Nitrogen Source Release Dynamics Drive the Carbon-Nutrient Coupling Mechanism in the Fast-Growing Tree Ochroma lagopus: Organ-Specific NSCs Allocation and Coordinated Adaptation of N-P-K Stoichiometry

This study investigated two-year-old seedlings of the fast-growing tree species Ochroma lagopus under a control (CK) and treatments with urea (N1–N3) and slow-release fertilizer (H1–H3) at rates of 300, 450, and 600 g per tree. We systematically revealed the organ-level “carbon‑nutrient coupling” strategies and physiological mechanisms driven by nitrogen‑source release dynamics. Nitrogen addition consistently increased non‑structural carbohydrate (NSCs) accumulation; leaf NSCs rose by 19.53% (N3) and 23.48% (H3) under high nitrogen. However, nitrogen‑source patterns dictated NSC allocation: urea favored starch (leaf starch increased by 81.90% in N2), forming a “high‑nutrient‑starch” growth‑oriented strategy, whereas slow‑release fertilizer increased soluble sugars (sugar/starch ratio rose 24.35% in H2), leading to a “homeostasis‑sugar” stress‑resistance strategy. Urea triggered rapid root N and P accumulation (root N up 70.60%) and K redistribution to leaves (leaf K up 44.39%), while slow‑release fertilizer maintained N homeostasis, preferentially supplying leaves and roots and reducing branch N. Stoichiometric ratios were the core regulators of carbon allocation, showing plasticity in the order root > branch > leaf, with root K/P being most plastic. Roots were the most responsive organ, and the soluble‑sugar/starch ratio acted as a key physiological hub. A growth model was constructed in which nitrogen dynamics set the strategy, organ functional differentiation shapes responses, stoichiometric balance serves as the central regulatory axis, and carbon and nutrients interact bidirectionally as “nutrients activate carbon fixation and carbon supply reciprocates nutrient metabolism.” For fast‑growing plantations, urea is recommended for rapid biomass accumulation and slow‑release fertilizer for enhanced stress resistance, providing theoretical support for carbon‑sink forest cultivation and sustainable management.