 |
D (300mL) | 0.98 | 0.58 | 0.73 | 0.91 | 0.09 | 0.63 |
E (control) | 3.54 | 0.82 | 1.68 | 0.49 | 0.35 | 1.32 |