Seasonal dynamics of olive mill wastewater behavior in soil: insights from a lysimeter experiment under semi-arid conditions

Abstract Olive mill wastewater (OMW), a by-product of olive oil production, is widely reused as an organic amendment in Mediterranean agriculture, yet its environmental behavior remains insufficiently characterized under realistic seasonal scenarios despite its growing agronomic relevance in many producing regions. OMW contains high concentrations of organic matter and polyphenolic compounds that can alter soil and groundwater quality. Understanding its fate in soil under seasonally variable climatic conditions is essential for evaluating environmental risks and valorization potential. OMW is widely reused as an organic amendment in Mediterranean agriculture, yet its environmental behavior remains insufficiently characterized under realistic seasonal scenarios despite its growing agronomic relevance in many producing regions. A laboratory lysimeter experiment was conducted to simulate OMW application to soil under semi-arid climatic conditions. Four seasonal phases two winters, one spring, and one summer were reproduced over 18 weeks. Leachate and soil properties were analyzed for soluble phenolic compounds (SPC), pH, electrical conductivity (EC), water drop penetration time (WDPT), and dissolved organic carbon (DOC) quality, specific ultraviolet absorbance at 254 nm (SUVA₂₅₄). Wet winter conditions enhanced OMW percolation, producing elevated SPC and EC levels in leachates, while moderate spring conditions promoted degradation processes, lowering SPC in leachates and reducing topsoil water repellency. Hot, dry summer conditions induced polymerization and accumulation of OMW-derived compounds at the topsoil, whereas the second winter simulation showed lower SPC values, indicating partial stabilization of the soil system after repeated exposure cycles Seasonal climatic variability thus exerts a strong control on OMW degradation and mobility in soil. These insights emphasize the need for season-specific guidelines for land application, particularly in semi-arid regions where rainfall distribution is highly irregular. The results provide a scientific basis for improving management strategies to minimize environmental risks and support the sustainable reuse of OMW as an organic soil amendment within integrated soil fertility programs.