Abstract: Spatiotemporal variations in the ammonium-to-nitrate ratio of atmospheric nitrogen deposition profoundly influence plant adaptation strategies. In this study, one-year-old Cunninghamia lanceolata seedlings were subjected to fourfour nitrogen form treatments （CK, NH₄⁺-₄⁺-N, NO₃^--₃^--N, and Mix-N at a 1:1 molar ratio） for 12 months. Rhizosphere soil properties, root morphology, nitrogen metabolism enzyme activities, photosynthetic physiology, and nutrient allocation were measured, and Partial Least Squares Structural Equation Modeling （PLS-SEM） was employed to disentangledisentangle the drivers of biomass accumulation. The results showed that: （1） Mixed nitrogen sources exhibited significant physiological synergistic effects, withwith the highest total biomass, net photosynthetic rate, and total root length. （2） Single NH₄⁺-₄⁺-N input maintainedmaintained high photosynthetic rates but triggered intensive GS/GOGAT cycling that depleted carbon skeletons; concurrently, severe rhizosphere acidification reducedreduced soil available phosphorus, elevating the leaf N:P ratio above the phosphorus limitation threshold and forcing thethe adoption of a high specific root length （SRL） foraging strategy, which further exacerbated carbon costscosts and constrained biomass. （3） NO₃^--₃^--N treatment induced a conservative root strategy withwith high root tissue density （RTD）. （4） PLS-SEM revealedrevealed thatroot morphological plasticity exerted a stronger direct effect on biomass （path coefficient = 0.75） than photosynthetic capacity （path coefficient = 0.26）. These findings indicate that root nutrientnutrient acquisition capacitycapacity, rather than photosynthetic carbon assimilation, is the primary determinantdeterminant of *C. lanceolata* seedling growth under varying nitrogen forms. A mixed ammonium-nitrate fertilization strategy is recommended forfor plantation management to optimize the root-soil interface and enhance productivity.