2. Department of Botany, Obafemi Awolowo University Ile-Ife, Osun State, Nigeria
Changes in soil properties due to contamination from petroleum-derived substances can lead to water and oxygen deficits as well as to shortage of available forms of phosphorus and nitrogen(Udo and Fayemi, 1975). Contamination of the soil environment can also limit its protective function, upset metabolic activities, unfavorably affect its chemical characteristics, and reduce fertility and negatively influence plant production(Gong et al.,1996). Oil pollution also causes an increase in soil acidity and this affects nutrient uptake, root growth, nitrogen fixation, and other physiological processes of plants, leading to poor growth(Black, 1995). All these negative effects of soil pollution with petroleum derivatives threaten human health and that of the organisms that are dependent on the soil(Aboribo, 2001).
The soil environment is increasingly exposed to changes resulting from indiscriminate disposal of oil-based contaminants, including waste engine oil. It has therefore become important to devise means to remediate soils polluted by these substances because of the effects of such contamination on important food crops, such as tomatoes. The tomato is one of the most popular and widely grown vegetables in the world. Its popularity is attributed in large part to its versatility and the variety it lends to the human diet. Ripe tomato fruit is consumed fresh and is utilized in the manufacture of a wide range of processed products, such as puree, paste, powder, ketchup, sauce, soup, and canned whole fruit. The unripe green fruits are processed into pickles and preserves. Moreover, tomato extracts have been used in traditional medicine to treat ulcers, wounds, hemorrhoids, and burns(FAO, 2005). Fawusi(1978) points out that commercial use of tomatoes is facing some major constraints due to the lack of primary processors; this is forcing food industries to process all tomatoes available in the open market, irrespective of the quality which is generally poor, with a high percentage of physical defects mostly due to environmental pollution.
Various methods of protecting plants against problems created by oil contamination include the use of sorbent materials, chemical dispersants, and burning, but most of these methods have undesirable ecological effects on the crops and the environment. Therefore, protecting the plants with vegetative components that form parts of the environment becomes necessary(i.e., biodegradable)(Gelta et al., 2004; Lin and Mendelsson, 2004). As a means of remediating soils polluted with petroleum derivatives or crude oil, soil amendments such as sawdust, peat, waste cotton, and organic manures are added to the soil(Tanee and Akonye, 2009)in order to increase the ability of the soil matrix to supply biologically available water and nutrients to microorganisms that are capable of degrading the target compounds. In addition, the amendments bind pollutants to reduce acute toxicity of the soil's aqueous phase, thereby allowing microorganisms to survive in soils containing very high concentrations of toxicants.
Pollution from spent engine oil is a major environmental problem in Nigeria and is more widespread than crude oil pollution(Victor and Sadiq, 2002). Engine oil is a petroleum product which is used to reduce the friction between engine surfaces; it is produced by vacuum distillation of crude oil. Obidike(1985) reported that engine oil contains many chemical additives, including amines, phenols, benzene, calcium, zinc, barium, magnesium, phosphorus, sulphur, and lead. The disposal of waste engine oil into gutters, water drains, open vacant plots, and farml and is a common practice in Nigeria, especially by motor mechanics who change oil in motor vehicles, power generators, and other machines. In Nigeria, the existing mode of indiscriminate disposal of waste engine oil increases pollution incidents in the environment and it has been shown to be more widespread than crude oil pollution(Atuanya, 1987). Spills arising from the uncontrolled disposal of waste engine oil are becoming a visible problem. Therefore, the usual improper disposal of waste engine oil generated by service stations and other users is now a problem that needs serious attention(Victor and Sadiq, 2002).
