Soil Seedbank Dynamics and Regeneration of Vegetation on Oil Spilled Sites in Baruwa Community,Alimosho Local Government Area of Lagos State.
The study determined the floristic composition of the standing vegetation and soil seedbank around oil spilled site, examined the relationship between standing vegetation and the soil seedbank as well as determined the effects of petroleum hydrocarbon pollutant on soil around the oil spilled site. This was with a view to determining the effects of oil spill on above and below-ground vegetation in oil polluted area. Sampling was carried out in five plots of 25 m x 25 m using purposive sampling techniques. Five plots were selected based on their physical characteristics such as severely burnt (Plot 1), burnt with scanty re-growth of grasses (Plot 2), burnt but dominated by grasses (Plot 3), burnt but dominated by herbs (Plot 4) and partially burnt with herbaceous plants (Plot 5). In each sample plot, plant species were identified in the field and those of unknown identity were collected and pressed for later identification at the IFE Herbarium, Department of Botany, Obafemi Awolowo University, Ile - Ife. The species were classified into grasses (annual or perennial), trees, herbs, shrubs, creepers and sedges. Individual plants were counted to determine the abundance of each species. The composition of trees and shrubs > 1 m height and their density was recorded. Species richness, diversity and dominance for each of the plot were determined. Soil samples for the soil seedbank were collected at two different seasons (raining and dry) at the same locations. Five quadrants of 5 m x 5 m were marked out in each 25 m x 25 m plot. Twenty five soil samples were collected at random in each sampling site at two soil depths (0 – 15 cm and 15 – 30 cm) and were pooled together, to form one composite sample for each of the two depths in each of the sampling sites. Germinated seeds were observed and counted till six months when no further seed germination was confirmed. The composite soil samples were analyzed for petroleum hydrocarbon and heavy metals (Lead, Iron, Copper and Cadmium) using standard procedures. Data collected was subjected to analysis of variance to compare the relationships between the soil seedbank and the floristic composition of the vegetation. The results showed that the frequency of occurrence of the vegetation were 917 herbaceous plants, 624 grasses, 186 creepers, 11 Sedges and 83 shrubs. Two (2) tree species were recorded in the oil spilled area. The total number of families recorded in the oil spilled area was sixteen (16). Shannon – Weiner index showed that species found in the burnt but dominated by herbs, were more diverse in their composition compared to other plots. Herbaceous species had the highest percentage contribution to the total seedbank density in both dry and raining seasons and at different depths, followed by grasses. No tree species emerged from the soil seedbank of the oil spilled area in both seasons. Sorenson index of similarity showed low similarity between the above-ground and below-ground vegetation of the oil spilled area. There was variation in the seedbank density in relation to soil depths in the oil spilled area in both seasons. The seedbank density was higher in the dry than the raining seasons at both soil depths. The seed density decreased with increase in soil depth by 14.5% during raining season and 32.1% during the dry season. The heavy metals (Lead, Cadmium, Iron and Copper) as well as the petroleum hydrocarbon in the soil of the oil spilled area were higher than the recommended level by Federal Ministry of Environment. The study concluded that oil spill impacted the species composition of the above and below-ground vegetation of the oil spilled areas.