Continuous Cropping Differentially Regulates Nitrogen Metabolism and Nodulation in Soybean Cultivars with Contrasting Continuous Cropping Tolerance Under Varied Nitrogen Levels

Continuous cropping obstacles severely restrict soybean production; nitrogen application can alleviate continuous cropping obstacles. However, it remains unclear how nitrogen metabolism and nodulation differ between soybean cultivars with contrasting continuous cropping tolerance under varied nitrogen levels. In this study, two soybean cultivars, Liaodou 14 (L14, continuous-cropping-tolerant) and Liaodou 10 (L10, continuous-cropping-sensitive), were cultivated in a sand culture system. Three nitrogen levels, low (LN), medium (MN), and high (HN), were set with three soybean soil extract treatments, namely continuous cropping (CC), continuous cropping + sodium orthovanadate (CS), and crop rotation (CR). Key nitrogen metabolic enzyme activities and gene expression in leaves and fine roots, nodule number, nitrogenase activity, and nitrogen content were systematically determined. The results show that the CC treatment significantly inhibited nitrogen metabolism enzyme activities, gene expression, and nitrogen accumulation, whereas the HN treatment partially alleviated the growth-related inhibitory effects induced by continuous cropping, although HN treatment did not fully restore symbiotic nitrogen fixation function. Under the CC treatment, L14 exhibited superior performance over L10 for most measured parameters, except for GDH activity and GmNADH-GOGAT expression. Specifically, the leaf NR activity of L14 was 8.51% and 8.47% higher than that of L10 at the R1 and R6 growth stages, respectively. The maximum nodule number was observed under the HN treatment, while the highest nitrogenase activity occurred under the LN treatment. Most indicators under the CS treatment showed intermediate values between the CC and CR treatments. Collectively, through optimized nitrogen management, continuous-cropping-tolerant cultivars maintain higher enzyme activities, stable gene expression and efficient nitrogen translocation and allocation, potentially alleviating continuous cropping obstacles and improving yield potential.