Quantifying the relationship between nitrogen nutrition index and radiation use efficiency in maize (zea mays L.)

Efficient nitrogen use is essential for maximizing profitability in maize production while minimizing environmental impacts. The nitrogen nutrition index (NNI) and radiation use efficiency (RUE) are key indicators used to assess nitrogen status and biomass production efficiency. However, their quantitative relationship remains insufficiently understood under diverse nitrogen conditions. In this study, field experiments were conducted in Morioka, Japan, during 2020 and 2021 using five nitrogen application rates and multiple maize varieties. Nitrogen concentration and NNI were determined, and RUE was calculated, at multiple growth stages through destructive sampling. A segmented regression model was applied to analyze the relationship between NNI and RUE based on 160 observations. Despite some variability, a consistent pattern was observed: RUE declined when NNI fell below the threshold. The segmented linear regression model captured the impact of nitrogen deficiency on RUE and identified a critical threshold value for NNI. RUE decreased by approximately 0.48 g MJ−1 for every 0.1-unit decrease in NNI below the threshold value of 1.07. This finding supports the use of NNI not only as an indicator of nitrogen status but also as a quantitative predictor of maize biomass production under varying nitrogen supply conditions. Fertilizer management strategies can be optimized by quantifying expected yield losses associated with suboptimal NNI and comparing them with current nitrogen application practices.