Nitrogenase Activity and Nitrogen-Fixing Bacteria Associated with Rhizosphere of Rice Cultivars with Varying N Absorption Efficiency

Abstract

In a field investigation, nitrogenase activity and nitrogen-fixing bacteria associated with rhizosphere of rice cultivars with varying N absorption efficiencies were examined at three N-fertiliser regimes. A significant difference among the cultivars with regard to the nitrogenase activity of rhizosphere bacteria was observed. Under no or low (30 kg 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 was stimulated, the differences with regard to nitrogenase activity among the cultivars were not significant. Microbiological analysis indicated that the cultivar with high N absorption efficiency harboured higher populations of nitrogen-fixing Azospirillum sp, Azotobacter sp and anaerobic bacteria. Under 60 kg N ha-1 fertiliser application, the rhizosphere of both cultivars, which differed in their N use pattern, harboured almost identical population densities of nitrogen-fixing bacteria. Interestingly, the anaerobic nitrogen-fixing bacteria were greatly stimulated following the application of fertiliser. Notwithstanding the effect of N fertiliser management on N2 fixation, N absorption efficiency of rice crop and the rhizosphere associated nitrogenase activity and nitrogen-fixing bacteria were found to be interrelated. Further investigations are required to evaluate the exact relationships including higher number of cultivars. © 1997 SCI.