Spectroscopic and Physicochemical Insights into Soil Organic Matter Humification Following Long-Term Olive Mill Wastewater Application in Mediterranean Arid Soil

Olive millwastewater (OMW), a by-product of olive oil production, is known for its high concentrations of organic matter, phenolic compounds, and nutrients, which can pose environmental risks if released untreated. However, OMW can also serve as a potential soil amendment under controlled application. This study investigated the long-term effects of repeated OMW applications on the physicochemical properties and humification of soil organic matter in a Mediterranean arid soil (Eutric Regosol). Soils were sampled at two depths (0–20 and 20–40 cm) in experimental plots receiving different OMW doses over nine years. Soil chemical parameters, including pH, electrical conductivity, cation exchange capacity and nutrient contents, were analyzed. Humic substances were extracted and characterized using spectroscopic techniques, including UV–Visible (E4/E6 ratio) and Fourier-transform infrared (FTIR) spectroscopy, to assess molecular changes and humification dynamics. Results revealed that OMW application enhanced soil organic matter content, improved nutrient availability, and increased cation exchange capacity, particularly in surface layers. Phenolic compounds accumulated mainly in the topsoil, while humification indices (HA/FA ratio) indicated an increased degree of soil organic matter stabilization under higher OMW doses. UV–Visible and FTIR analyses confirmed enhanced condensation of aromatic structures and incorporation of nitrogen- and carboxyl-containing compounds in soils receiving OMW. These findings demonstrate that long-term OMW applications can promote the formation of stable humic substances, improving soil quality and fertility in arid Mediterranean environments.