Effect of N fertilizer top-dressing on N accumulation and N2 fixation of supernodulating soybean mutant

Increased application of nitrogen (N) fertilizer top-dressing during growth is an effective option for enhancing N supply to soybean plants. SS2-2 was characterized by the superior ability of symbiotic N₂ fixation at the level of 30 kg N ha⁻¹. But, the response of nitrogen fixation ability of supernodulating soybean mutant, SS2-2, to N fertilizer application rate remains unclear. The objective of this experiment was to compare the response of N fertilizer top-dressing on N accumulation and N₂ fixation between supernodulating mutant, SS2-2, and wild-type, Sinpaldalkong 2. The effect of N fertilizer top-dressing (0.6 g N pot⁻¹ top-dressing) on the nitrogen accumulation and redistribution were compared between SS2-2 and Sinpaldalkong 2. N fertilizer top-dressing at R1 stage increase in plant dry weight, relative growth rate (RGR), net assimilation rate (NAR), nitrogen harvest index (NHI), and N redistribution (NR). SS2-2 showed highest N concentration, 65.0 mg N g DW⁻¹, followed by Sinpaldalkong 2 and En1282, and the N content per plant did not show a significant difference between SS2-2 and Sinpaldalkong 2. The N₂ fixation rate was significantly reduced by N top-dressing, but the amount of N₂ fixation was not changed due to an improved dry weight without changes of N concentration. In addition, SS2-2 showed higher NHI, NR and NRE than Sinpaldalkong 2. These results suggested that supernodulating soybean mutants, SS2-2, could be characterized by high N concentration and N₂ fixation regardless of N fertilizer top-dressing due to a higher nitrate tolerance of supernodulating mutants than that of wild-type.