Growth and Dinitrogen Fixation, of Soybean Root System Affected by Partial Exposure to Nitrate

(1) When a portion of the root system of soybean is exposed to NO3−N at concentrations below 100 ppm, the growth and NO3−N absorption of the roots at that portion are accelerated, although the growth and dinitrogen fixing activity of the nodules at that portion are retarded. (2) The nitrogen absorbed as NO3−N by a portion of roots, which is directly exposed to NO3−N, translocates to the other portion of the roots and promotes the growth of roots and also the growth and dinitrogen fixing activity of the nodules there. If this portion of roots is also exposed to NO3−N, the translocation is less active. (3) The majority of nitrogen in the nodules consists of fixed nitrogen. When the nodules are exposed to NO3−N directly, some amount of nitrogen absorbed as NO3−N translocates into these nodules, and then the translocation of nitrogen from the nodules becomes active. The nitrogen absorbed as NO3−N at a portion of the root system translocates to the nodules at the other portion and this translocated nitrogen can accumulate in the nodules considerably. (4) When fertilizer nitrogen is applied to a portion of the root system at an adequate amount, the amount of nitrogen accumulated in the plant is higher than that when the same amount of fertilizer nitrogen is applied to the whole root system by increasing fertilizer nitrogen absorption at that portion of roots and maintaining the dinitrogen fixing activity relatively high at the other portion.