Optimized nitrogen fertilizer management enhances soybean (Glycine max (L.) Merril.) yield and nitrogen use efficiency by promoting symbiotic nitrogen fixation capacity

Abstract

Introduction: Although the mulched drip irrigation system combined with high nitrogen input (240∼310 kg ha^-1) in Xinjiang, China, frequently achieves record-high soybean yields (6855 kg ha^-1), this practice is not conducive to symbiotic nitrogen fixation and compromises agricultural sustainability.

Methods: Under the mulched drip irrigation, this study evaluation four nitrogen application treatments (N₀: 0 kg ha^-1, N₁₂₀: 120 kg ha^-1, N₁₈₀: 180 kg ha^-1, and N₂₄₀: 240 kg ha^-1) were evaluated over two consecutive growing seasons to investigate their effects on nodule morphological and physiological traits, stem ureide content, and the percentage of nitrogen derived from the atmosphere (%Ndfa) during the reproductive growth stage.

Results: The application of 180 kg ha^-1 nitrogen significantly increased nodule number, nodule dry weight, nodule sucrose content, and nodule starch content, while improving soybean yield and nitrogen agronomic use efficiency. Conversely, the application of nitrogen exceeding 180 kg ha^-1 inhibited nitrogenase activity, suppressed leghemoglobin synthesis, disrupted the glutamine synthetase/glutamate synthase metabolic pathway, and reduced ureide translocation from nodules to stems, leading to significant accumulation of ureides in nodules. Correlation and path analyses indicated that nitrogenase activity, leghemoglobin content, urate oxidase activity, and stem ureide content were significantly positively correlated with %Ndfa, whereas nodule ureide content showed a significant negative correlation with %Ndfa. Stem ureide content exhibited a strong direct positive effect on %Ndfa (path coefficient = 0.95), confirming its validity as a robust indicator for assessing SNF capacity.

Discussion: In conclusion, mulched drip irrigation, applying 180 kg ha^-1 nitrogen at the beginning pod stage (R₃) effectively enhances root nodulation, promotes carbohydrate allocation to nodules, sustains symbiotic nitrogen fixation activity, and ultimately increases soybean yield and nitrogen use efficiency. Thus, under mulched drip irrigation system, applying the correct rate of nitrogen fertilizer is beneficial for enhancing soybean yield and mitigating environmental risks, which holds significant importance for promoting sustainable agricultural development.

Keywords: high yield; nitrogen fertilizer; nodulation; soybean; symbiotic nitrogen fixation.

Figure 1

Daily minimum temperature, maximum temperature and rainfall from April to October in 2022 (A) and 2023 (B).

Figure 2

Effects of nitrogen application on nitrogen metabolism enzyme activities in nodules in 2022 (A) and 2023 (B). Bars represent means and error bars standard error (n=3). Different letters represent significant differences (p<0.05) between treatments at the same growth stage.

Figure 3

Effect of nitrogen application on the activities of urate oxidase in soybean nodules in 2022 and 2023. Bars represent means and error bars standard error (n=3). Different letters represent significant differences (p<0.05) between treatments at the same growth stage.

Figure 4

Effects of nitrogen application on ureide content (A) and nodule carbohydrate content (B) in 2022 and 2023. Bars represent means and error bars standard error (n=3). Different letters represent significant differences (p<0.05) between treatments at the same growth stage.

Figure 5

The relationship between the parameters of symbiotic nitrogen fixation in soybean and %Ndfa at the full pod stage (R₄).

Figure 6

Structural equation modeling (SEM) of the relationships between symbiotic nitrogen fixation traits and nitrogen derived from atmosphere (%Ndfa) at R₄ stage in 2022 and 2023. Solid and dashed arrows indicate significant and nonsignificant path coefficients, respectively. NNA, Nodule nitrogenase activity, LB, Leghemoglobin content, UO: Urate oxidase activity. Values above arrows represent standardized path coefficients. ** denote significance at the 0.01 probability levels.