Abstract
It has been shown that the nodulation of soybeans is inhibited by the application of nitrogen fertilizers. However, the influence of the nitrogen fertilizers on the nitrogen-fixation activity (nodule activity : measured as acetylene-reduction activity) of soybean nodules is not always clear. Therefore, the influence of applied nitrogen on the activity of nodules was examined under several conditions including the amount of nitrogen fertilizer applied and the methods of fertilization. The results are summarized as follows. 1) When soybeans were grown in a split-root system, and fertilizer was applied to only one side, the decreases in nodule dry mass and nodule activity on the fertilized side were significant with increasing nitrogen application. The nonfertilized side of the root system also produced less nodule dry mass and had reduced nodule activity ; however, the decrease was not as drastic as that on the fertilized side. In addition, the nitrogen concentration of the leaves showed a negative linear correlation to the specific activity of the nodules on the total root system. 2) The concentration of nitrate nitrogen in the soybean roots was increased by increasing the amount of basal nitrogenous fertilizer ; however, this trend disappeared two weeks after the flowering stage. In this time, there was no correlation between the concentration of nitrate nitrogen of the roots and the nodule activity ; however, the nitrogen concentration of the leaves correlated negatively to the specific activity of the nodules. 3) The application of nitrogen to the plant leaves was found to decrease nodule activity. In view of the above results, it is suggested that the concentration of nitrate nitrogen to the roots does not affect the activity of soybean nodules.
