Effect of Soil Management on Terbuthylazine Behaviour in an Olive tree crop Soil at Southwest Spain

Comunicación presentada en el 17th International Symposium on Environmental Pollution and its Impact on Life in the Mediterranean Region, celebrado en Estambul del 28 de septiembre al 1 de octubre de 2013.Contamination of surface and groundwater in the Guadalquivir River basin by herbicides has been shown to be closely related to the very intensive and extensive olive crop in that area (Hermosin et al. 2013). Also, the South Spain climate and geographic characteristics contribute to make a worse scenario. The aim of this work was to evaluate two soil managements (cover crops and conventional tillage) in the soil behavior of the herbicide terbuthylazine, very widely used in olive crop, and as related to its water presence. Particularly, to know the soil processes affecting the persistence of terbuthylazine herbicide in a soil devoted to olive crop (Benacazón, Seville, SW Spain) by comparing the same soil under cover crops (SCC) and conventional tillage (PA and PB) managements. These soils were sampled at the field and the experimental assays were carried out in the lab to measure the herbicide soil sorption (adsorption-desorption), dissipation and leaching processes. In parallel, a complementary field experiment was carried out at the conventional tillage area by herbicide application in two selected plots (PA and PB) which were sampled during three months. Soil PB is the same as PA but without previous terbuthylazine field application and SCC is the same as PB and PA but with cover crops established during 8 years; all of them in the same area of study. In lab experiments, higher adsorption was found in SCC soil than in PB soil, mainly due to the higher SOM (soil organic matter) content of SCC. Furthermore, this adsorption was very irreversible in both soils. Soil incubation experiments (62 days) did not show relevant differences in terbuthylazine dissipation between PB soil and SCC soil, which is a surprising result, because SCC management generally enhance soil microorganism population and usually it could to increase the herbicide degradation. Soil column leaching experiment showed higher and faster terbuthylazine leaching in PB soil (68%) as compared to SCC soil (20%), also due to the higher SOM content of SCC versus PB, and as a consequence of the enhanced irreversible soil adsorption process. The field study showed a terbuthylazine half-life of 35 days, recovering 15% of herbicide applied after 85 sampling days. The temporal evolution in the soil profile of field herbicide residues versus time and rain showed that field losses are mainly related to the amount of rain and hence to the leaching and runoff processes. The lab results suggest that cover crops are not only are relevant to decrease pesticide runoff process, soil losses or erosion, but also they could to contribute in decreasing the herbicide soil leaching losses, by increasing soil irreversible adsorption and hence decreasing groundwater contaminationPeer Reviewe