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

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. This study aimed to elucidate the impact of plant density on NUE and its physiological components. A meta-analysis was conducted, including 237 peer-reviewed studies and 2226 observations. 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 %. 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. 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.