However, since most of the methods used for protecting plants against oil contamination have undesirable effects on the crops and the environment, this study aims to provide information on the use of sawdust as an alternative, by determining the effects of sawdust soil amendment using the growth, fruit yield, and fruit quality of Solanum esculentum Linn.. 2 Materials and methods
Topsoil was collected r and omly to a depth of about 15 cm from an area on the campus of Obafemi Awolowo University in Ile-Ife, Nigeria, and was thoroughly mixed to obtain homogeneity. The soil samples were air dried and later sieved to remove the non-soil particles. Fresh waste engine oil was pooled from an auto mechanic workshop that specializes in the repair of heavy-duty trucks in Sabo, Ile-Ife and was stored in a 25-L jerry can. For the amendment, fresh sawdust was collected from a sawmill. It was air dried and passed through a 2-mm sieve. Sawdust less than 2 mm in size was retained for use.
The entire experiment was divided into three regimes: control(air-dried soil without waste engine oil and with sawdust), polluted(waste engine oil-contaminated soil), and amended(polluted soil treated with sawdust)(Table 1). For the control regime, 3 kg of soil was measured into perforated plastic buckets and further divided into two treatments(CT1, only air-dried soil; and CT2, air-dried soil with sawdust). For the polluted regime, 3 kg of soil was carefully measured onto a flat platform covered with cellophane, mixed with 30 mL, 60 mL, 90 mL, 120 mL, and 150 mL of waste engine oil, and further divided into five treatments(PT1, PT2, PT3, PT4, and PT5)with the contamination levels corresponding to each treatment respectively. Each treatment was replicated five times in a completely r and omized design consisting of 30 pots. For the amended regime, soil samples were also mixed with 30 mL, 60 mL, 90 mL, 120 mL, and 150 mL of waste engine oil, 2 kg of sawdust, and further divided into five treatments(AT1, AT2, AT3, AT4, and AT5), with each treatment corresponding to the respective waste engine oil contamination levels(Table 1). They were also replicated five times in a completely r and omized design consisting of 30 pots.
|1||CT1(air-dried soil only)|
|2||CT2(air-dried soil with sawdust)|
|1% WEO(30 mL)||PT1||AT1|
|2% WEO(60 mL)||PT2||AT2|
|3% WEO(90 mL)||PT3||AT3|
|4% WEO(120 mL)||PT4||AT4|
|5% WEO(150 mL)||PT5||AT5|
|WEO, waste engine oil.|
A nursery bed was prepared for a hybrid tomato variety(Roma V F)obtained from the National Horticultural Research Institute(NIHORT)in Ibadan, Nigeria. The tomato seeds were planted in the nursery bed at the rate of five seeds per hole and at the depth of 3 cm. The nursery bed was watered daily for two weeks to ensure germination of the seeds. The seedlings were transplanted into perforated buckets established for each treatment at the rate of two seedlings per bucket. The entire setup was left in a screened house for two months. During this period the soil was carefully watered every day with 200 mL of water.
During the maturation period, the growth parameters of plant height, number of leaves, and number of branches per plant were determined. At harvest, the plants were pulled out and oven dried at 70 °C for 48 hours, and then the dry weights of the plants were determined(Nottidge et al.,2005).
In order to determine the effects of the sawdust amendment on the soil, the soil samples were analyzed for pH, nitrogen(N), phosphorus(P), potassium(K), organic carbon, and heavy metals such as chromium(Cr), lead(Pb), and cadmium(Cd)before applying the treatment and after the harvest. Soil organic matter contents were determined through the determination of carbon. The soil pH was measured in a 1:1 soil-water suspension, or 1:2 soil 0.01 M CaCl2 solution, using a glass electrode pH meter. Total N was determined by the Kjeldahl method. Total P was determined by using Bray-1 solution(Bray and Kurtz, 1945). K was determined on the leacheate by a flame photometer, while Cr, Pb, and Cd were determined using an atomic absorption spectrophotometer.
