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Influence of Available Nitrogen and Rice Genotype on Associative Dinitrogen Fixation

Soil Microbiology, Soil and Water Sciences Division, International Rice Research Institute (IRRI), P.O. Box 933, 1099 Manila, Philippines

*Corresponding author (J.K.Ladha{at}cgnet.com).

ABSTRACT

Rice (Oryza sativa L.) genotypes with a high ability to support biological N₂ fixation are of interest because they would enhance N gains in the rice-soil system without changing existing cultural practices. Pot experiments were conducted in a greenhouse during the 1994 wet season to determine the amount of N₂ derived from the atmosphere (Ndfa) by 10 rice genotypes grown in three soils with varying levels of initial soil-NH⁺₄-N. Enriched ¹⁵N and natural abundance ¹⁵N dilution methods were employed. Flooded soils were puddled with and without 99.5 atom ¹⁵N%-labeled urea to minimize spatial variation of the isotope. The integrated ¹⁵N enrichment of plant available N, calculated using a model that accounts for temporal changes in the isotopic composition, was used as a reference to estimate genotypic differences in Ndfa. The ¹⁵N enrichment of soil available N declined exponentially and was half the original level at 122 d. The ¹⁵N values correlated with atom ¹⁵N% excess of enriched ¹⁵N for the rice genotypes grown in two out of three soils. Percentage Ndfa for the genotypes across three soils ranged from 4.2 to 32.2 when calculated using as a reference plant, and from 10.8 to 35.9 when calculated using ¹⁵N enrichment of soil available N as a reference. Genotypic differences were significant and more pronounced at low soil-NH⁺₄-N (11 mg kg^-1) than at higher soil-NH⁺₄-N (79 and 92 mg kg^-1); they ranged from 14.9 to 35.9% at low N and from 10.8 to 23.6% at high N. Some genotypes consistently gave either high or low Ndfa. Because of the low levels and narrow range of genetic differences in associative N₂ fixation, the potential for its improvement through breeding appears to be limited.

Received for publication September 22, 1997.

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