Influence of repeated applications of carbofuran on nitrogenase activity and nitrogen-fixing bacteria associated with rhizosphere of tropical rice

In a greenhouse study, the effect of repeat applications of the insecticide carbofuran (2,3- dihydro 2,2- dinethyl-7 benzofuranyl-N- methylcarbanate), on nitrogenase activity and nitrogen-fixing bacteria associated with rice rhizosphere was investigated. Repeat applications of carbofuran effected a significant stimulation of rhizosphere associated nitrogenase activity. Thus the nitrogenase activity, which was stimulated following first application of carbofuran, was further enhanced after the second addition of carbofuran. Interestingly, no further enhancement in the nitrogenase activity occurred due to subsequent applications. Populations of nitrogen-fixing Azospirillum sp., Azotobacter sp. and anaerobic nitrogen-fixing bacteria increased progressively upto third application of carbofuran. Results indicate that repeated applications of carbofuran stimulated nitrogenase and the populations of nitrogen-fixing bacteria associated with rice rhizosphere.