Analysis of variance and a Duncan multiple-range test were employed to test significant differences in the soil properties of the three regimes. 3 Results 3.1 Plant height
Growth performance of Solanum esculentum Linn. in the control regime showed that the plant height was higher in treatment CT1 than in treatment CT2(Figure 1). When the soil was contaminated with 30 mL and 60 mL of waste engine oil, respectively, the plant height in the amended soil was significantly higher than that in the polluted soil throughout the growing period(Figure 2). However, at the 90-mL contamination level, the effect of the sawdust amendment on the plant height was not significantly different from the polluted regime(Figure 2). At the 120-mL and 150-mL contamination levels, the plant height in the amended soil was higherthan in the polluted soil. In the amended regime, 30-mL and 60-mL contamination levels had higher plant heights, while those at contamination level levels of 90 mL, 120 mL, and 150 mL showed no significant difference in heights(Figure 2).3.2 Number of leaves
In the control regime, the number of leaves on Solanum esculentum Linn. increased over the five weeks. In weeks 4 and 5, there was no significant difference in the number of leaves obtained in CT1 and CT2(Figure 3). The number of plant leaves in both the polluted and amended regimes decreased with the increase in contamination level(Figure 4), except at the 90-mL(AT3) and 120-mL(AT4)contamination levels, where the numbers of leaveswere almost the same. There was significant difference in the numbers of leaves obtained in the two non-control regimes(Figure 4). The amended regime recorded the highest number of leaves in the five contamination levels(Figure 4).3.3 Number of branches
In weeks 1 and 2 there was no significant difference in the number of branches of Solanum esculentum Linn. obtained in CT1 and CT2, but the number of branches obtained in weeks 2, 3, and 5 were significantly different(Figure 5). There was no significant difference in the number of plant branches obtained in the polluted and the amended regime at 30-mL, 60-mL, 90-mL, and 120-mL contamination levels(Figure 6). At the 150-mL pollution level, the influence of the sawdust as an amendment on the number of plant branches became significant(Figure 6). The number of plant branches in both the polluted and the amended regime decreased with an increase in contamination, except at 90 mL and 120 mL in the amended regime where the same values were recorded(Figure 6).3.4 Plant dry weight
The plant dry weights clearly showed the effect of the sawdust soil amendment. Treatment CT1(only air-dried soil)had a higher plant mean dry weight than treatment CT2(air-dried soil with sawdust(Table 2). The plant dry weights decreased with increasing contamination levels(Table 2), but the sawdust was able to enhance the plant dry weight in the amended regime. For example, at the 150-mL contamination level, the mean values for plant dry weight were 0.28 g in the polluted regime and 1.02 g in the amended regime.
|CT1(air-dried soil only)||4.04|
|CT2(air-dried soil with sawdust)||2.92|
|WEO, waste engine oil.|
The chemical properties of the soil and sawdust used in the study before waste engine oil was added are presented in the Table 3. The pH for the topsoil indicates a slightly acidic soil condition. The organic carbon content of the soil was 50.10 g/kg, while the N, P, and K values were 3.20 g/kg, 10.40 mg/kg, and 2, 605.00 mg/kg, respectively. The heavy metal contents were Pb, 0 mg/kg; Cd, 0 mg/kg; and Cr, 10.75 mg/kg(Table 3). The organic carbon content of the sawdust was 53.9%, while the total N content was 0.07%. The available P content of the sawdust sample was 0.013%, while the K content was 0.058%. The selected heavy metals contents of the sawdust sample were as follows: Pb, 2.31 mg/kg; Cd, 0.48 mg/kg; and Cr, 37.20 mg/kg(Table 3).
|pH in water(H2O)||5.90||-|
Analysis of the soil samples after harvest showed that the pH of the soil was affected by the addition of waste engine oil; treatment CT1 soil had the highest pH value of 6.60, which was significantly different from the pH values at other levels of waste engine oil contamination(Table 3). The pH value reduced significantly(P ˂0.05)with increasing contamination levels of 30 mL(6.17), 60 mL(5.63), and 90 mL(5.33), but it was not significantly different at the 120-mL(5.10) and 150-mL(PT5)(5.13)contamination levels.
In the polluted regime, there was no significant difference in the organic carbon values obtained at the 30-mL(PT1) and 60-mL(PT2)contamination levels, but the values increased with increasing contamination levels(Table 3). The N content of the soil was clearly affected by the addition of waste engine oil. While the control soil had the lowest N value, the value increased significantly with increasing concentrations of waste engine oil and was highest in soil polluted with 150 mL(PT5)(Table 3).
