Incorporating Nonextractable Atrazine Residues into Soil Size Fractions as a Function of Time

Abstract

AbstractSoil organic matter is directly implicated in pesticide nonextractable (bound) residues formation, but the role of different soil organic constituents is unknown. Physical soil fractionation allows separation, with minimal chemical modification, of organic constituents whose nature and properties are dependent on particle size. Physical fractionation was used to determine the role of soil organic constituents in atrazine (6‐chloro‐N‐ethyl‐N′‐(1‐methylethyl)‐1,3,5‐triazine‐2,4‐diamine) bound residues formation. 14C‐ring‐labeled atrazine was applied to the top of soil lysimeters (0.3 by 1.2 m) in the field. The lysimeters were periodically 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 µm. The largest proportion of total bound residues in the whole soil was in the clay size (0.2–2 µm) fraction, which also contained 50% of the total soil organic C (OC). The ratio of bound residues to OC content decreased with the particle size, and it was highest in fractions >50 µm, those rich in nonhumified organic matter. The low capacity of humified organic matter in the finest size fractions to form bound residues is presumably due to the chemical nature of the humified organic matter; however, accessibility of organic compounds stabilized on the fine mineral surfaces or included in aggregates structures is also a possibility. The most stable bound residues are associated with humified organic matter, especially in clay size fraction (0.2–2 µm). 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.