Effects of soil attributes and straw accumulation on the sorption of hexazinone and tebuthiuron in tropical soils cultivated with sugarcane

Brazil is the largest sugarcane producer in the world in which hexazinone (3-cyclohexyl-6-dimethylamino-1-methyl-1,3,5-triazine-2,4-dione) and tebuthiuron (1-(5-tert-butyl-1,3,4-thiadiazol-2-yl)-1,3-dimethylurea) are heavily used. Sugarcane harvesting is changing from the manual system with previous straw burning to the mechanized system without straw burning. The lack of burning results in soil organic carbon accumulation mainly in clayey soils, which should affect herbicides availability and fate. Therefore, we evaluated sorption of these herbicides in soil samples with and without straw burning. Both herbicides presented low apparent sorption coefficients (mean K-d,K-app= 0.6 and 2.4 L kg(-1) for hexazinone and tebuthiuron, respectively), suggesting that they may leach to groundwater. Moreover, their sorption correlated primarily with soil organic carbon (SOC), but iron oxide contents extracted with ammonium oxalate (Fe2O3AOX) also affected it (K-d,K-app = -0.228 + 0.0397 SOC + 0.117 Fe2O3AOX for hexazinone and K-d,K-app = -1.407 + 0.201 SOC + 0.348 Fe2O3AOX for tebuthiuron). Soil organic carbon accumulation due to straw maintenance in the field positively affected sorption of both herbicides, but its effects were not enough to classify them as "non-leachers."