Short-term effects of olive mill wastewater application on the hydrological and physico-chemical properties of a loamy soil

Wastewater from the olive oil industry is often spread on soils, but this practice requires caution because of the possible harmful hydrological and physico-chemical effects on soils treated. While research has mainly focussed on the long-term changes in soil properties, very few studies refer to short-term effects, which appear a few weeks or months after Olive Mill Wastewater (OMW) application. Mediterranean areas are particularly prone to runoff and soil erosion risks, and the evaluation of the short-term effects of OMW on hydraulic conductivity and other physico-chemical properties of soils in this area are scarce. To fill this gap, this study has evaluated the effects of OMW spreading on the hydrology of a loamy soil at one and three weeks after application. More specifically, irrigation tests with OMW were carried out in olive field plots in Calabria, Southern Italy; the infiltrability and physico-chemical properties of treated soils were compared with unirrigated soils and soils irrigated with clean water, such as groundwater. A temporary reduction in the soil infiltration rate was detected immediately after OMW irrigation, although it was followed by a significant increase after three weeks. In the topsoil, irrigation with OMW produced an increase in phosphorous, potassium and organic matter contents as well as a higher stability of aggregates. In the deeper layer of soil treated with OMW the contents of organic matter and nutrients as well as of polyphenols were lower than in the unirrigated plot. Potassium, calcium and sodium contents were higher in the soil irrigated with OMW both in topsoil and deeper layers. Principal Component Analysis highlighted a noticeable influence of treatments on soil micronutrient contents, Cation Exchange Capacity and the stability of aggregates with slight differences between topsoil and deeper soil layers. Conversely, the concentration of organic matter, polyphenols and nutrients is noticeably stratified in both the topsoil and the deeper layers regardless the type of treatment.