Optimizing nitrogen management in grain rotations: balancing retention and photosynthesis

Diversified cropping systems exhibited enhanced resource utilization efficiency, yet the nitrogen (N) allocation mechanisms and utilization patterns in economic-grain rotation systems remain poorly understood. This study investigated the cross-seasonal N allocation dynamics and photosynthetic responses to N reduction in a garlic-maize rotation system through a three-season field experiment with graded N treatments (garlic, 300 and 240 kg N ha^-1; maize, 220, 175, and 130 kg N ha^-1). N reduction increased soil water-filled pore space (WFPS) by 5.2%-8.7% during maize seasons but decreased it in garlic seasons. It also reduced topsoil (0-20 cm) NO₃ ^--N accumulation by >15% compared to deeper layers. Leaf physiological parameters-including leaf area index (LAI), SPAD values, and net photosynthetic rates-declined by 18%-32% under N reduction, with garlic demonstrating higher sensitivity. Residual N from preceding garlic crops stabilized maize LAI (± 6.5%), indicating compensatory inter-season adjustments. Critical thresholds were identified: maize achieved optimized grain nitrogen partitioning (65% to 72%) and a 22% improvement in nitrogen use efficiency (NUE) with a reduction of 45 kg N ha^-1 without a significant yield penalty (less than 5%). Conversely, garlic experienced a 23% increase in stem nitrogen translocation when nitrogen was reduced by 60 kg N ha^-1, which resulted in a 34% decrease in bulb allocation. Annual N reduction (8.65%-28.85%) enhanced maize agronomic efficiency (+40%) but reduced garlic yields (4.2%-27.5%) and partial factor productivity (-18%). These results reveal contrasting, crop-specific N allocation strategies and support the development of demand-driven N management to balance productivity and economic outcomes in multi-cropping systems.