Effects of Nitrogen–Phosphorus Co-Application on Biomass Allocation and Accumulation in Two-Year-Old Pinus yunnanensis Seedlings

<i>Pinus yunnanensis</i> is a significant native tree species in southwestern China, contributing substantially to the area's ecological stability and economic growth. However, its growth rate tends to be relatively slow during the seedling stage, and fertilization is crucial to promote seedling growth. This study used two-year-old <i>P. yunnanensis</i> seedlings as experimental materials and applied a 3 × 3 factorial design, combining three nitrogen (N) levels (0, 0.4, and 0.8 g·plant^-1) supplied in the form of urea with three levels of phosphorus (P) (0, 0.8, and 1.6 g·plant^-1) supplied in the form of superphosphate to form nine treatments, denoted as T1 to T9. This study was carried out in the open-air nursery of Southwest Forestry University, with fertilization beginning in July and observations continuing until December of the same year. Using allometric growth analysis and constructing the fertilizer response regression equation, we investigated the effects of fertilization on biomass accumulation in <i>P. yunnanensis.</i> The findings revealed that fertilization significantly increased the biomass allocation ratio to roots but decreased the allocation to needles and aboveground parts (<i>p</i> < 0.05). Allometric growth analysis showed that root growth was more rapid than stem and needle growth, and the growth rate of belowground parts exceeded that of aboveground parts. Allometric growth between organs differed among treatments, whereas the allometric growth relationship between aboveground and belowground biomass showed no significant difference across treatments. Moderate N and P fertilizer application promoted biomass accumulation in all organs, with T5 (N: 0.4 g·plant^-1; P: 0.8 g·plant^-1) exhibiting the highest biomass accumulation. Based on the comprehensive analysis of optimal N and P fertilizer requirements for biomass accumulation across different organs, the recommended fertilizer application rates are as follows: N 0.5-0.6 g·plant^-1 and P 0.5-0.9 g·plant^-1, with an optimal N:P ratio ranging from 1:0.8 to 1:1.8. The results establish a scientific rationale for enhancing fertilization methods in <i>P. yunnanensis</i> seedling cultivation, contributing to the slow growth issue during the seedling stage.