Effects of the Concentrations of Nitrogen Supplied From Both or One-Half of the Dual-Root System on the Nitrogen Fixation of Soybean Nodules and the Distribution of Absorbed Nitrogen From Roots

This study aimed to investigate the effects of the different concentrations of ammonium or nitrate supplied from both or one-half of the dual-root system on the nitrogen fixation of soybean nodules and the distribution of absorbed nitrogen from roots. The soybean plants with a dual-root system were prepared, the treatments consisted of two N sources (ammonium or nitrate N), three N concentration levels (25, 50, or 100 mgN L^− 1), and two N application methods (unilateral: N was applied only one-half of roots, or bilateral: N was applied for both half roots). As N concentrations increased, both forms of N were able to inhibit reduced soybean nodule numbers, nodule dry weight, nitrogenase activity, leghemoglobin content, and percent N derived from the atmosphere (Ndfa%). The Ndfa% in the shoots of unilateral N-supply treatments were lower than those of bilateral N-supply. The total N accumulation and N accumulation from fertilizer in soybean plants increased with the concentration of supplied N, while the N accumulation derived from the atmosphere first increased and then declined with rising concentrations of applied N, peaking at 50 mgN L^− 1. The unilateral N-supply in 25 mgN L^− 1 N concentrations and the bilateral N-supply in 100 mgN L^− 1 N concentrations improved the total N accumulation in soybean plants. The ratio of N derived from atmosphere in soybean plants decreased with the increase in N concentration. At the levels of 50 and 100 mgN L^− 1, bilateral N-supply improved the ratio of N derived from the atmosphere than those in unilateral N-supply treatments. Overall, the results from this study indicate that peak of the nodule N₂ fixation accumulation in plants was observed when N fertilizer was applied at a concentration of 50 mg L^− 1 either from ammonium or nitrate. This study provides an appropriate comprehensive N supply can help maximize nodule N fixation activity and total N assimilation.