Effect of nitrogen application levels on the photosynthetic nitrogen distribution and use efficiency in leaves of soybean seedlings

Abstract Background Nitrogen nutrition is closely related to crop growth and development. The nitrogen application level affects leaf size, nitrogen content in leaves, and nitrogen distribution between photosynthetic and non-photosynthetic systems. Nitrogen distribution in leaves alters the photosynthetic nitrogen use efficiency (PNUE) and photosynthetic rate, ultimately affecting crop yield. In this study (Heilongjiang Province, 2021–2022), Jinyuan 55 and Keshan 1 soybean varieties were treated with different nitrogen levels in the form of urea: N0, 0 kg·hm− 2; N0.5, 60 kg·hm− 2; N1, 120 kg·hm− 2; and N1.5, 180 kg·hm− 2. We compared the effects of different nitrogen levels on plant morphology, biomass, photosynthetic physiology, nitrogen distribution, PNUE, and other indicators of soybean seedling leaves. Results The maximum carboxylation, maximum electron transfer, net photosynthetic rates, and PNUE of both soybean varieties first increased significantly with the increase in nitrogen application rate, and then stabilized. The PNUE, carboxylation system components, electron transport components, and non-photosynthetic system distribution ratios in the photosynthetic system increased, and then decreased with the increase in nitrogen application rate. The proportion and content of components in the light-harvesting system decreased and increased gradually, respectively, with the increase in nitrogen application rate. The nitrogen ratios between the carboxylation and electron transport systems of both soybean varieties were positively correlated with the PNUE. Furthermore, the nitrogen ratio in the light-harvesting and non-photosynthetic systems was linearly negatively correlated with the PNUE Conclusions Overall, an appropriate nitrogen level maintained a high photosynthetic nitrogen ratio, whereas low or high nitrogen conditions increased or decreased the nitrogen ratio in non-photosynthetic and photosynthetic systems, respectively, thus decreasing the PNUE and photosynthetic capacity. Moreover, an increase in the nitrogen application rate can lead to a decrease in nitrogen ratio of the light-harvesting system and an increase in the nitrogen ratio of electron transport and carboxylation systems. Our results provide a theoretical basis for optimizing leaf nitrogen distribution, determining optimum nitrogen levels, and promoting soybean seedling growth.