INCORPORATING NONEXTRACTABLE ATRAZINE RESIDUES INTO SOIL SIZE FRACTIONS AS A FUNCTION OF TIME

Soil organic matter is directly implicated in pesticide nonextractable (bound) residues formation, but the role of different soil organic co nstituents is unknown. Physical soil fractionation allows separation, with minimal chemical modification, of organic constituents whose natu re and properties are dependent on particle size. Physical fractionati on was used to determine the role of soil organic constituents in atra zine hyl-N'-(1-methylethyl)-1,3,5-triazine-2,4-diamine) bound residues formation. C-14-ring-labeled atrazine was applied to the top of soil lysimeters (0.3 by 1.2 m) in the field. The lysimeters were periodical ly removed during 16 mo, and the soil was physically fractionated into seven size fractions by sieving, sedimentation, and centrifugation: > 200, 50 to 200, 20 to 50, 5 to 20, 2 to 5, 0.2 to 2, and <0.2 mu m. Th e largest proportion of total bound residues in the whole soil was in the clay size (0.2-2 mu m) fraction, which also contained 50% of the t otal soil organic C (OC). The ratio of bound residues to OC content de creased with the particle size, and it was highest in fractions >50 mu m, those rich in nonhumified organic matter. The low capacity of humi fied organic matter in the finest size fractions to form bound residue s is presumably due to the chemical nature of the humified organic mat ter; however, accessibility of organic compounds stabilized on the fin e mineral surfaces or included in aggregates structures is also a poss ibility, The most stable bound residues are associated with humified o rganic matter, especially in clay size fraction (0.2-2 mu m). Physical soil fractionation as a function of incubation time allows definition of the different capacity of soil organic constituents to form bound residues in relation to their localization in the size fractions.