Optimized nitrogen application rate based on soil residual nitrogen significantly increased the yield and biological nitrogen fixation of fresh faba bean as vegetables

Optimizing nitrogen (N) application rate during faba bean (Vicia faba (L.) Linné) growth season might increase biological N fixation (BNF) and decrease soil residual N which will be benefit to the growth of following N sensitive crops. A 4-year field study was conducted with six N rates (0, 45, 90, 135, 180, and 225 kg N ha⁻¹). Yield, agronomic traits, root and nodule characteristics, N uptake, and biological N fixation were measured. Our results showed that (1) pod and grain yield, nodule weight and count, and BNF of faba bean initially increased with N rates, peaking at 90 kg N ha⁻¹, and then declined. (2) BNF measured by ¹⁵N natural abundance strongly correlated with ¹⁵N isotope dilution method. The maximum BNF of 61 kg N ha⁻¹ occurred at the N rate of 90 kg N ha⁻¹, which was significantly greater than that at the other N rates. (3) Root nodule dry weight and count were positively correlated with BNF, as were shoot dry weight and pod yield. (4) The N equation, which is based on the sum of N rate plus soil residual N before sowing, revealed that the pod yield and BNF peak when the total N is <200 kg N ha⁻¹. In conclusion, to improve the yield and quality of the next N-sensitive crops and reduce environmental pollution risk, N rate for faba bean should be significantly reduced and adjusted on the basis of soil residual N before sowing. This maximizes the BNF potential of faba bean.