Optimizing nitrogen supply promotes biomass, physiological characteristics and yield components of soybean (Glycine max L. Merr.)

Abstract

Avoidable or inappropriate nitrogen (N) fertilizer rates harmfully affect the yield production and ecological value. Therefore, the aims of this study were to optimize the rate and timings of N fertilizer to maximize yield components and photosynthetic parameter of soybean. This field experiment consists of five fertilizer N rates: 0, 75, 150, 225 and 300 kg N ha^-1 arranged in main plots and four N fertilization timings: V₅ (trifoliate leaf), R₂ (full flowering stage) and R₄ (full poding stage), and R₆ (full seeding stage) growth stages organized as subplots. Results revealed that 225 kg N ha^-1 significantly enhanced grain yield components, total chlorophyll (Chl), photosynthetic rate (P _N), and total dry biomass and N accumulation by 20%, 16%, 28%, 7% and 12% at R₄ stage of soybean. However, stomatal conductance (g _s ), leaf area index (LAI), intercellular CO₂ concentration (Ci) and transpiration rate (E) were increased by 12%, 88%, 10%, 18% at R₆ stage under 225 kg N ha^-1. Grain yield was significantly associated with photosynthetic characteristics of soybean. In conclusion, the amount of nitrogen 225 kg ha^-1 at R₄ and R₆ stages effectively promoted the yield components and photosynthetic characteristics of soybean.

Keywords: Ci, intercellular CO2 concentration; DW, dry weight; E, transpiration rate; GM, grain mass; GNP, grain number per pod; GY, grain yield; Grain yield; J, journal; LAI, leaf area index; Nitrogen rates; PN, photosynthetic rate; PNP, pod number per plant; PPFD, photosynthetic photon flux density; Photosynthetic characteristics; R2, R4 and R6, reproductive stage; TCC, total chlorophyll contents; TN, total nitrogen; Timing; V5, Vegetative stage of five trifoliate leaf; g, grams; gs, stomatal conductance.

Fig. 1

Effect of different nitrogen rates and timings on N contents of soybean crop. The data is the average of the three repeats. V₅, R₂, R₄, and R₆ refer to three trifoliate, full flowering, full pod and full seeding stages of the soybean, N0-N300 kg ha⁻¹ represent the nitrogen application rates. Different lowercase letters indicate the significant (p£0.05) differences among the N rates and growing stages of soybean, means with similar letters denotes the no significant different among the treatments p£0.05 using the HSD test in SPSS. Statistical bars indicate ± standard errors, (n = 3).

Fig. 2

Effect of different nitrogen rates and timings on total chlorophyll contents of soybean crop. The data is the average of the three repeats. V5, R2, R4, and R₆ refer to three trifoliate, full flowering, full pod and full seeding stages of the soybean, N0-N300 kg ha⁻¹ represent the nitrogen application rates. Different lowercase letters indicate the significant (p£0.05) differences among the N rates and growing stages of soybean, means with similar letters denotes the no significant different among the treatments p£0.05using the HSD test in SPSS. Statistical bars indicate ± standard errors, (n = 3).

Fig. 3

Effect of different nitrogen rates and timings on leaf area index of soybean crop. The data is the average of the three repeats. V₅, R₂, R₄, and R₆ refer to three trifoliate, full flowering, full pod and full seeding stages of the soybean, N0-N300 kg ha⁻¹ represent the nitrogen application rates. Different lowercase letters indicate the significant (p£0.05) differences among the N rates and growing stages of soybean, means with similar letters denotes the no significant different among the treatments p£0.05using the HSD test in SPSS. Statistical bars indicate ± standard errors, (n = 3).

Fig. 4

Effect of different nitrogen rates and timings on total dry biomass of soybean crop. The data is the average of the three repeats. V5, R2, R4, and R6 refer to three trifoliate, full flowering, full pod and full seeding stages of the soybean, N0-N300 kg ha⁻¹ represent the nitrogen application rates. Different lowercase letters indicate the significant (p£0.05) differences among the N rates and growing stages of soybean, means with similar letters denotes the no significant different among the treatments p£0.05 using the HSD test in SPSS. Statistical bars indicate ± standard errors, (n = 3).

Fig. 5

Relationship of grain yield with photosynthetic traits and yield components of soybean. Total dry biomass (TDB), Photosynthetic rate (P_N), Stomatal Conductance (g_s), Intercellular CO₂ Concentration (Ci), Transpiration Rate (E), Total Nitrogen (TN), 100 grain weight (g).

Fig. 6

Correlation coefficient of grain yield with other growth and photosynthetic traits. Grain yield (kg ha⁻¹) (GY), Pod number per plant (PNP), Grain number per pod (GNP), 100 grain weight (g) (GW), Total chlorophyll contents (TCC), Total dry biomass (TDB), Leaf area index (LAI), Photosynthetic rate (P_N), Stomatal Conductance (g_s), Intercellular CO₂ Concentration (Ci), Transpiration rate (E), Nitrogen Contents (NCS). Correlation matrix with numeric values and their graphical representation. Orange bubbles show significant relation, Blue bubbles show negative relation, white color values are less 0.01 so values are disappear and values not significant (p > 0.05 level) are marked with an X.