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 kin etics of the herbicides atrazine loro-4-ethylamino-6-isopropylamino-1, 3,5-triazine) and metolachlor 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 contr olled moisture (field capacity) and temperature (25-degrees-C) conditi ons in the laboratory. The dissipation of metolachlor and atrazine wer e also examined in repacked cores of the loam soil adjusted to the fie ld bulk density. The rates of atrazine or of metolachlor dissipation i n 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 re sidue formation in soils incubated in flasks and intact cores were lik ewise the same. However, the rate of dissipation of metolachlor, the r ate of accumulation of atrazine transformation products, and the rate of formation of nonextractable atrazine residues were all significantl y slower in repacked cores of the loam soil. These results indicate th at, under controlled laboratory conditions, atrazine and metolachlor d issipation occured at the same rate in undisturbed soil cores and in f lask incubations with soil that had been disrupted by previous air-dry ing and sieving. We conclude that, at least in some soils, structure n eed not necessarily be maintained in laboratory soil pesticide dissipa tion experiments.