Microbial ecotoxicity and persistence in soil of the herbicide bensulfuron-methyl

Changes in microbial numbers and activities in two non-flooded soils treated with bensulfuron-methyl at 16 and 160 mu g/kg were studied after 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 fixers, autotrophic nitrifiers, aerobic and anaerobic cellulolytic microorganisms) and two activities (respiration and nitrification) were also studied. With respect to microbial numbers, only cellulolytic microorganisms in soil which had never been previously treated with pesticides significantly decreased in number. Only the higher dose of bensulfuron-methyl in the second soil (which had previously received bensulfuron treatment) substantially inhibited nitrification, possibly because of the greater persistence of the herbicide in the soil. Soil respiration (CO2 emission) was not influenced by the herbicide in any of the samples. 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 which is known to favor abiotic degradation. The lower persistence of the herbicide in the other soil, which was alkaline and which contained more microflora, could be attributed to a higher rate of biodegradation. 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 absence of leaching and runoff.