Improved Cultivars and the Application of Combined Fertilizer Improve the Grain Yield and the Nitrogen Uptake and Utilization in Continuously Cropped Soybean (Glycine max (L.) Merr.)

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.