ATRAZINE AND METOLACHLOR DISSIPATION IN SOILS INCUBATED IN UNDISTURBED CORES, REPACKED CORES, AND FLASKS

This study compared three different laboratory methods for determining soil pesticide dissipation kinetics and pathways. The dissipation kinetics of the herbicides atrazine (2-chloro-4-ethylamino-6-isopropylamino-1,3,5-triazine) and metolachlor (2-chloro-6'-ethyl-N-(2-methoxy-1-methylethyl)acet-o-toluidine) were examined in three soils (sandy loam, loam, clay loam) incubated for 63 d in flasks or intact cores (20 by 20 cm) under controlled moisture (field capacity) and temperature (25-degrees-C) conditions in the laboratory. The dissipation of metolachlor and atrazine were also examined in repacked cores of the loam soil adjusted to the field bulk density. The rates of atrazine or of metolachlor dissipation in soils incubated in flasks or in intact cores were indistinguishable (P < 0.05). The rate of net accumulation of extractable transformation products from radiolabeled atrazine and the rate of nonextractable residue formation in soils incubated in flasks and intact cores were likewise the same. However, the rate of dissipation of metolachlor, the rate of accumulation of atrazine transformation products, and the rate of formation of nonextractable atrazine residues were all significantly slower in repacked cores of the loam soil. These results indicate that, under controlled laboratory conditions, atrazine and metolachlor dissipation occured at the same rate in undisturbed soil cores and in flask incubations with soil that had been disrupted by previous air-drying and sieving. We conclude that, at least in some soils, structure need not necessarily be maintained in laboratory soil pesticide dissipation experiments.