Responses of Nitrogen Metabolism Pathways to Low-Phosphorus Stress: Decrease in Nitrogen Accumulation and Alterations in Protein Metabolism in Soybeans

Phosphorus is an indispensable nutrient for nitrogen metabolism in soybeans. In this study, two P levels were established, 1 mg/L (low-P stress) and 31 mg/L (normal P, CK), by combining 15N labeling with real-time quantitative PCR and the UHPLC-MS/MS method, to analyze soybean nitrogen accumulation, 15N abundance, nodule nitrogen fixation accumulation, nodule nitrogen fixation rate, soluble protein content, the relative expression of phosphorus transporters, amino acid changes, and metabolic pathways. The impacts of phosphorus stress on soybean nitrogen metabolism were explored from the perspectives of nitrogen accumulation and protein metabolism. The results demonstrated that low-P stress promoted the absorption of fertilizer nitrogen by aboveground parts, roots, and nodules of soybeans. However, it significantly inhibited nitrogen accumulation (11.09–95.41%), nodule nitrogen fixation accumulation (21.54–96.21%), and nodule nitrogen fixation rate (2.95–37.75%). The soluble protein content in both leaves and nodules decreased remarkably, while the relative expression of GmPT7 was upregulated in leaves, roots, and nodules under low-P stress. A total of 70 amino acids exhibited alterations, among which 26 amino acids were involved in 37 metabolic pathways, playing a crucial role in regulating the effects of low-P stress on soybean nitrogen metabolism. This study identifies significant alterations in nitrogen accumulation, nodule nitrogen fixation, and the expression of phosphorus transporter genes, providing insights into the metabolic pathways involved in soybean’s adaptation to phosphorus deficiency. This research provides a solid theoretical foundation for further in-depth investigations into the physiological and molecular mechanisms of soybean response to low-P stress.