The amended control soil(treatment CT2)soil had a lower pH value than the control soil that was only air-dried(treatment CT1). There was no significant difference(P <0.05)in the soil pH at 30-mL(AT1) and 60-mL(AT2)contamination. Similarly, at the 150-mL(AT5)pollution level, the sawdust significantly increased the pH value of the soil.
Organic carbon values were lower in the amended soil samples than in the polluted soil samples at all of the levels of contamination. The N concentration was higher in the polluted soil compared to the soil amended with sawdust. At the 30-mL contamination level, the sawdust did not improve the P content of the soil sample because the polluted soil sample had a higher P value than the amended soil sample, but the effect of sawdust on the P content of polluted soil increased at the 150-mL level. Also, at the 150-mL contamination level, the effect of the sawdust on the K value of the soil became significant, with the amended soil sample having a higher K value than the polluted soil sample(Table 4).
|Control||CT1||6.60e||47.88a 27.02a||2.62a 2.36a||33.35c||3.19b||12.20b||Nd||Nd|
|Note: Columns and groups with the same letter(s)in each are not significantly different at P <0.05. OC, organic carbon; WEO, waste engine oil; Nd, not detected.|
Pb and Cd were not detected in the soil samples after harvesting of Solanum esculentum Linn., but Cr was detected. There was no significant difference in Cr concentration at 90-mL(AT3) and 120-mL(AT4)contamination in the amended soil sample, but both were significantly different(P ˂0.05)from the values obtained at other pollution levels(Table 4). There was no significant difference in Cr values at 90-mL(PT3), 120-mL(PT4), and 150-mL(PT5)contamination levels in the polluted soil samples. 4 Discussion
The presence of waste engine oil in the soil-plant microenvironment appears to have affected normal soil chemistry wherein nutrient release and uptake as well as amount of water have been reduced. In this study, there was a remarkable decrease in the growth of Solanum esculentum Linn. plants growing in waste engine oil-polluted soil relative to control soil. Seedlings transplanted into the control soil grew better than those in the polluted soil samples. The growth of Solanum esculentum Linn. was inhibited with increasing waste engine oil-pollution levels. This result agrees with the works of Kinghorn(1983), Anoliefo and Vwioko(1995) , and Sadiq(2002) , who noted poor growth of Capsicum annum and Solanum esculentum Linn. when treated with 4% and 5% waste engine oil.
Plant heights, number of leaves, and number of branches of the control soil plants were significantly higher than those in the polluted soils. This agrees with the work of Hazel(2011) , who reported that oil in soil creates unsatisfactory conditions for plant growth, ranging from heavy metal toxicity to insufficiency in aeration of the soil. The physical properties of the oil may have imposed some stressful conditions which may have interfered with water uptake and gaseous exchange(Amakiri and Onofeghara, 1984). These conditions in turn result in physiological drought(Anoliefo and Edegbai, 1995). Damage toSolanum esculentum Linn. plants may also have resulted from increase in temperature due to the dark color of contaminated soils. It was observed in this study that the contaminated soils were darker than the control soil, and dark soils absorb more heat than light ones(Donahue et al.,1990).
Plant height, number of leaves, and number of branches of Solanum esculentum Linn. were found to be affected by the application of sawdust as an amendment in the waste engine oil-polluted soil. At the 120-mL contamination level, the height of Solanum esculentum Linn. was 6.8 cm, compared to 10.5 cm in the amended soil. Also at the 120-mL contamination level, the number of leaves and branches in the polluted soil were 6 and 3, respectively, but were 13 and 4, respectively, in the amended soil. The improvement in the growth and yields of Solanum esculentum Linn. observed in the amended soil compared to the polluted soil sample may have been due to an increase in soil nitrate. This agrees with Tanee and Kinako(2008) and Tanee and Akonye(2009) , who reported an increase in yields of cassava in a crude oil-phytoremediated soil amended with sawdust. It is also in line with Starbuck(1994) and Davies and Wilson(2005) , who reported that sawdust has been found to be a good soil amendment because of its ability to improve soil properties. The plants grown in the soil without waste engine oil contamination grew better than those in the contaminated soil, irrespective of whether or not sawdust was added to the soil. This is similar to the findings of Victor and Sadiq(2002) , Adedokun et al.(2007), Nwoko et al.(2007) , Ikhajiagbe and Anoliefo(2010) , and Tanee and Albert(2011).
