How does increasing planting density affect nitrogen use efficiency of maize: A global meta-analysis

Context: Obtaining high grain yield and nitrogen use efficiency (NUE) is imperative in maize (Zea mays L.) production. Optimization of planting density is recognized as a key strategy to promote grain yield. However, its impacts on NUE have not been well investigated. Objectives: This study aimed to elucidate the impact of plant density on NUE and its physiological components. Methods: A meta-analysis was conducted, including 237 peer-reviewed studies and 2226 observations. Results: Globally, increased planting density boosts grain yield and affects NUE-related indicators. NUE shifts are attributed to increased fertilizer-N recovery efficiency (REferN, by ,14.5 %), enhanced N remobilization efficiency (NRemE, by 8.4 %), and promoted physiological N utilization efficiency (NUtE, by 2.3 %). High planting density reduces root to shoot ratio (R/S), root biomass (RB), root length (RL), and nodal root number (NR), showing decreases ranging from 5.1 % to 43.2 %. A 13.3 % increase in specific root length (SRL) is found. High planting density prompts changes in N allocation. At silking stage, there is a 20.6 % reduction in stalk N accumulation and a 16.0 % decline in leaf N accumulation per plant. At per hectare level, increased planting density results in a 12.8 % increase in pre-silking N accumulation (PrS-N) but a 7.1 % decrease in post-silking N accumulation (PoS-N). Optimizing N rate with a split-N application increases grain yield, REferN, NRemE, NUtE, NHI, and grain N concentration, with gains ranging from 6.5 % to 29.2 %. Implementing fertigation enhances grain yield and REferN by 13.1 % and 27.8 %. Adopting new cultivars increases NUtE by 2.7 %. Conclusion: In a high-yielding system with increasing planting density, efficient root N uptake, despite constraints on root size, contributes to greater population PrS-N accumulation and fertilizer-N recovery efficiency. Increased population PrS-N accumulation and its remobilization into grains contribute to elevated NUtE, and finally NUE. Implications: The findings highlight the importance of developing a robust root system for enhanced N uptake and coordinated N partitioning to improve NUE. The results provide valuable insights for advancing field management practices and breeding efforts.