Optimizing seasonal nitrogen reallocation enhances productivity and fertilizer-N recovery while reducing nitrogen losses in a rapeseed-rice rotation

Context Optimizing seasonal nitrogen (N) allocation in crop rotations is essential for improving system productivity and reducing environmental N losses, yet most N management strategies remain focused on individual crop seasons. Objective A two-year field experiment was conducted in a plateau lake region of Southwest China to evaluate whether reallocating a fixed annual N input between seasons could improve yield, fertilizer N recovery, and environmental performance in a rapeseed-rice rotation. Methods Five N treatments were examined: no N fertilizer application as the control, farmers’ conventional practice with 210 kg N ha −1 for rapeseed and 180 kg N ha −1 for rice (T1; 390 kg N ha −1 ), and three optimized treatments with a fixed annual N input of 330 kg N ha −1 but different rapeseed/rice allocations: 150/180 (T2), 180/150 (T3), and 210/120 kg N ha −1 (T4). Crop yield, N use efficiency, NH₃ volatilization, runoff N loss, and leaching N loss were measured. Fertilizer N fate was further quantified using 15 N-labeled urea in treatments T2 to T4. Results and conclusions At a fixed annual nitrogen fertilizer application rate, reallocating more N to the rapeseed season significantly increased rapeseed yield without reducing rice yield. Compared with rice-prioritized T2, T4 increased rapeseed yield by 42.96–49.80% and annual system yield by 15.83–16.45%. Meanwhile, T4 reduced annual NH 3 volatilization loss by 32.66–36.84%, runoff N loss by 12.80–26.30%, and leaching N loss by 16.41–18.34%. The 15 N results showed that cumulative fertilizer N recovery increased from 35.78% in T2–47.56% in T4, whereas unaccounted-for fertilizer N decreased from 51.41% to 38.99%. Soil residual fertilizer N remained similar among treatments. Significance These results indicate that, under the conditions of this study, reallocating N at the rotation scale can simultaneously improve system productivity and fertilizer N recovery while reducing the measured N-loss pathways in a rapeseed–rice rotation.