Optimizing NPK fertilization for Prunus mira seedlings: growth thresholds, physiological driving factors, and nutrient accumulation

Prunus mira Koehne is an endemic and ecologically significant tree species of the Qinghai-Tibet Plateau. However, the harsh alpine climate and barren soil conditions in this region significantly restrict its early seedling growth and the success of artificial afforestation. To systematically elucidate how nitrogen (N), phosphorus (P), and potassium (K) fertilization regulate the growth and physiological/ecological adaptability of P. mira seedlings, a “3,414” incomplete orthogonal experimental design was implemented using 1-year-old seedlings. We comprehensively evaluated the effects of different nutrient configurations on morphological development, biomass allocation, photosynthetic gas exchange, physiological homeostasis, and nutrient accumulation. The results revealed that (1) the morphological development and biomass accumulation of P. mira seedlings exhibited pronounced non-linear threshold effects and diminishing marginal returns in response to nutrient inputs. Moderate single-nutrient supplies (N2, P2, K2) were most conducive to promoting overall plant growth. Notably, under high-P conditions, the seedlings evolved a unique “P-driven root foraging” allocation strategy to adapt to nutrient-poor habitats. (2) Network analysis based on the Mantel test and principal component analysis (PCA) confirmed that the effective uptake of exogenous nutrients served as the core driver for stimulating photosynthetic potential and reshaping physiological homeostasis. Optimal fertilization significantly enhanced the activities of superoxide dismutase (SOD) and peroxidase (POD), effectively alleviated membrane lipid peroxidation (MDA), and relieved stomatal limitation, thereby leading to a comprehensive surge in photosynthetic carbon assimilation efficiency and net photosynthetic rate. (3) P. mira seedlings demonstrated the highest dependence on nitrogen, but the growth-promoting potential of any single nutrient was restricted due to strong synergistic interactions among N, P, and K. Based on the ternary quadratic fertilizer response model, the optimal fertilization formula for P. mira seedlings under specific potting conditions (20 × 15 × 15 cm) was quantitatively deduced for the first time: N at 1.42 g/pot, P at 2.05 g/pot, and K at 1.36 g/pot, achieving a theoretical maximum total biomass of 78.4 g. In summary, this study provides novel theoretical insights into the physiological mechanisms of P. mira in response to alpine and arid habitats. Furthermore, it offers precise mathematical model support and quantitative guidance for artificial propagation and precision nutrient management during large-scale ecological restoration practices on the Qinghai-Tibet Plateau.