Breeding and cultivating low-N-efficient maize varieties to obtain high yields with less N fertilizer is important for addressing food demands and environmental pollution. However, few studies have investigated the physiological characteristics of low-N-efficient maize varieties. Therefore, we performed an experiment over four years to test two maize varieties (low-N-efficient variety: JNK728, and high-N-efficient variety: XY335) and five N application rates (N120: 120 kg·ha⁻¹, N180: 180 kg·ha⁻¹, N240: 240 kg·ha⁻¹, N300: 300 kg·ha⁻¹, and N360: 360 kg·ha⁻¹). The optimal N application rates for JNK728 and XY335 were N180 and N300, which obtained the highest yields (11,754 and 12,752 kg·ha⁻¹, respectively), N uptake efficiencies (1.32 and 0.93 kg·kg⁻¹), and N harvest index (67.94% and 61.98%), compared with other N application rates. The key period for plant N accumulation was the R1–R6 stage, which contributed 35.2–49.7% and 40.8–53.8% to plant N accumulation at the maturation stage in JNK728 and XY335, respectively. In addition, N accumulation in the grain accounted for more than half (51.8–63.2%) of the total N accumulation in plants, and the leaf N transport amount after the post-silking stage was the primary source of grain N accumulation in both JNK728 and XY335. We also explored the key enzymes and genes related to the N transport amount and efficiency in leaves in the two maize varieties, and found that GOGAT was the key enzyme and GOGAT2 was the key gene for JNK728, whereas the AS enzyme and AS1 and AS3 genes were most important for XY335. Therefore, we suggest that molecular breeding programs should focus on the GOGAT2 gene in low-N-efficient maize varieties, and cultivation techniques should aim to improve the GOGAT enzyme activity after the post-silking period to achieve high yields and N utilization efficiencies with less N fertilizer.

In recent years, continuous cropping has become a major constraint on soybean production in China, and thus, researching methods to improve soybean yield under this cropping pattern has become a research hotspot. This study aimed to explore whether cultivar improvement and different fertilization regimes could enhance the nutrient uptake and resource utilization of continuously cropped soybean, thereby elevating its yield potential. A total of 11 soybean cultivars were subjected to different fertilization treatments, with the leaf area index (LAI), net photosynthetic rate (Pn), radiation use efficiency (RUE), dry matter accumulation, crop growth rate (CGR), nitrogen content (NC), grain yield, yield components, and harvest index (HI) analyzed. Compared with early cultivars, current cultivars increased LAI, Pn, CGR, RUE, NC, dry matter accumulation, grains per plant, 100-seed weight, HI, and grain yield by 26.22%, 10.07%, 34.13%, 22.65%, 20.43%, 29.44%, 30.09%, 9.80%, 13.69%, and 15.88%, respectively, while decreasing the nitrogen requirement per 100 g grain by 20.08%. Similarly, compared with unfertilized plants, fertilized plants increased these indices by 23.93%, 14.08%, 53.38%, 39.01%, 29.53%, 42.49%, 16.95%, 23.35%, 10.49%, and 26.50%, respectively, while decreasing the nitrogen requirement per 100 g grain by 14.40%, with the highest yield observed by the 2006-era cultivar (Dennison) fed compound fertilizer. In conclusion, cultivar improvement and fertilization can improve the yield potential of continuously cropped soybean by enhancing light energy and optimizing nitrogen accumulation and consumption, and future research should focus more on breeding to further tap the production potential of continuously cropped soybean.

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.