Seasonal change of N2-fixing rate in rice field assayed byIn Situacetylene reduction technique

Abstract

The contribution of blue-green algae and heterotrophic bacteria in or around the rice roots to nitrogen (N₂) fixation in a submerged rice soil was assessed by in situ acetylene-reduction assay. A 24-hr assay was conducted every 3 weeks for 48 weeks in the International Rice Research Institute (IRRI) long-term fertilizer plots at Los Baños, Philippines. Unfertilized plots and NPK fertilizer plots were used. In unfertilized plots, the highest peak of in situ acetylene-reduction activity appeared late in the growing season, in both wetand dry-season crops, when the activity and biomass of blue-green algae in the flooded water were highest. In the unfertilized plot, a close correlation was found between in situ Na-fixation activity and in vitro photo-dependent N₂-fixation activity of the flood water, which included floating algae and epiphytic algae on the weeds. N₂-fixing activity of blue-green algae assayed both in the field and in the laboratory was lower in fertilized plots than in unfertilized plots. When the activity of the blue-green algae was high, the replacement of the flood water including surface soil with disti11ed water, to eliminate the bulk of algal activity, greatly reduced the in situ acetylene-reduction activity. That suggests that blue-green algae contribute more to acetylene-reduction activity than do the microorganisms in the proximity of rice roots.