Effects of N management on growth, N 2 fixation and yield of soybean

Abstract

Soybean (Glycine max) is one of the most importantfood and cash crops in China. Although soybean has the capacity to obtain alarge proportion of its N from N2 fixation, it is commonfarmer's practice to apply an N top dressing to maximize grain yield. Afield experiment was conducted to study the effects of N application as urea atvarious stages during the vegetative and reproductive phases on crop biomass,N2 fixation and yield of two soybean genotypes, Luyuebao and Jufeng.Starter N at 25 kg ha−1 resulted in minimumbiomass and pod yield while starter N at 75 kgha−1 had no significant effect and N top dressing, ateither the R1 or R5 stage, resulted in increased biomass and pod yield. Maximumbiomass and pod yield were obtained when a top dressing of 50 kgha−1 was applied at the flowering stage. The effects oftop dressing on the capacity to fix N2 were complex. The proportionof plant N derived from N2 fixation (Pfix) was highest when onlystarter N at 25 kg ha−1 was applied. Any topdressing reduced nodulation and Pfix, but increased biomass, so that totalN2 fixed increased for top dressing at the flowering or pod fillingstage. Common farmer's practice of applying 75 kg Nha−1 at the V4 stage, resulted in a significantreduction in N2 fixation. To evaluate the application of Nfertilization at various stages ofdevelopment on growth, nodulation and N2 fixation in more detail, anexperiment in nutrient solution with or without 20 mMNO3− was conducted with genotype Tidar. The N-freetreatment gave the lowest biomass and total N accumulation, as in the fieldexperiment. A continuous nitrate supply resulted in the highest biomass,associated with an increase in total leaf area per plant, maximum individualleaf area, branch and node number per plant, shoot/root ratio and leaf arearatio, compared to the N-free treatment. R1 was the most responsive stage fornitrate supply as well as for interruption of the nitrate supply. Since theresults from the field experiment were in agreement with thosefrom the experiment in nutrient solution in a greenhouse, the latter can beusedto predict crop performance in the field.