Soil faunal activity of an oil-polluted tropical alfisol amended with organic wastes as determined by micromorphological observations

Abstract

This study evaluates the effect of used-lubricating oil (spent oil) and organic nutrient supplements as derived from animal droppings on faunal activity as expressed in the incidence of faunal excrement and related micromorphological features. This approach allows the effects of the treatments to be evaluated even if the organisms are no longer present. The main-plot treatments included control (C), cow dung (CD), poultry droppings or manure (PM) and pig droppings or waste (PW) each at 10 Mg ha −1 each. The sub-plot treatments were control (no applied contaminant), 0.5%, 2.5% and 5% spent oil (SP) applied at 10, 50 and 100 Mg ha −1 (w/w), respectively. Residual oil blobs were clear in soil thin sections and the results from point counting demonstrated a significant reduction in oil content following the addition of organic wastes to the polluted soils. Spheroids of organic material interpreted as excrement from enchytraeids dominated thin sections from the control and treated plots. Other observed excrement types were undifferentiated (fused) excrements formed from coalesced or “aged” excrements. The occurrence of the total excrements showed no faunal activity in plots polluted with 0.5%, 2.5% and 5% SP, indicating elimination of soil fauna by oil pollution. Results from point counting of excremental features indicated significant faunal activity with CD, PM and PW treatments, and in oil-polluted plots supplemented with organic wastes. The treatments with PM and PW were better than CD in stimulating oil degradation. Treatments also showed a reduction in total porosity with an increase in oil concentrations whereas no significant improvement was observed in oil-polluted soils amended with organic wastes. However, addition of the organic wastes to oil-polluted soils showed evidence of increased decomposition in the order PM > PW > CD. In conclusion, the substantial reduction in soil faunal activity as a result of oil treatment confirmed the deleterious effect of petroleum products on living organisms, but the addition of PM and PW were effective in ameliorating this negative impact.