Influence of two-phase olive mill waste application to soil on terbuthylazine behaviour and persistence under controlled and field conditions

Terbuthylazine is one of the most common herbicides used to control weeds in olive groves. Application of two-phase olive mill waste (OW) to soils may play a fundamental role in the management of leaching losses of pesticides, especially in Mediterranean areas where soils are characterized by low organic matter levels. We evaluated the impact of OW amendments on the sorption-desorption, degradation, leaching, and persistence of the herbicide terbuthylazine in a representative olive grove soil from Portugal. The soil was amended in the laboratory with OW at the rates of 5% and 10% (w/w), and in the field at the rates of 0.7% and 1.4% of OW for 7 years. Cumulative and residual effects were evaluated in the last year and 2 years after the last OW field application. A batch equilibration method was used to determine terbuthylazine adsorption-desorption. Leaching experiments were studied in hand-packed soil columns. Half-lives (t (1/2)) were calculated with incubation studies. In the field study, in order to determine the persistence and distribution of terbuthylazine down the soil profiles, at selected times after the herbicide application, soil subsamples were taken from different soil depths. The OW amendments significantly increased the adsorption of terbuthylazine to the soil. Applications of OW to soil significantly reduced the amount of terbuthylazine leached in the field-amended soil columns, with a major residual decrease (from 19.6% to 2.9% at the greater application rate) 2 years after the last OW application. Leaching losses of the herbicide, however, were increased by up to 69.3% for the laboratory-amended soil columns, mainly promoted by an increase in water-soluble organic carbon content. Addition of OW retarded the degradation of terbuthylazine from 6.7 days for the unamended soil to 15.7 and 89 days at the greater application rate in the field- and the laboratory-amended soils, respectively. In the field study, OW addition decreased terbuthylazine's vertical movement through the amended soils with increasing OW rate. The study has shown that OW amendment of olive grove soils may be an effective management practice for reducing the vertical movement of terbuthylazine through the soil. However, leaching loss of this pesticide could potentially increase in fresh OW-amended soils, especially if humification of the organic matter is not favoured. In addition, the use of OW could also increase the risk of surface water contamination, especially if application rates are greater than 30 t ha(-1).