Ecotoxicity of measured concentrations of soil-applied diesel: Effects on earthworm survival, dehydrogenase, urease and nitrification activities

Diesel is an important petroleum product, and a common pollutant in soil caused by leaks and accidental spills. Studies dealing with the ecotoxicity of diesel towards earthworms always relied on growth inhibition endpoint (EC50) values that were determined based on the spiked concentrations (nominal), ignoring the substantial portion of hydrocarbons volatilized from soil. In the present study we used, for the first time, the measured concentrations of total petroleum hydrocarbons (TPHs) from soil-applied diesel to assess earthworm (Eisenia fetida) survival, and the activities of dehydrogenase, urease and nitrification as indicators of soil health. The mortality endpoint (LC50) value for initially measured concentrations after exposure of earthworms to diesel for 28 days was 916 +/- 10 mg TPHs kg(-1) soil which was equivalent to the nominal (initially added) concentration of 1426 +/- 19 mg TPHs kg(-1) soil. Morphological abnormalities such as clitella swelling and curling were noticed when the measured concentrations of diesel were more than 971 mg kg(-1) soil. Significant increases in activities of soil dehydrogenase (38-58%) as well as urease were observed in the diesel-applied soil. Presence of earthworms further enhanced the activities of these soil enzymes. Nitrification was sensitive to application of diesel to soil, and it was inhibited in a dose-related manner even in the presence of earthworms. The differential response of the toxicity criteria to diesel-contaminated soil observed in the present study clearly warrants more studies involving several soil health parameters to arrive at a generalization of ecotoxicity of an environmental pollutant.