MICROBIAL ECOTOXICITY AND PERSISTENCE IN SOIL OF THE HERBICIDE BENSULFURON-METHYL

Changes in microbial numbers and activities in two non-flooded soils t reated with bensulfuron-methyl at 16 and 160 mu g/kg were studied afte r 1 and 4 weeks of incubation under laboratory conditions. In addition to herbicide persistence, six general and functional microbial groups (numbers of culturable aerobic and anaerobic bacteria, aerobic N-2 fi xers, autotrophic nitrifiers, aerobic and anaerobic cellulolytic micro organisms) and two activities (respiration and nitrification) were als o studied. With respect to microbial numbers, only cellulolytic microo rganisms in soil which had never been previously treated with pesticid es significantly decreased in number. Only the higher dose of bensulfu ron-methyl in the second soil (which had previously received bensulfur on treatment) substantially inhibited nitrification, possibly because of the greater persistence of the herbicide in the soil. Soil respirat ion (CO2 emission) was not influenced by the herbicide in any of the s amples. The half-life of the herbicide in the soils studied was 1 to 3 weeks. Persistence was greater in the soil with pH 5.5, a condition w hich is known to favor abiotic degradation. The lower persistence of t he herbicide in the other soil, which was alkaline and which contained more microflora, could be attributed to a higher rate of biodegradati on. The data suggest that concentrations of bensulfuron-methyl greater than those present under normal agricultural practice can alter some aspects of the structure and activity of the soil microbial community, and that the persistence of the herbicide can be low even in the abse nce of leaching and runoff.