Degradation of atrazine in two soils as a function of concentration

Dissipation of atrazine (2-chloro-4- thylamino-6-isopropylamino-s- triazine) in a Webster clay loam soil (fine loamy, mixed, mesic Typic Haplaquoll), and Estherville sandy loam (sandy, mixed, mesic typic Hapludoll) was determined over a concentration range of 5 to 5000 mg kg^-1 in field and laboratory experiments. Over the first 6 mo in the clay loam soil, the persistence of atrazine (based on percent of applied) was greater for the high-rate treatments than the low-rate treatments. However, in the laboratory, there was no effect of concentration on dissipation; the amount of atrazine degraded increased proportionally with the increase of concentration. In the sandy loam, persistence was greater at high concentration in both field and laboratory studies. Mineralization was the most important pathway for the dissipation of atrazine at all concentrations in the clay loam soil and from 5 to 500 mg kg^-1 in the sandy loam. It was postulated that soil microorganisms were able to use the N or C from the s- triazine ring. Atrazine at 500 and 5000 mg kg^-1 may have increased soil microbial growth and activity and thus increased the degradation of atrazine based on the increase in soil respiration in the clay loam soil. Degradation pathways in both soils apparently were not influenced by concentration. Ring cleavage and hydrolysis were the major metabolic pathways in both soils, with dealkylation of less importance. Addition of a dairy manure amendment increased the rate of atrazine mineralization, while corn meal decreased and (NH₄)₂HPO₄ amendments prevented mineralization.