Nitrogen Assimilation Traits and Dinitrogen Fixation in Soybean and Common Bean

Abstract

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 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 fixation 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.

Alteration in Intra-plant Distribution ofδ15N in Response to Shading in Legumes

“…In common bean under control conditions, %Ndfa ranged from 37 to 38%, which is lower than that in cowpea and soybean in the present experiment, but falls within the range determined elsewhere under fi eld conditions without fertilizer (Kimura et al, 2004). The lower N 2 -fi xing capacity of common bean may be due to the N assimilation traits favoring mineral N uptake rather than N 2 -fi xation (George and Singleton, 1992), and poor nodule formation.…”

Nitrogen-Fixing Activity of Root Nodules in Relation to Their Size in Peanut (Arachis hypogaea L.)

“…3). Nitrogen accumulation during early reproductive stage is considered important for the yield of legume crops (George and Singleton, 1992). The present study indicates that the nitrogen-fi xing activity of root nodules at fl owering stage can be estimated from their diameter.…”

N2 fixation by soybeans grown with different levels of mineral nitrogen, and the fertilizer replacement value for a following crop

“…Nonetheless, the presence of a high level of mineral N adversely affects N 2 fixation. The number and dry weight of nodules decrease with increasing mineral N (Chen et al 1992;Jefing et al 1992;Koutroubas et al 1998), and the percentage of N derived from atmosphere decreases, with a concomitant increase in the percentage of N derived from fertilizer (Deibert et al 1979;Hardarson et al 1984;Herridge and Brockwell 1988;George and Singleton 1992). …”

“…There is usually a net depletion of soil N after a soybean crop. Depending on whether or not the stover is removed, and on the amount of N fertilizer applied, the balance of soil N between sowing and harvest can vary from +9 to -133 kg N ha -1 , excluding the root system (Zapata et al 1987;George and Singleton 1992;Jefing et al 1992). However, even if the crop fails to provide a clear net benefit in terms of total N in the soil, the N balance is usually assumed to be more positive than with a non-legume crop.…”