NITROGEN FIXATION IN SOME TROPICAL RICE SOILS

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Abstract
Fixation of atmospheric nitrogen by 3 submerged Philippine rice soils was studied. Significant fixation was detected when the unamended soils were incubated in the light for a period of 28 days in an atmosphere enriched with N215. Additions of rice straw and roots at levels approximating crop residues led to increased fixation in 2 of the soils when incubated in the light. Fifteen comparisons of nitrogen fixation by dark- versus light-incubated soil samples were made. Of these, 8 comparisons showed significantly more fixation in the light, 2 showed significantly less fixation in the light, and 5 showed essentially no effect of light. Significant fixation was, however, obtained with dark-incubated samples, indicating a role for the heterotrophic nitrogen-fixing bacteria in the nitrogen-fixing process of flooded soils,, Soil inoculation with a blue-green algae led to increased fixation in 2 of the soils, whereas inoculation with a heavy suspension of a nitrogen-fixing Clostrid-ium did not increase fixation in any of the soils.

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