Characterization and Metabolism of Bound Residues of Three Herbicides in Soils Amended with Sugarcane Waste

This study evaluated the remobilization, mineralization, and metabolism of herbicide-bound residues in soils amended with various types of sugarcane waste. Soil with bound residues of three herbicides and fresh soil samples were added to the biometric flasks, followed by the addition of vinasse, filter cake, or sugarcane straw in order to reactivate the microbial activity. In sandy loam soil, higher mineralization was observed where filter cake was added, and the maximum mineralized percentage was 7.7, 46.7, and 8.1% of diuron-, hexazinone-, and metribuzin-bound residues, respectively. Conversely, this soil presented a greater percentage of re-extractable hexazinone and metribuzin residues when vinasse was added. Among the examined herbicides, a higher percentage of bound metribuzin residues remained in the soil (57.5–75.6%). It was possible to identify both metabolites and parent compounds in the re-extracted residues, implying either species could bind to the soil. Therefore, this study has shown that bound residues of three herbicides and their metabolites can become bioavailable, and mineralized or returned to the soil solution, which could adversely affect subsequent crops or non-target organisms. Hence, the remobilization of bound residues must be taken into account when assessing the environmental risk of herbicides in soils in registration processes.