Optimizing Nitrogen Application Enhances Sugar Beet (Beta vulgaris L.) Productivity by Modulating Carbon and Nitrogen Metabolism

Optimizing fertilization practices can reduce the application of chemical nitrogen fertilizers, thereby enhancing crop yield while mitigating environmental impacts. In 2021–2022, we conducted field experiments in the Inner Mongolia region, evaluating the effects of different nitrogen application methods on sugar beet growth and productivity. A two-factor randomized complete block design was applied for the experiments with the nitrogen application rates (0, 75, 150, 225, and 300 kg ha−1, denoted as N0, N75, N150, N225, N300, respectively) as the primary factor and the nitrogen application methods (single basal application, S1; and split application, S2) as the second factor. The results indicate that increasing nitrogen application rates significantly enhances leaf growth, chlorophyll content, dry matter accumulation, nitrogen metabolism, and yield. In comparison with a nitrogen-free treatment, the N225S1 treatment significantly improved sugar beet development and nitrogen metabolism, resulting in an average yield increase of 29.36% over two years. Notably, by optimizing the root–shoot balance and carbon–nitrogen metabolism, N150S2 achieved root and sugar yields comparable to N225S1. Therefore, our study demonstrates that split nitrogen application can maintain sugar beet yield while reducing nitrogen fertilizer inputs, thereby providing valuable insights for sustainable sugar beet cultivation in Inner Mongolia, the main sugar beet growing area in China.