NITROGEN ASSIMILATION TRAITS AND DINITROGEN FIXATION IN SOYBEAN AND COMMON BEAN

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 N2 fixation and yield, and to identify plant traits supporting high N2-fixation. These interactions were examined in inoculated soybean [Glycine mar 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 N-15 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 fixation rate. Our N-15 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.