Nitrogen application rates affect nitrogen metabolism, allocation, and fine root morphology in juvenile rubber trees (Hevea brasiliensis): A ¹⁵N tracing study

Storage nitrogen (N) plays a critical role in sustaining tree growth and metabolism during periods of N deficiency, but how exogenous N application affects the utilization of internal stored N reserves in juvenile rubber trees ( Hevea brasiliensis ) remain poorly understood. To address this, a pot experiment was conducted with 2‑year‑old rubber trees pre‑labeled with ¹ ⁵N to track the fate of stored N. In March 2024, during dormancy, an in‑situ soil replacement procedure was performed to remove exogenous ¹ ⁵N from the soil, ensuring that subsequent plant ¹ ⁵N signals originated solely from internal storage. Three N rates were established: N0 (0 g N plant⁻¹, control), N1 (50 g N plant⁻¹, medium), and N2 (100 g N plant⁻¹, high). Medium N significantly enhanced leaf antioxidant enzyme (CAT, POD, SOD) and N metabolism enzyme (NR, GS, GDH) activities, increasing total free amino acids content by 22.04% over N0. ¹ ⁵N tracing revealed that N1 increased the accumulation of storage-derived N in leaves (1314.40 mg plant⁻¹) and main stems, 68.47% higher than N0. In contrast, high N reduced storage N utilization in leaves (385.12 mg plant⁻¹) and induced root thickening (+4.19% vs. N0) without improving root length or surface area. Correlation network analysis further showed that moderate N strengthened the synergy between storage N remobilization and leaf metabolic processes, whereas excessive N disrupted this coordination. Soil N distribution indicated that N1 retained more fertilizer-derived N in the 0–40 cm layer, while N2 increased the accumulation of fertilizer-derived N in deeper soil layers (60–80 cm). Among the three N rates tested, 50 g N plant⁻¹ produced the most favorable combination of the measured responses, providing a basis for further field validation of N management in rubber plantations.