Soil metabolism of the herbicide rimsulfuron under laboratory and field conditions

Procedures were developed for soil analysis of rimsulfuron and its metabolites by means of GC-ECD, GC-FPD and GC-MS with a detection limit of 1 microgram of rimsulfuron equivalents per kilogram of dry soil, after purification of the soil extracts by TLC. Soil containing an exaggerated rimsulfuron concentration was incubated in the laboratory, in order to adjust the analytical procedures. To evaluate the role of different manuring managements, rimsulfuron was applied postemergence (10 g ha-1 on a corn field in 1994 and 1995) on plots treated with (1) green manure, pig slurry applied in (2) November and (3) March, and cow manure applied in (4) November and (5) March and (6) untreated control plots without organic fertilizer. Neither rimsulfuron nor its metabolites were detected at soil depths lower than 8 cm. The results show that manure managements prolong rimsulfuron half-life in the 0-8 cm surface soil layer from a minimum of 14 days (control) to a maximum of 46 days (pig slurry in March). At corn harvest, rimsulfuron and its metabolites were not detected in soil. Similar soil degradation pathways were observed in the field as in the laboratory. N-(4,6-Dimethoxypyrimidin-2-yl)-N- [3-(ethylsulfonyl)-2-pyridinyl]urea 2 was a transient soil degradation product. N-[3-(ethylsulfonyl)-2-pyridinyl]- 4,6-dimethoxy-2-pyrimidineamine 3 attained a maximum soil concentration after 20 days of incubation and then progressively disappeared and could not be detected after 40 days. The high-molecular weight amine 3 did not accumulate in soil, eliminating the concern for potential formation of nitroso amino compounds. 2-Hydroxy-3-(ethylsulfonyl)pyridine 4 became the major rimsulfuron soil degradation product. The soil concentrations increased and attained a maximum after 40 days of incubation and then decreased; its isomerization into 2-pyridone, followed by hydrolysis, could transform it into low molecular weight nontoxic products. The concentrations of 2-amino-4,6-dimethoxypyrimidine 5 were somewhat lower than those of compound 4.