Objective The northwestern region of Liaoning Province is located in a wind-water erosion crisscross zone, where severe soil erosion and continuous decline in soil fertility have significantly constrained plant growth. In this context, this study aims to investigate the synergistic regulatory effects of different microbial inoculant combinations on the physio-ecological characteristics of Populus simonii and the quality of the soil inter-root environment, thereby providing scientific support and technical solutions for vegetation restoration and ecological rehabilitation in this region.

Methods Based on a field experiment of Populus simonii Carr., three microbial inoculants: Arbuscular mycorrhizal fungi (AMF), Trichoderma harzianum (TH), and Bacillus mucilaginosus (BM) were aplied using a completely randomized block design. A total of eight different treatments were established: AMF, TH, BM, AMF + BM, AMF + TH, TH + BM, AMF + TH + BM, and a non-inoculated control (CK). The effects of these treatments on poplar growth were assessed, together with changes in soil aggregate stability, mechanical composition, physicochemical properties, and available nutrient content.

Results (1) All treatments improved poplar growth compared with the control, with the TH + BM treatment exhibiting the strongest synergistic effect. Relative to CK, plant height, stem thickness and net photosynthetic rate of poplar increased by 50%, 51.75% and 65.38%, respectively, and the plant growth index reached a peak value of 0.90. (2) Soil aggregate analysis indicated that particles sized 0.25–0.05 mm were the dominant fraction. In May, the AMF + TH treatment produced the highest sand content (58.06%). During May and October, the soil texture under the sole application of AMF shifted from sandy to silty soil, while the soil under the AMF + BM treatment changed from silty to sandy soil.(3) The AMF + TH treatment most effectively increased soil organic matter in May, while TH + BM treatment had the greatest improvement in soil pH. In October, TH + BM treatment significantly enhanced total nitrogen, total phosphorus, and total potassium improvement by 58.82%, 90.91%, and 17.31%, respectively, compared with CK. Regarding available nutrients, AMF + TH treatment most effectively increased alkaline nitrogen, TH + BM treatment maximized available potassium, and AMF + TH + BM (May) and AMF + BM (October) produced the greatest increases in available phosphorus. Soil quality index (SQI) analysis indicated that TH + BM had the strongest overall improvement effect, with a peak SQI value of 0.85, representing a 1.58-fold increase relative to CK.

Conclusion The combined application of Trichoderma harzianum and Bacillus mucilaginosus (TH + BM) exerts pronounced synergistic effects on promoting plant growth and improving soil quality. This microbial strategy effectively alleviates constraints on vegetation development and soil degradation in wind–water erosion-prone areas, providing an efficient and sustainable biotechnological approach for ecological restoration in such fragile environments.