NITROGENASE ACTIVITY AND NITROGEN-FIXING BACTERIA ASSOCIATED WITH RHIZOSPHERE OF RICE CULTIVARS WITH VARYING N ABSORPTION EFFICIENCY

In a field investigation, nitrogenase activity and nitrogen-fixing bac teria associated with rhizosphere of rice cultivars with varying N abs orption efficiencies were examined at three N-fertiliser regimes. A si gnificant difference among the cultivars with regard to the nitrogenas e activity of rhizosphere bacteria was observed. Under no or low (30 k g N ha(-1)) fertiliser application, the cultivar with high boot leaf-N and high apparent N recovery had a higher nitrogenase activity. At 60 kg N ha(-1) fertiliser application, though the nitrogenase activity w as stimulated, the differences with regard to nitrogenase activity amo ng the cultivars were not significant. Microbiological analysis indica ted that the cultivar with high N absorption efficiency harboured high er populations of nitrogen-fixing Azospirillum sp, Azotobacter sp and anaerobic bacteria. Under 60 kg N ha(-1) fertiliser application, the r hizosphere of both cultivars, which differed in their N use pattern, h arboured almost identical population densities of nitrogen-fixing bact eria. Interestingly, the anaerobic nitrogen-fixing bacteria were great ly stimulated following the application of fertiliser. Notwithstanding the effect of N fertiliser management on N, fixation, N absorption ef ficiency of rice crop and the rhizosphere associated nitrogenase activ ity and nitrogen-fixing bacteria were found to be interrelated. Furthe r investigation are required to evaluate the exact relationships inclu ding higher number of cultivars.