Exploring the Impacts of Pre-Stem Elongation Nitrogen Status on Winter Wheat’s Subsequent Growth and Nitrogen Status Using Nitrogen Nutrition Index

Crop nitrogen (N) status governs winter wheat growth and productivity. This study tested the hypothesis that pre-stem elongation N status determined subsequent N dynamics, with leaf area index (LAI) serving as an associative physiological mediator. Using the nitrogen nutrition index (NNI), we quantified relationships between pre- and post-stem-elongation N status and explored mechanisms underlying post-stem elongation NNI variation. Three field experiments were conducted over multiple seasons using two winter wheat cultivars, with basal and topdressing N rates ranging from 0 to 300 kg N ha−1. We measured shoot biomass, tiller number, LAI, plant N concentration, critical N concentration, and yield. Generalized linear mixed models revealed significant positive relationships between spring-regrowth NNI and NNI at stem elongation and booting across experiments. Simple linear regression analysis yielded corresponding R2 values ranging from 0.50 to 0.61. Under the tested conditions, treatments without basal N showed severe N deficiency, reflected by low NNI values (0.44–0.56). This deficiency constrained growth and persisted despite topdressing. In contrast, basal N application (60–90 kg N ha−1) alleviated these limitations. Structural equation modeling suggested that LAI was an associative mediator linking early- and late-season N status. These results highlight the value of early-season N assessment for winter wheat N management.