Influence of the herbicide bentazon on soil microbial community

Changes in microbial numbers and activities in a soil in response to bentazon applied at 10 and 100 ppm were studied after 4 and 30 weeks of incubation in laboratory conditions. As regards the eight general and functional microbial groups studied (aerobic and anaerobic bacteria, fungi, aerobic and anaerobic N2-fixing bacteria, nitrifiers, aerobic and anaerobic cellulolytic microorganisms), only the number of anaerobic N2-fixing bacteria significantly decreased, in the presence of the highest herbicide concentration for 30 weeks. At both the incubation times, only the higher dose of bentazon markedly inhibited soil nitrification and CO2 emission. Methanogenesis was inhibited by 1000 ppm bentazon added to anaerobic liquid cultures containing 5% soil for at least 2 weeks. There was an incomplete recovery of the herbicide at the two incubation times: < 5% of 10 ppm after 4 weeks and about 30% of 100 ppm after 30 weeks. No biodegradation of the compound was observed in liquid cultures under aerobic or anaerobic conditions. It is concluded that a bentazon concentration no higher than the field rate distributed within a 2-cm layer of soil does not considerably affect the microflora even in the absence of microbial degradation.