Spreading raw olive mill wastewater is compatible with the growth and the beneficial functions of the earthworm Octodrilus complanatus

Abstract Recycling Olive Mill Wastewater (OMW) by its application on agricultural land is considered to be a sustainable practice for soil enrichment and waste management, and recent legislation permits large scale OMW recycling in Mediterranean countries. Therefore, studies on the effects of OMW application on earthworms under experimental conditions that closely resemble those occurring in the field are urgently required to facilitate the safe and ecologically friendly use of OMW at large scale. In this study, the effects of three-phase OMW liquid on the survival and growth of the earthworm Octodrilus complanatus (Duges, 1828), commonly present in olive groves, were assessed. The liquid was applied at a rate i) equal to the maximum authorized by Greek legislation for olive groves fertilization (80 m3 ha−1) and ii) half of this amount (40 m3 ha−1). The effects were tested in an indoor experiment conducted in plastic containers. The results showed no adverse effect on the survival of the earthworms over a period of 56 days post-treatment, at both application doses. The growth of earthworms, as estimated by their body weight change at 28 or 56 days post-treatment, was not negatively affected; instead, there was a relative increase seen at the higher dose (80 m3 ha−1), although the difference between the treatments was not significant. Qualitative observations indicated feeding of earthworms on the OMW liquids. Organic matter decomposition was not affected and there was no significant alteration in the soil properties, except the electrical conductivity that was increased at the higher level of OMW application. Therefore, the application of OMW liquids at the doses recommended by the legislation does not affect the survival and growth of the earthworms and is compatible with their activities to promote organic matter decomposition.