Response Characteristics of Nitrogen Metabolism and Yield Formation in Oilseed Rape Driven by Silicon–Nitrogen Synergy

Silicon deficiency is widespread in soils of Southeastern China and may constrain nitrogen (N)-use efficiency and yield formation in oilseed rape; therefore, this study aimed to identify an N-reduction window enabled by silicon (Si) fertilization and to clarify the underlying mechanisms. Field experiments were conducted in Putian, Fujian Province (2023–2025), with five treatments: conventional N (T1, 300 kg·N·ha−1), conventional N plus Si (T1+Si, 150 kg·Si·ha−1), and three N rates (120%, 80%, and 60% of conventional N; T2+Si, T3+Si, and T4+Si) under a fixed Si input. Yield, N-use efficiency, plant physiological traits, soil quality index (SQI), and nitrogen-cycle ecosystem multifunctionality were assessed. Compared with T1, T1+Si and T3+Si increased yield by 6.55% and 6.06%, respectively, accompanied by higher dry matter translocation (27.20% and 34.60%) and improved N-use efficiency (28.86% and 39.66%). SQI increased by 31.42% (T1+Si) and 33.03% (T3+Si), while nitrogen-cycle multifunctionality increased by 32.42% and 58.42%, respectively. Correlation and path analyses indicated that Si promoted yield formation by simultaneously alleviating soil constraints (lower exchangeable acidity and Al3+; higher cation exchange capacity) and enhancing plant assimilation and allocation processes, thereby reducing potential N losses and strengthening N cycling. Overall, applying 150 kg·Si·ha−1 combined with a 20% reduction in N (240 kg·N·ha−1) achieved stable yield gains and coordinated improvements in soil quality, providing an operational fertilization window for Si-enabled N management in regional oilseed rape systems.