Response of Young Hazelnut Trees to Different Nitrogen Fertilization Rates

Nitrogen (N) plays a central role in supporting photosynthesis, vegetative growth, and early productivity in fruit crops. For hazelnut (Corylus avellana L.), Italian regional guidelines recommend relatively low N inputs during the establishment phase, particularly under non-irrigated conditions. However, information on optimal N requirements for young, irrigated hazelnut orchards remains limited. This study aimed to identify the optimal N rate for young, irrigated hazelnut orchards and to relate N supply to leaf N concentration, gas exchange, vegetative growth, and early yield. A three-year experiment (2023–2025) was conducted in an irrigated ‘Tonda di Giffoni’ orchard in northern Italy. Four fertilization regimes combining two N doses and two application frequencies were compared with an unfertilized control. Leaf N concentration, gas exchange parameters, vegetative growth, and yield were monitored throughout the study. N supply had limited influence on seasonal physiological patterns, while leaf N concentration showed a clear dose–response relationship. Maximum photosynthetic rates were associated with leaf N concentrations between 20 and 22 g kg− 1 dry weight. Growth and yield responses indicated that intermediate–high N applications (approximately 75–100 kg N ha⁻¹ at the 2nd − 3rd year) resulted in the greatest trunk growth and highest yield at the 4th year. In the final year, despite fertilizer rates exceeding current recommendations, leaf N concentrations remained below the optimal threshold, suggesting a dilution effect associated with increasing canopy size and structural biomass. The results indicate that young hazelnut trees require higher N inputs than those commonly recommended by regional guidelines. However, increasing N supply beyond the optimal range does not proportionally enhance photosynthesis, vegetative growth, or early yield.