Influence of Application Rate on Atrazine Fate in a Silt Loam Soil

Abstract

In Wisconsin, reduced application rates of atrazine (2-chloro-4-ethylamino-6-isopropylamino-s-triazine) are a major component of restrictions enacted to protect groundwater, but the relationship between rate of application and atrazine fate has not previously been determined in any Wisconsin soil. Therefore, laboratory experiments were conducted to determine the movement and degradation of atrazine applied at rates from 0.56 to 4.48 kg a.i. ha−1 in nondisturbed Plano silt loam (fine-silty, mixed, mesic; Typic Argiudolls) soil columns contained in 90-cm long by 20-cm diameter lysimeters. Over 180 d, a total of 62.5 cm of simulated rainfall was applied uniformly to the soil surface of each column. Rate of atrazine application did not influence time of initial detection of atrazine, deethylatrazine (2-chloro-4-amino-6-isopropylamino-s-triazine), or deisopropylatrazine (2-chloro-4-ethylamino-6-amino-s-triazine) in drainage water, but as application rate increased, these compounds leached over a longer period of time, up to twice as long for atrazine applied at 4.48 than 0.56 kg a.i. ha−1. The frequency of detection, average concentration, and mass of atrazine, deethylatrazine, and deisopropylatrazine in drainage water increased as the rate of atrazine application increased. The total mass of atrazine and its metabolites in drainage water was sevenfold greater for atrazine applied at 4.48 than 0.56 kg a.i. ha−1. In contrast, the percent of applied atrazine recovered as total atrazine and metabolites was not influenced by rate of application and was <0.15% of applied atrazine for any rate. These results indicated that reduced rates of atrazine application are associated with reduced movement of atrazine and metabolites through the root zone of Plano silt loam soil and suggest that reduced rates of application are a valid approach to groundwater protection.