The Role of Sugarcane Residues in the Sorption and Leaching of Herbicides in Two Tropical Soils

Sugarcane is a major crop in Brazil that generates huge amounts of organic residues that are usually left deposited in, or applied to the soil, and thereby affect the behavior of herbicides. This study assessed the effects of sugarcane residues (straw, ash, and compost) and residence time (“aging”) on the sorption of alachlor and diuron in two contrasting soils (LVd and LVAd), as well as the effects of these residues on the leaching of alachlor. Adding straw and compost had no effect, whereas adding ash significantly increased sorption of both herbicides. Aging (28 days) increased apparent sorption distribution coefficients (Kd,app values) by 1.2 to 2.3 times. Straw and ash amendments resulted in less leaching of alachlor (<1.0 % of the applied amount) than compost or control soil (~6 % of the applied amount). The straw retained ~80 % of the applied alachlor during leaching. Although this may be overrated due to an artifact of the methodology adopted, alachlor retention in the straw could not be predicted by the use of Kd,app. The transport potential of alachlor may be overestimated if aging and sugarcane straw management are not factored into the models.