Scope and Influence of Enhanced Triazine Degradation in U.S. Soils

The triazines are one of the most widely used herbicide classes ever developed, and play a role in managing herbicide-resistant weed populations in sustainable agricultural production systems. The triazines are traditionally valued for their persistence and season-long weed control in over 50 crops including corn, soybeans, wheat, and vegetables. The literature suggests that atrazine, the most widely used triazine, may no longer remain persistent in soils due to enhanced microbial degradation. Experiments examined the rate of degradation of atrazine and two other triazine herbicides: simazine and metribuzin in both atrazine adapted and non-adapted soils from across the United States. Additional studies examined the rate of atrazine dissipation in flooded and non-flooded soils, as well as soils with varying history of atrazine use. In soils with a history of atrazine use, the t_1/2 [half-life] of atrazine was up to 40 times more rapid than in soils with no history of atrazine use. Simazine t_1/2 was at least 2.4-15 times more rapid in history soils than non-history soils, and metribuzin was degraded at 0.6, 0.9, and 1.9 times the rate in the same soils. These results indicate cross-enhancement of the symmetrical triazine simazine, but not for metribuzin, an asymmetrical triazine. In soils with 3, 5, and 10 years of previous atrazine use, atrazine t_1/2 was 2.66, 4.44, and 2.14 respectively, indicating that atrazine adapted soils may develop rapidly. Finally, atrazine dissipation in flooded and non-flooded soils appears rapid, indicating that soybeans are a viable option when replanting production con fields previously treated with atrazine.