Uptake and distribution of combined nitrogen and its incorporation into seeds of nodulated soybean plants as revealed by15N studies

Pot experiments were carried out with soybean plants grown in N-free solution and supplied with NO3 - or NH4 + before 15N administration, in order to study the uptake, distribution, and redistribution of absorbed nitrogen. To determine the utilization and incorporation of combined nitrogen in the seed yield, as influenced by the N form and application time, another series of experiments was conducted in which plants were grown in N-free solution before 15N feeding at pod setting (P.S.) or initial pod filling (I.P.F.), as 15NO3 - or 15NH4 +. By comparing the nitrate-treated plants (Nt-plant) with the ammonium ones (Am-plant), and P.S. with I.P.F., the following results were obtained. 1. Nt-plants accumulated higher amounts of dry matter in different plant parts, particularly the pods which proved to be an active sink for the newly metabolized nitrate nitrogen.2. Both the 15N abundance and 15N incorporation in the whole plant were much higher for Nt-plants, and the same was true for different plant parts except the nodules.3. A large portion of the root share of 15N was redistributed to the top parts in Nt-plants, but the same was not true for Am-plants which distributed a higher percent of the absorbed N in their nodules.4. Irrespective of the N form, the 15N abundance in different plant parts after direct distribution obeyed the order: roots>stems>leaves>pods>nodules.5. Application of labeled combined nitrogen at P.S. resulted in a higher 15N abundance and labeled N content in the seeds.6. The labeled N content in the seeds per plant treated with nitrate was higher, but the same was not true with respect to the concentration.7. P.S. application gave a higher seed yield, number of pods, and protein content in the seeds per plant. Nt-plants showed the higher seed yield, number of seeds per pod, and protein content per plant seed. The absorbed nitrogen percent in the total N was correlated with the seed yield.