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The dissipation rate of seven currently used soybean and corn pesticides in two tropical soils (Ustox and Psamments) of Brazil was studied in a laboratory incubation experiment. Dissipation half-lives of pesticides ranged between 2 (monocrotofos) and 90 days (endosulfan-beta). The contrasting clay contents of the studied tropical soils (130 versus 470 g of clay kg(-1) of soil) did not influence the dissipation dynamics of pesticides substantially. Mineralization to CO(2) was high [up to 78% of the applied radioactivity (AR)] for the studied organophosphorus compounds and deltamethrin, which also formed considerable amounts of bound residues (>20% of AR) during the 80 days of incubation. The highest portion of nonextractable residues was found for alachlor and simazine (55-60% of AR). In contrast, the nonpolar trifluralin and endosulfan formed only small amounts of bound residues (mostly <20% of AR) but showed the highest dissipation half-lives (>14 days) in the studied soils, also due to a low mineralization rate. When endosulfan-sulfate, as the main metabolite of endosulfan, was considered, the half-life time of endosulfan compounds (sum of -alpha, -beta, and -sulfate) was enhanced to >160 days in both soils. In comparison with the laboratory experiments, dissipation half-life times of chlorpyrifos, endosulfan-alpha, and trifluralin were shortened by a factor of 10-30 in field trials with the same soils, which was related to the volatilization potential of pesticides from soils.

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