Oxyfluorfen is an herbicide used in olive orchards as an alternative to diuron and terbuthylazine recently banned in Andalusia (Spain) for  this cultivar. Soil processes affecting the behavior of oxyfluorfen and the effect of oil mill waste (OMW) amendment were assessed at laboratory conditions. Oxyfluorfen photodegradation, dissipation and leaching experiments were studied in two soils (P2 and SJ) with different textural properties. These soils were amended with composted OMW (5% w/w). Photodegradation of this herbicide applied to amended and unamended soils was recorded under xenon irradiation on during 96 h. In general, low oxyfluorfen photodegradation was observed (SJ 15% and P 20% photodegraded). Moreover, addition of OMW did not affect  the photodegradation process, although natural soil organic matter seems to have a slight protection effect upon herbicide adsorption. Dissipation study was carried out under two differents soil moisture contents, 20% and 40% of field capacity (F.C.).at 25ºC for 84 days. Higher dissipation of oxyfluorfen was found at 40% soil moisture than at 20% soil moisture in both amended soils. The amendment of soil  produces an increase in the persistence of oxyfluorfen in both soils and at both moisture contents. Herbicide persistence lab experiments showed that dissapearance was directly related with soil moisture and it seems is mainly due to biodegradation processes, which are balanced by herbicide adsorption  on soil organic and inorganic colloidal components. Leaching   study was carried out at 25º C in handpacked soil columns. No oxyfluorfen residues were found in leachates. An increase in OMW content has a different effect depending on the type of soil. It seems that O.M. plays a protective action on the chemical degradation, increasing the persistence of oxyflourfen and the risk of leaching.

Acknowledgement: This work has been financed by Junta de Andalucía through the project P06.AGR-01565, Research GroupAGR-246 and FEDER funds.