Developing high-efficiency mixed forests or converting low-efficiency pure forests into near-natural mixed forests with optimal structure and function is a crucial aspect of forest management. In the initial stages of afforestation or stand improvement, fertilization and planting distance significantly influence the formation and development of mixed forests. This study investigated how nutrients and planting distance affect root competition between five-year-old Chinese pine (Pinus tabuliformis) and one-year-old Chinese white poplar (Populus tomentosa) and identified the factors influencing the competitive ability of these two species. Field planting experiments used three fertilization gradients (63 g·m−2, 125 g·m−2, and 250 g·m−2) of Stanley compound fertilizer with an N:P:K ratio of 2:1:1 and two planting distances (25 cm and 50 cm). Each experimental group was planted in circular plots with a radius of 0.5 m, yielding a total of nine plots. The effects of different fertilization concentrations and planting distances on root distribution were analyzed both horizontally and vertically. Pearson correlation analysis was used to assess the relationship between roots and soil nutrients, while Levins’ niche overlap formula evaluated the differences in root competition between the species. Furthermore, principal component analysis quantified the relationships between impact factors and the root competitiveness of the two species. Results indicated that both species primarily allocated their fine root biomass to the shallow 0–10 cm layer. Pinus tabuliformis primarily extended to the southwest, while Populus tomentosa predominantly grew to the north. Both species exhibited enhanced root growth at moderate nutrient concentrations of 125 g·m−2. At a planting distance of 50 cm from Pinus tabuliformis, Populus tomentosa seedlings demonstrated superior root growth compared to those planted 25 cm apart. Pinus tabuliformis demonstrated greater competitive ability in the deeper 10–30 cm soil layers compared to Populus tomentosa, which showed the opposite pattern in the shallow 0–10 cm layers. Furthermore, available phosphorus (contribution rate of one impact factor on the competitiveness indexes, CR: −0.998), organic matter (CR: −0.978), total nitrogen (CR: −0.947), and alkali-hydrolysable nitrogen (CR: −0.937) significantly negatively impacted the competitiveness indexes of Pinus tabuliformis. The fine root surface area, volume, and length of Populus tomentosa also significantly negatively affected its competitiveness indexes, with all contribution rates exceeding an absolute value of 0.847. Results indicated that the root distributions of Pinus tabuliformis and Populus tomentosa overlapped spatially, with each species exhibiting advantages in different regions. Therefore, in future plantation reconstruction and forest management, it is essential to thoroughly evaluate root, soil, and fertilizer factors, adjusting planting distances accordingly, to effectively mitigate competition between the two species and successfully establish a mixed forest.
