Sustainability assessment of nitrogen nutrition index based topdressing nitrogen application

Context: Precision nitrogen (N) management (PNM) serves as a promising strategy for meeting the dual goals of food security and agricultural sustainable development by matching crop N demand with soil N supply in time and space. Previous studies have focused on in-season estimation of crop N nutrition index (NNI) and N requirement (NR) to diagnose plant N status and facilitate PNM. However, direct application of NNI and NR in N fertilizer management remains limited. Objective: The objective of this study was to develop an NNI-based N recommendation algorithm (NNI-PNM) for wheat and evaluate its effectiveness compared with local management and multi-objective optimization strategies in terms of agronomic, economic benefits and environmental impacts. Methods: The dynamic relationships between NR and NNI were analyzed across wheat growth stages and integrated into an N recommendation model. Based on the yield response to N fertilizer, the performance of the NNI-PNM strategy was simulated under different N status scenarios from four site-year N plot experiments with different N levels and varieties. Then, Pareto-based ranking and life cycle assessment approaches were used to investigate the comprehensive potential of the NNI-PNM strategy in five farmers’ field experiments. Results: The results indicated that NNI and NR were significantly correlated (R² = 0.54–0.98), with their slopes increasing as days after sowing increased and N recovery efficiency decreased. Based on this pattern, the estimated NR values closely matched the observed NR values (RMSE = 3.0–13.7 kg ha⁻¹, RE = 23–36%). The topdressing N rates simulated by the NNI-PNM strategy varied with N status, with increasing yield (0.36–0.66 t ha⁻¹) and N partial factor productivity (16.8–21.6%) under N deficiency and N surplus conditions, respectively, compared with regional optimal N management. Moreover, the NNI-PNM strategy reduced N rates by 42% and global warming potential by 28.8%, and increased N use efficiencies by 78.4% without any reduction in grain yield, grain protein concentration, and net ecosystem economic benefits compared with farmer's conventional practices. Conclusion: The NNI-PNM strategy based on a robust relationship between NR and NNI is a simple yet effective tool for aiding smallholders in decision-making about fertilizer application. The developed strategy performed better in achieving multi-objective optimization (food security, economic benefits, and environmental quality) than farmers’ current practices. Significance: These findings demonstrate the potential of the NNI-PNM strategy in optimizing wheat production for multi-objective benefits and provide valuable insights for promoting agricultural sustainability in small-scale farming systems.