Abstract
The development of under-forest economy within agroforestry systems can enhance the efficiency of land and forest resource utilization, protect the ecological environment, and promote the sustainable development of forests. Forest understory American ginseng (Panax quinquefolius L.) often experiences spatial variability in light availability. However, the impact of light intensity under reduced spatial heterogeneity on soil microenvironments and the regulation of plant physiological traits to promote P. quinquefolius growth remains insufficiently explored. This study adopts field experiments in forest ecosystems to explore how light intensity influences P. quinquefolius physiology, soil biochemical properties, and microbial communities under reduced light spatial heterogeneity. The results indicate that, under reduced spatial heterogeneity with a light transmittance of 13.1%, P. quinquefolius exhibits coordinated physiological responses. The 13.1% light transmittance treatment decreases hydrogen peroxide content, while increasing net photosynthetic rate and chlorophyll content. These changes further improve root dry weight, which is significantly higher than that under 0.7%, 6.4%, and 19.8% light transmittance (with increases of 600.0%, 50.0%, and 5.0%, respectively). Additionally, soil urease activity, pH, and alkali-hydrolyzable nitrogen content are elevated, along with the relative abundance of pH-related microorganisms (e.g., unclassified_Leotiomycetes) and denitrifying bacteria (e.g., unclassified_Gemmatimonadaceae, Anaeromyxobacter, and Bradyrhizobium). The results of the Partial least squares path modeling analysis show that under reduced spatial light heterogeneity in the agroforestry system, light intensity directly promotes the growth of P. quinquefolius by modulating its physiological characteristics. Furthermore, light intensity indirectly fosters the growth of P. quinquefolius by improving soil biological properties (Urease, Sucrase, Catalase, Dehydrogenase, Acid phosphatase) and chemical properties (pH, Alkali-hydrolyzable nitrogen, Available potassium) through the regulation of its physiological characteristics. In conclusion, the optimal light transmittance for P. quinquefolius growth under reduced spatial heterogeneity is 13.1%. These findings offer practical insights for managing P. quinquefolius cultivation in forest ecosystems, contributing to the sustainable development of agroforestry systems.