Plant growth stimulator formulations improve Michelia macclurei seedling performance under a controlled compacted-soil condition

Soil compaction restricts root development and resource acquisition, limiting tree establishment and productivity. Michelia macclurei is a valuable timber species that may encounter compacted soils in subtropical forestry. This study screened plant growth stimulator formulations for improving M. macclurei seedlings performance under a defined moderately compacted-soil condition. An L9 (3 4 ) orthogonal array experiment was conducted using potassium indole-3-butyric acid (IBA-K), sodium naphthaleneacetate (NAA-Na), potassium fulvic acid, and thiamine hydrochloride. Several formulations improved multiple growth and physiological traits. Among the tested treatments, T8 (50 mg·L −1 IBA-K, 40 mg·L −1 NAA-Na, and 300 mg·L −1 Thiamine hydrochloride) achieved the highest PCA-based composite score and ranked above the commercial rooting agent under the tested conditions. Compared with the untreated control, T8 increased shoot and root biomass by 185.74% and 227.87%, respectively, and increased shoot and root P concentrations by nearly 300% to 1.78 g·kg −1 and 232.99% to 1.95 g·kg −1 , respectively. T8 also improved root morphology and photosynthetic traits and increased antioxidant enzyme activities, accompanied by lower malondialdehyde content. Intrinsic water-use efficiency did not differ significantly among treatments, while none of the PGS formulations significantly altered the root-to-shoot biomass ratio relative to the untreated control. Notably, these seedling responses occurred without detectable changes in the measured bulk-soil physical properties. Main-effect analysis indicated that NAA-Na was associated with responses across a relatively broad range of physiological domains within the tested formulation space. Overall, T8 was the highest-ranked formulation tested and represents a promising candidate for improving early M. macclurei seedling performance under the compacted-soil conditions examined.