Objective This study aimed to provide a comprehensive technical reference for improving the growth quality and soil micro-ecology of Pinus sylvestris var. mongolica plantations in the Three-North region by investigating the synergistic effects and underlying mechanisms of the combined application of Suillus luteus and biochar on seedling growth, physiology, and rhizosphere microecology.

Method One-year-old P. sylvestris var. mongolica seedlings were used to examine the effects of individual and combined applications of S. luteus and biochar. Four treatments were established: a blank control (CK), biochar alone (C), S. luteus inoculation alone (S), and their combined application (C + S).

Result The combined application (C + S) treatment yielded the most significant effects. Regarding growth, the (C + S) treatment significantly increased seedling height, ground diameter, fresh mass, and dry mass, which increased by 51.32%, 68.47%, 215.42%, and 87.37%, respectively, compared with the CK, while the mycorrhizal colonization rate reached 75.24%. In terms of physiological metabolism, the (C + S) treatment effectively increased chlorophyll a, soluble sugar, and soluble proteins contents, as well as root vigor, while promoting lignin and cellulose synthesis. Regarding rhizosphere microecology, the (C + S) treatment significantly increasedrhizosphere soil populations of bacteria, fungi, and actinomycetes enhanced urease, catalase, and acid phosphatase activities, and elevated soil nitrogen, phosphorus, and potassium contents. Overall, performance indicators ranked as follows: (C + S) treatment > single treatments (S or C) > CK, indicating that the combined application experted a stronger promotional effect than individual applications. These results demonstrate that the combined application of S. luteus and biochar exerts a significant synergistic promotion effect on P. sylvestris var. mongolica seedlings by facilitating mycorrhizal symbiont formation, improving physiological metabolism and enhancing rhizosphere soil microecology.

Conclusion The combined application of S. luteus and biochar effectively promotes the growth of P. sylvestris var. mongolica seedlings, improves the rhizosphere soil microecology, and systematically enhances seedling vigor, thus serving as a potential technical measure for seedling cultivation and woodland soil improvement of P. sylvestris var. mongolica.