Contact-time-dependent atrazine residue formation in surface soils

The formation of nonextractable atrazine residues was evaluated in sterilized agricultural and woodland soils pre-loaded with 14C-atrazine for contact periods of 1 h (0.046 days), and 1, 7, 14, 28, 56, and 84 days. Extractability of the pre-loaded atrazine and nonextractable residue formation were determined by subjecting the soils to sequential fill-and-draw extractions with water, ethylacetate/water, and alkali. Nonextractable atrazine residues associated with the fulvic acid (FA), humic acid (HA) and humin/mineral (HM) components of soil were determined by separating FA and HA from the soils and measuring the 14C-activity associated with each fraction. Longer herbicide–soil contact times resulted in attenuated water extractability and enhanced nonextractable residue formation. At the longest contact periods, residues recovered in the FA, HA and HM components of soil accounted for 35–50% of the pre-loaded herbicide. The woodland soil contained significantly larger amounts of HA and humin than the agricultural soil, and appeared to have contributed significantly to nonextractable residue formation. Results from this study indicate that physicochemical processes occurring at intra-mineral and intra-organic matter sites continue to influence the fate of organic pesticides long after their application on soils.