Mineralisation studies of C-14-labelled metsulfuron-methyl, tribenuron-methyl, chlorsulfuron and thifensulfuron-methyl in one Danish soil and groundwater sediment profile

Bacterial mineralisation of four sulfonylurea herbicides at 20 mug kg(-1) i n a sandy soil from nine different depths in a sandy soil horizon (5-780 cm ) was investigated in laboratory studies. Metsulfuron-methyl, chlorsulfuron , and tribenuron-methyl were C-14-labelled in the sulfonamide ring, while t hifensulfuron-methyl was labelled in the thiophene ring. The highest minera lised amount in 126 days was observed for metsulfuron-methyl (40%) followed by tribenuronmethyl (25%), and thifensulfuron-methyl (11%), Chlorsulfuron showed low mineralisation in all the soils tested (<4%). Mineralisation of the herbicides metsulfuron-methyl and tribenuron-methyl varied according to soil depth (upper profile: 5-70 cm, and lower profile: 165-780 cm) and wer e proven faster in soil taken from depths 5-7 and 30-35 cm, and slower in d epths 45-50 and 70-75 cm. Mineralisation was absent in the lower profile (1 65-780 cm). As an indicator of microbial activity bacterial counts were tak en at the experimental start; these counts grouped in three levels: highest in the surface layer (5-7 cm), slightly lower in the depths 30-75 cm, and lowest in the lower profile (165-780 cm). Residual concentrations of metsul furon-methyl correlated to the accumulated amount mineralised, with high re sidual concentrations in soil showing low mineralisation. Also chlorsulfuro n showed high residual concentrations with increasing depth in the upper pr ofile, but the relatively high dissipation at 30-35 cm and lower one at 45- 50 cm could not be related with the lack of mineralisation. This shows that hydrolysis occurs, but mineralisation of the chloro-substituted sulfonamid e is restricted. Tribenuron-methyl and thifensulfuron-methyl could not be d etected due to interference with other compounds. (C) 2001 Elsevier Science Ltd. All rights reserved.