Abstract
Nitrogen fixation by soybeans is reduced by high soil nitrate levels. This reduced nitrogen fixation capacity increases the dependence on soil nitrogen, which may result in depletion of soil nitrogen and decrease soybean yield. Therefore, we examined cultivar differences in the dependence on nitrogen fixation in nitrogen-fertile fields by measuring the relative abundance of ureide value in root bleeding sap, nodule number, nodule weight and root weight in four Japanese, two American and one supernodulation cultivars. The small precipitation in August 2002 led to a substantial reduction in nitrogen fixation, which was shown by the smaller values for relative abundance of ureide (%, RU) in 2002 when compared with 2003. There was a significant cultivar difference in RU, and the supernodulation cultivar Sakukei 4 maintained a relatively high RU during all growing periods. Among the common cultivars, American cultivars showed lower RU values (11-48%) than Japanese cultivars (14-67%) all through the seasons. Nodule number and weight at 63 days after sowing was higher in Japanese cultivars than in American cultivars. Nodule number and weight exhibited a significant positive correlation with the amount of ureide in root bleeding sap. We thus concluded that Japanese cultivars are superior to American cultivars with regard to nodule formation ability, and that they have a higher dependency on nitrogen fixation under nitrogen fertile soil conditions.
