Effects of N management on growth, N₂ fixation and yield of soybean

Soybean (Glycine max) is one of the most important food and cash crops in China. Although soybean has the capacity to obtain a large proportion of its N from N-2 fixation, it is common farmer's practice to apply an N top dressing to maximize grain yield. A field experiment was conducted to study the effects of N application as urea at various stages during the vegetative and reproductive phases on crop biomass, N-2 fixation and yield of two soybean genotypes, Luyuebao and Jufeng. Starter N at 25 kg ha(-1) resulted in minimum biomass and pod yield while starter N at 75 kg ha(-1) had no significant effect and N top dressing, at either the R1 or R5 stage, resulted in increased biomass and pod yield. Maximum biomass and pod yield were obtained when a top dressing of 50 kg ha(-1) was applied at the flowering stage. The effects of top dressing on the capacity to fix N-2 were complex. The proportion of plant N derived from N-2 fixation (Pfix) was highest when only starter N at 25 kg ha(-1) was applied. Any top dressing reduced nodulation and Pfix, but increased biomass, so that total N-2 fixed increased for top dressing at the flowering or pod filling stage. Common farmer's practice of applying 75 kg N ha(-1) at the V4 stage, resulted in a significant reduction in N-2 fixation. To evaluate the application of N fertilization at various stages of development on growth, nodulation and N-2 fixation in more detail, an experiment in nutrient solution with or without 20 mM NO3- was conducted with genotype Tidar. The N-free treatment gave the lowest biomass and total N accumulation, as in the field experiment. A continuous nitrate supply resulted in the highest biomass, associated with an increase in total leaf area per plant, maximum individual leaf area, branch and node number per plant, shoot/root ratio and leaf area ratio, compared to the N-free treatment. R1 was the most responsive stage for nitrate supply as well as for interruption of the nitrate supply. Since the results from the field experiment were in agreement with those from the experiment in nutrient solution in a greenhouse, the latter can be used to predict crop performance in the field.