The Role of Natural Weed Species from Soil Seed Bank in the Natural Attenuation of a Petroleum Hydrocarbon Polluted Soil

Aims: The present study investigated the effect of the presence of natural weeds in the remediation of oil polluted soils that have not been disturbed or modified anthropogenically.
Study Design: The experimental design chosen was the completely randomized design (CRD) following assumption of homogeneity of the experimental plot in use.
Place and Duration of Study: Botanic Garden, University of Benin, Benin City. Study period spanned from April 2011 through December 2012.
Methodology: Top soil was collected from a marked plot and thoroughly mixed with waste engine oil on weight basis to obtain 2.5%w/w oil-in-soil concentrations. Ten kilograms of the contaminated soil were measured each into experiment bowls. The entire set up was divided into 4 sets. The natural weeds that eventually emerged in the first set (Wwds) were left undisturbed throughout the duration of the experiment. Those in the second set (W4mw) were removed once every four months. Those in the third set were manually removed as soon as they appeared, leaving none on the soil surface (Wnon). The soils in the fourth set were sterilized before amendment with waste engine oil. The entire set up was exposed to the various treatment conditions for 20 months.
Results: There were significant reductions in composition of soil heavy metals and polyaromatic hydrocarbon contents in the all the treatments. The treatment showing most enhanced remediation was Wwds, followed by W4mw, Wnon, and Wctr/ste in that order. Seven out of the nineteen weed species that originally made up the soil seed bank, were identified in the polluted soil. These included Euphorbia hirta, Fluerya aestuans, Panicum maximum, Phyllanthus amarus, Spigelia anthelmia, and Tridax procumbens. The predominant weed species was Euphorbia heterophylla. These weed species are likely oil tolerant species.
Conclusion: The study further affirms that weed composition of any soil is to be reckoned with as an important factor in the natural attenuation of a petroleum hydrocarbon-polluted soil.