Sorption, leaching and persistence of metribuzin in Mediterranean soils amended with olive mill waste of different degrees of organic matter maturity

• Olive mill waste (OW) influenced soils' microbial activity and behaviour of metribuzin.
• The effects were especially dependent on the organic matter's maturity.
• Only fresh OW could prolong the persistence of metribuzin.
• Use of OW may be an option for the control of metribuzin leaching in Mediterranean soils.

Metribuzin is a widely used herbicide, and worldwide is one of the most important contaminants in ground and surface waters. The aim of this study was to evaluate the impact amendment with fresh, composted, and field-aged olive mill waste (OW, COW, and AOW, respectively) had on the behaviour of this herbicide in four typical Mediterranean soils. A batch equilibration method was used to determine metribuzin adsorption–desorption. Leaching experiments were studied in hand-packed soil columns. Half-lives were calculated with incubation studies. Soil dehydrogenase activity (DHA) was also monitored. Metribuzin adsorption in the soils increased not only with increasing amounts of amendment, but especially with increasing degree of organic matter humification. Compared to unamended soils, the adsorption capacity increased by between 81% and 216%, 135% and 193%, and by 363% for the OW, COW, and AOW amended soils, respectively, at a 5% rate of application. The addition of COW enhanced metribuzin degradation in all the soils. In contrast, OW addition increased metribuzin persistence, attributable mainly to the inhibitory effect of this amendment on microbial activity, especially in the acidic soils. The AOW-amended soils, which had the smallest labile fraction of soil organic matter and greatest degree of humification, showed the shortest herbicide persistence. The OW and COW amendments significantly reduced the amount of metribuzin leached. This was especially so in the latter case because of the higher sorption capacity and the faster degradation of the pesticide. The use of OW as organic amendment, especially when it has a high degree of organic matter humification, may be a useful management practice for reducing the risk of groundwater contamination by metribuzin in soils with low organic matter content.