Effects of Nitrogen Application Rate on Bulb Yield, Nitrogen Use Efficiency, and Normalised Difference Red Edge-Based Nitrogen Diagnostics in Garlic Varieties

Optimising nitrogen (N) management is crucial for improving garlic (Allium sativum L.) productivity and nitrogen use efficiency (NUE). This study evaluated the effects of N application rate on bulb yield, NUE, and the links between N status, Normalised Difference Eed Edge (NDRE), and yield. A field experiment was conducted using nine N rates (0–360 kg N ha−1) across three garlic varieties (Glenlarge, Southern Glen and AV08). Foliage and bulb N concentrations were measured at key growth stages, and yield components were determined at 185 days after planting (DAP). NDRE values were collected at 147 DAP to evaluate their potential as non-destructive indicators of crop N status and yield. Nitrogen rate significantly affected bulb yield, with maximum yield achieved at 160–240 kg N ha−1. Increasing the N application rate reduced dry matter content and NUE across three garlic varieties. NDRE showed strong positive correlations with N rate (r2 = 0.96), leaf N concentration (r2 ≥ 0.82), and bulb yield (r2 = 0.85), demonstrating its sensitivity to in-season N variability and its potential for yield prediction. This study provides a systematic assessment of NDRE for N status diagnosis in garlic and presents initial evidence of varietal differences in N response, contributing to improving the understanding of management in this crop.