GENOTYPIC VARIATION IN PROMOTION OF RICE DINITROGEN FIXATION AS DETERMINED BY N-15 DILUTION

Rice (Oryza sativa L.) genotypes capable of stimulating associative N- 2 fixation (NFS) are desirable to reduce dependence on fertilizer N. A pot experiment was conducted during the 1993 dry season to assess dif ferences in NFS among 70 genotypes of diverse origin and growth durati on and to validate the use of N-15 enrichment of mineralized soil N as reference for N-2 fixation measurement. The soil used had a stable N- 15 enrichment that was obtained by incubating and frequent mixing with ((NH4)-N-15)(2)SO4 (6.3 kg N ha(-1)) in submerged condition for 6 wk. The N-15 enrichment of soil NH4+-N dropped exponentially, showing an initial rapid decline to half of the original level within 2 wk, follo wed by a slower, nonsignificant rate of decline after 5 wk. Whole plan t N-15 atom % excess was inversely correlated with growth duration and ranged from 1.78 in Oking Seroni (late genotype) to 2.45 in PTB-18 (e arly genotype). It was, therefore, necessary to compare NFS among geno types of the same growth duration. Enrichment was lowest in the roots, suggesting this as the site of active N-2 fixation; it was highest in the shoot and intermediate in grain. Nitrogen derived from air (Ndfa) estimated using N-15 enrichments of mineralized soil N and reference rice genotype correlated highly (r = +0.998*). Percentage Ndfa ranged from 1.5 to 21.0, with an average of 10.2. Yelsulbyeo, Pankaj, and Ok ing Seroni had the highest Ndfa among early, medium, and long growth d uration genotypes, respectively. Genotypes with high Ndfa, grain yield , and N uptake are Oking Seroni, IR29337-36-3, OR-142-99, and Pankaj.