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 insect icide carbofuran (2,3- dihydro 2,2- dimethyl-7 benzofuranyl-N- methylc arbanate), on nitrogenase activity and nitrogen-fixing bacteria associ ated with rice rhizosphere was investigated. Repeat applications of ca rbofuran effected a significant stimulation of rhizosphere associated nitrogenase activity. Thus the nitrogenase activity, which was stimula ted following first application of carbofuran, was further enhanced af ter the second addition of carbofuran. Interestingly, no further enhan cement in the nitrogenase activity occurred due to subsequent applicat ions, Populations of nitrogen-fixing Azospirillum sp., Azotobacter sp, and anaerobic nitrogen-fixing bacteria increased progressively upto t hird application of carbofuran. Results indicate that repeated applica tions of carbofuran stimulated nitrogenase and the populations of nitr ogen-fixing bacteria associated with rice rhizosphere.