Soil-dissipation kinetics of twelve herbicides used on a rain-fed barley crop in Spain.

This study evaluated the dissipation kinetics under actual field conditions of twelve herbicides in a typical xerofluvent soil in Castilla y León (north central Spain) sustaining barley. The type of soil selected was that typically used in the Castilla y León region to cultivate barley under a rain-fed alternating crop-fallow rotation regimen. Treatments were conducted in spring as two replicates and the soil was sampled every day during the first week, once a week for the following few weeks and thereafter once every month. Soil samples were extracted with a suitable mixture of acetone, water and acetic acid (30:7.5:0.3) before their analysis by GC-MS (gas chromatography-mass spectrometry). Dissipation of the herbicides was well described by a biphasic kinetics pattern. The dissipation times DT50 and DT90 were in general lower than those reported in the literature, owing to a high initial dissipation rate because of volatilization and photolysis processes caused by high environmental temperatures. Herbicide degradation was also enhanced by their lack of sorption by this low colloid-content soil. However, the most persistent herbicides, triallate, flamprop, pendimethalin, terbutryn, and isoproturon, remained for 286 to 372 days in the soil, because low water and organic carbon content impaired microbial growth. In contrast, the phenoxy acid herbicides dissipated rapidly, with no detectable residues detected on harvesting the crop.