Nitrogen fixers make up a large percentage of the total microflora in the rhizosphere of lowland rice. There are more aerobic nitrogen fixers than there are anaerobic ones.When soil crumbs from the root zone were pIaced on a nitrogen free agar medium and inoculated at 0, 5, 10, and 21 percent oxygen concentration, colonies of aerobic nitrogen fixers reached their greatest diameter at 5 and 10 percent oxygen.In acetylene reduction assays rice plants grown in paddy fields and in solution culture were tested for the nitrogenase activities of their roots at different oxygen tensions. Nitrogenase activity was highest at 3 percent oxygen, lower at 0 percent, and far lower at 21 percent.When rice was grown in solution culture the redox potential of the nutrient solution strongly influenced nitrogenase activity. With declining redox potential, nitrogenase activity increased to a maximum value but dropped sharply as redox potential further decreased.Ten ppm of combined nitrogen as urea depressed nitrogenase activity on excised roots. Combined nitrogen applied to one part of the root system affected, to some extent, nitrogen fixation on other roots kept in a solution without nitrogen.Nitrogenase activity in a fertility trial with lowland rice, examined at several dates, showed no inhibitory effect of fertilizer nitrogen, however, presumably because the nitrogen concentration in the soil solution rapidly decreased. Instead, an overall stimulating effect of nitrogen dressing was noticeable.Diurnal fluctuations of nitrogenase activity in the rhizosphere, with a peak in the afternoon and low fixation rates after low solar radiation, suggest a photosynthetic effect on nitrogen fixation.
I N T R O D U C T I O NAS p r e v i o u s l y r e p o r t e d 20 excessive m i c r o b i a l a c t i v i t y i n t h e rhizos p h e r e seems to be m o r e d e t r i m e n t a l to t h a n f a v o r a b l e for, g r o w i n g
