Olive oil production generates large amounts of olive mill wastewater (OMW). OMW has a high nutrient content and could serve as fertilizer, but its fatty and phenolic constituents induce soil water repellency, phytotoxicity, and acidification. An appropriate season of OMW application may mitigate negative consequences while preserving beneficial effects. In order to investigate this, a field study was conducted, in which OMW was applied to an olive orchard in Israel either in winter or summer. Soil-water interactions (water drop penetration time, hydraulic conductivity), soil physicochemical parameters, phenolic compounds, and soil biological activity (bait-lamina test) were determined 12 to 18 months after OMW application. The results showed elevated K + contents in all treatments, but all other soil properties of winter treatments were comparable to the control, which suggested a certain recovery potential of the soil when OMW is applied in winter. By contrast, summer treatments revealed a ten-fold higher soil water repellency, a three-times lower biological activity, and a four-fold higher content of phenolic compounds, independently of whether the soil was kept moist by irrigation or not. Thus, the OMW constituents were neither degraded nor leached by winter rain when applied during the hot season. Further research is needed to distinguish leaching and biodegradation effects, and to understand the development of the composition and degradation kinetics of organic OMW constituents.

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Although olive mill wastewater (OMW) is often applied onto soil and is known to be phytotoxic, its impact on soil fauna is still unknown. The objective of this study was to investigate how OMW spreading in olive orchards affects Oribatida and Collembola communities, physicochemical soil properties and their interdependency. For this, we treated plots in two study sites (Gilat, Bait Reema) with OMW. Among others, the sites differed in irrigation practice, soil type and climate. We observed that soil acidity and water repellency developed to a lower extent in Gilat than in Bait Reema. This may be explained by irrigation-induced dilution and leaching of OMW compounds in Gilat. In Bait Reema, OMW application suppressed emergence of Oribatida and induced a community shift, but the abundance of Collembola increased in OMW and water-treated plots. In Gilat, Oribatida abundance increased after OMW application. The effects of OMW application on soil biota result from an interaction between stimulation of biological OPEN ACCESSAgriculture 2015, 5 858 activity and suppression of sensitive species by toxic compounds. Environmental and management conditions are relevant for the degree and persistence of the effects. Moreover, this study underlines the need for detailed research on the ecotoxicological effects of OMW at different application rates.

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The controlled application of olive mill wastewater (OMW) as a by-product of the olive oil extraction process is widespread in olive oil-producing countries. Therefore, a sustainable approach necessarily targets the positive effects of soil resilience between successive annual applications to exclude possible accumulations of negative consequences. To investigate this, we applied 50, 100, 100 with tillage and 150 m3 OMW ha−1 y−1 for five consecutive seasons to an olive orchard in a semi-arid region and monitored various soil physicochemical and biological properties. OMW increased soil water content with concentration of total phenols, cations, and anions as well as various biological and soil organic matter indices. Soil hydrophobicity, as measured by water drop penetration time (WDPT), was found to be predominantly in the uppermost layer (0–3 and 3–10 cm). OMW positively affected soil biology, increased the activity and abundance of soil arthropods, and served as a food source for bacteria and fungi. Subsequent shallow tillage reduced the extent of OMW-induced changes and could provide a simple means of OMW dilution and effect minimization. Despite potentially higher leaching risks, an OMW dose of 50–100 m3 ha−1 applied every two years followed by tillage could be a cost-effective and feasible strategy for OMW recycling.