Our soil analysis showed a significant decrease in soil pH values with increasing waste engine oil contamination levels. According to Alex and er(1999) and Eweis et al.(1999) , a decrease in soil pH with the addition of waste engine oil, could be due to the degradation of hydrocarbons which may cause the release of acidic intermediates and final products that probably lowered the pH of the soil. Our results showed that in the amended soil, at high contamination levels, sawdust significantly increased the pH of the soil. This was attributable to the decomposition and mineralization of the sawdust.
Several studies(e.g., Agbim and Adeoye, 1994 ; Ayuba et al.,2005)have reported that organic wastes incorporated into the soils are capable of increasing the soil pH because they contain exchangeable cations. In our study, after harvesting of the Solanum esculentum Linn. plants, there was significant increase in the pH of the amended soil compared to the polluted soil. This agrees with the work of Ikhajiagbe and Anoliefo(2010) , who reported increased pH of waste engine oil-polluted soil amended with sawdust. Those researchers concluded that this increase could have been from high metabolic activities possibly due to production of intermediate metabolites in the compost system.
Our results also showed that there was a significant increase in the organic carbon of the soil with increasing concentration of waste engine oil. This may be attributed to the high carbon content in the oil, which might have been converted to soil organic carbon. Similar findings have been reported by Benka-Coker(1989) and Ekundayo and Obuekwe(1997). In our study, the organic carbon content was higher in the polluted soil than in the amended soil samples. This may be attributed to the high nutrient level, especially nitrate, in the amended soil samples which stimulated microbial populations and activities. This agrees with the work of Tanee and Albert(2011).
The N level was higher in the waste engine oil-polluted soil than the control soil in our study. This agrees with the finding of Odu(1972) , who reported an increase in N content of an oil-polluted soil. This could have been due to high organic matter in the soil. Our data also showed that N was higher in the polluted soil than in the amended soil. This, according to Tanee and Albert(2011) , was probably due to the increase in the microbial activities and populations in the polluted soil samples amended with sawdust, which invariably means increase in nutrient dem and by the microorganisms for metabolic activities. Olayinka and Adebayo(1985) also attributed this to the high C:N ratio in sawdust, which leads to N immobilization and a retardation of the decomposition process.
Starbuck(1994) also reported decreases in both N and P in greenhouse and field studies with tomatoes using sawdust as soil amendment. However, in our study there was no significant difference in the P content of the soil at the 30-mL and 60-mL waste engine oil contamination levels. This is in contrast with the work of Odu(1972) , who reported an increase in P content of an oil-polluted soil.
Our results also showed a higher K content in the amended soil than in the polluted soil. This agrees with the work of Starbuck(1994) , who reported an increase in K content of soil amended with sawdust.
After plant harvesting in our study, the Cr content of the soil was higher in the amended soil than in the polluted soil sample at the 30-mL contamination level. However, at the 150-mL contamination level, the sawdust significantly remediated the Cr content of the soil. This is in line with the work of Ikhajiagbe and Anoliefo(2010) , who reported a decrease in heavy metal concentrations in waste engine oil-polluted soil after remediation with sawdust. 5 Conclusion and recommendation
Considering the effects of sawdust on the growth performance of Solanum esculentum Linn. vis-à-vis height, number of leaves and number of branches, it can be concluded that sawdust has the potential of amending waste engine oil-polluted soil for increased tomato performance because it is capable of increasing the soil nutrient content and reducing the soil total hydrocarbon toxicity. Sawdust is therefore recommended as a secondary treatment option for waste engine oil-polluted habitats.
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