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Nitrogen Assimilation Traits and Dinitrogen Fixation in Soybean and Common Bean

NifTAL Project Dep. of Agronomy and Soil Sci., Univ. of Hawaii, 1000 Holomua Ave., Paia, HI96779

^* Corresponding author.

Nitrogen derived from symbiotic and mineral sources by a legume is determined by the interactions between mineral N supply, plant N demand, and N assimilation traits. These interactions need to be understood to maximize legume N, fixation and yield, and to identify plant traits supporting high N₂-fixation. These interactions were examined in inoculated soybean [Glycine max Merr. (L.)] and common bean (Phaseolus vulgaris L.) by varying N supply (9, 120, and 900 kg N ha^–1) at two field sites. Nitrogen fixation was measured by ¹⁵N dilution method. Plants were sampled at full bloom (R2), 21 to 25 d from R2, and physiological maturity (R7). Total N of both legumes at R7 was 25% greater with 900 than 9 kg N ha^–1. With 900 kg N ha^–1, soybean N accumulation at R7 (271 kg N ha^–1) was 42% more than common bean but was 22% less at R2 (78 kg N ha^–1). Applied N had the largest impact on N accumulation rates before R2 and from the second sampling to R7, both periods of minimal N₂ fixation. Rate of N accumulation by common bean (0.19 g N m^–2 d^–1) was more uniform over the growth cycle than by soybean (0.26 g N m^–2 d^–1) which peaked (0.58 g N m^–2 d^–1) between R₂ and the second sampling. This peak also coincided with maximum N₂ fixstion rate. Our ¹⁵N uptake and extractable soil N data indicate that common bean derived more N from the mineral source than soybean because of more efficient uptake. Greenhouse data indicated greater root weight and uptake of mineral N per unit root weight for common bean than soybean. Maximizing both N₂ fixation and yield might entail timing the mineral N supply during early vegetative and late reproductive phases. The limited N₂ fixation capacity of common bean might be due to N assimilation traits favoring mineral N uptake.

Received for publication April 29, 1991.