Objectives Nitrogen application promote soybean growth and increase yields, but inhibit nodule formation and nitrogen fixation. The resource of nitrogen in root and nodules were studied, and the effect of supplying NO3– and NH4+ on the nitrogen absorption and distribution of soybeans was systematically investigated.
Methods The grafting method was used to generate soybean plants with dual root systems, in which two modulated roots shared one symbiotic shoot. Two experimental treatments were conducted with NO3– and NH4+ as nitrogen sources under sand culture conditions. Experiment I, supplying one side of root with 50 mg/L of 15NO3– or 15NH4+ (side A), and no nitrogen on the other side (side B). Experiment II, supplying one side of root with 50 mg/L 15NO3– or 15NH4+ (side A), and supplying the other side with 50 mg/L NO3– or NH4+. At R1 (initial flowering) and R5 (initial seeding) stages of soybean, the plant samples were collected and divided into separate parts for the analysis of N contents.
Results The 15N abundance in the nodules on both the side A and side B were higher than natural (0.365%), which indicated that the nitrogen in the nodules was derived from both self-nitrogen fixation and root absorption. The rates of nodule-fixed N in both side A and B of experiment Ⅱ were significantly lower than those in experiment Ⅰ, indicating that the fertilizer N was preferentially absorbed and used by soybeans. No significant differences were observed in both the 15N abundance and N accumulation in all parts of soybean when supplied with NO3– or NH4+, which indicated that soybean was not sensitive to N forms under the experimental N level of 50 mg/L. In experiment Ⅰ, the 15N abundance of root and nodules on the side B was higher than the natural but lower than that in the tested fertilizers (3.63%), suggesting that the N absorbed from root of side A was transferred to the root and nodules in side B via the shoot. Considering the shoots, root and nodule in the dual root systems as a system, we proposed a method for calculating the amount of N translocation from shoots to roots and nodules during the R1–R5 stages based on the difference in the 15N abundance. When adding 50 mg/L of N, the translocated N from the shoots accounted for 28.4%–40.8% of the N accumulation in roots and 14.4%–17.2% of that in nodules of soybeans.
Conclusions The N required for nodule growth and development is derived from both self-nitrogen fixation and root absorption. The fertilizer N will be preferentially absorbed and used by soybeans in the presence of fertilizer N. N forms, namely NO3– and NH4+, will not affect the N absorption and distribution of soybean plants under the tested N supply concentration (50 mg/L). All the N acquired by the roots and nodules will be transported to the shoots, and a portion of them is then redistributed to the roots and nodules.