Nitrogen and Phosphorus Additions Alleviate Nitrogen Limitation in Pinus yunnanensis Seedlings by Reshaping Organ-Specific Stoichiometry

Nitrogen (N), phosphorus (P), and potassium (K) are essential elements for plants and are also the main limiting elements in the growth process. In order to investigate the effects of fertilization on the nutrient content and stoichiometric ratio of Pinus yunnanensis seedlings, we implemented a complete 3 × 3 factorial design incorporating three nitrogen levels (0, 0.4, and 0.8 g·plant-1) and three phosphorus levels (0, 3, and 6 g·plant-1), yielding nine distinct treatment combinations. The contents of N, P, and K in different organs of P. yunnanensis were determined, and their stoichiometric ratios were calculated. N content ranked as needle > root > stem; P content was in the order of root > stem > needle; and K content showed the pattern of stem > needle > root. Fertilization did not alter the above nutrient distribution patterns; it significantly influenced nutrient concentrations and stoichiometry. Single applications of nitrogen fertilizer had the most significant effect on nutrient content; compared with the unfertilized control, single nitrogen application increased needle N content by 1.13-fold. Combined nitrogen and phosphorus application preferentially promoted the accumulation of N relative to P in stems; specifically, under combined N-P treatment, stem N content increased by 1.11-fold while stem P content increased by 1.03-fold, indicating a greater relative increase in N. In terms of stoichiometric ratio, fertilization increased N:P, particularly in the root and needle; root N:P increased by 1.19-fold under T8 (0.8 g N + 3 g P) and needle N:P increased by 1.17-fold under T2 (3 g P alone), both indicating a mitigation of N limitation. Consistent with the nutrient content results, single nitrogen application exerted the greatest effect on stoichiometric ratio, followed by nitrogen and phosphorus combined application, and single phosphorus application had the least effect. These findings suggest that fertilization can alter nutrient allocation and stoichiometric relationships in Pinus yunnanensis, which may affect its growth and metabolic processes, but further studies are needed to link these changes to seedling growth.