Leachable Fraction of Olive Mill Waste Contaminated Soil: Effect on Earthworm Dendrobaena veneta and Treatment with Additives

Disposal of olive mill wastes presents a major threat to the environment due to the extremely high level of polyphenols unsuitable for plants and soil organisms. Data about the impact of wastes coming from the olive oil industry on non-target organisms are still scarce. Nevertheless, recently, we started paying attention to study the effect of olive mill waste liquid and solid fraction on soil organisms such as earthworms. Therefore, this study aimed to determine the effects of the leachable fraction extracted from an olive mill waste contaminated soil (OMWCS) on neurotoxicity (Acetylcholinesterase activity, AChE) and oxidative stress (Catalase activity, CAT) in the earthworm Dendrobaena veneta obtained from a laboratory culture at the department of biology (Osijek, Croatia). Moreover, the treatment of OMWCS by adding diatomaceous earth (D) or clay (C) at different rates has been investigated. Incubation of OMWCS with D (at 15 and 30% ratios) and with clay (at 15 and 30% ratios) lasted seven days. Sub-lethal bioassays were based on a filter paper contact test (OECD, OECD—Organization for Economic Co-operation and Development. (1984). Earthworm, acute toxicity tests. Test guideline 207. OECD Guidelines for the Testing of Chemicals, 207, 9. 10.1787/9789264070042-en) with extracted leachates from untreated and treated OMWCS during 72 h. For the untreated soils, induction in the molecular biomarkers has been observed compared to non-contaminated soil. In fact, the induction of AChE indicated that earthworms were consuming more energy in order to avoid stress and/or were due to a tissue inflammation. Those negative effects are likely consequences of the high phenol content. After the application of D additive at a ratio of 15%, CAT and AChE activity were recovered to the levels observed in the leachate from the clean soil indicating a restoration of homeostasis. No significant changes were observed using clay as an additive. Besides, additives effectively reduced polyphenols concentration up to 45% for clay and 55% for D. Even though additives could reduce the harmful effects of olive mill wastes, they should be carefully selected and quantified to not damage the soil indigenous organisms.