Direct and residual effects on diuron behaviour and persistence following two-phase olive mill waste addition to soil: Field and laboratory experiments

Two-phase olive mill waste (TPOMW) is a by-product of olive oil extraction, contains up to 90% organic matter, and may be used as a soil amendment. In order to investigate the impact of TPOMW amendments to soil on the sorption-desorption, degradation, leaching, and persistence of the herbicide diuron, field and laboratory experiments were conducted on a representative olive grove soil. The soil was amended in the laboratory with TPOMW at the rates of 5% and 10%, and in the field with 30 and 60 Mg ha− 1 of TPOMW for seven years. Direct and residual effects were evaluated in the last year and two years after the last TPOMW field application (2005 and 2007, respectively). Significant increases in diuron sorption were observed with the greater amount of TPOMW in the laboratory and field-amended soils, mainly promoted by an increase in humic acid content, this process being more reversible in the laboratory than in the field amended soils. The TPOMW soil application only significantly increased the half-life of diuron in the laboratory amended soils, ranging from 8.5 days for the original soil to 32 days at the greater application rate. The TPOMW amendments significantly reduced the downward mobility of diuron, and reduced the amount of herbicide leached in the laboratory and field-amended soils, with a major residual decrease two years after the last waste addition since no herbicide leached through these amended-soil columns. In the field persistence study, TPOMW addition increased diuron retention mainly in the upper 30 cm of the soils, decreasing the herbicide's vertical movement through the amended soils. The residence time of diuron increased with increasing TPOMW rate, especially in the direct year (2005). The results revealed major changes in diuron's behaviour due to the different TPOMW loading rates and the transformation of the amendment during two years of biodegradation in the soil. This study has shown the usefulness of TPOMW for reducing groundwater contamination by pesticides for at least 24 months following its application.