Soil salinization is a major cause of land degradation along the western coast of Taiwan.
Afforestation has the potential to improve soil quality, enhance ecological functions, and increase
land resilience. In this study, a planting experiment was conducted in the salt-affected degraded
land of Qigu, using three native coastal tree species. The experiment aimed to investigate the
effects of microbial inoculants under different soil salinity conditions by applying Trichoderma
harzianum Rifai (strain T2). Results showed that Trichoderma Pers. significantly increased the
activity of most rhizosphere soil enzymes in low-salinity areas and enhanced invertase and alkaline
phosphatase activity in Hibiscus tiliaceus L. and Calophyllum inophyllum L., respectively, under
high-salinity conditions. Additionally, Trichoderma promoted arylsulfatase activity across all
tree species and salinity conditions, demonstrating its potential to enhance sulfate transformation
in soils. In terms of seedling growth, Trichoderma improved the Fv/Fm and root collar diameter
of Melia azedarach L. under high salinity, indicating enhanced salt tolerance. It also promoted
the ФPSII of all species across different salinity levels, suggesting widespread benefits to the
photosynthetic system. Furthermore, microbial biomass carbon was highly positively correlated
with enzyme activity, particularly alkaline phosphatase and urease, which were closely related
to plant height growth. These findings indicated that soil microorganisms play a crucial role in
improving soil quality and promoting plant growth. In conclusion, although excessive salinity
inhibits tree growth, the application of Trichoderma improves rhizosphere enzyme activity and
promotes seedling growth and physiological health, making it a promising soil amendment for
enhancing afforestation success in coastal salt-affected areas.