Alleviating Microbial Carbon Limitation in Pinus armandii Forests Through Panax notoginseng Cultivation

The cultivation of Panax notoginseng (Sanqi) within the Pinus armandii forest understory has been widely promoted in Yunnan, China. However, relatively little is known about how Sanqi cultivation influences microbial metabolic limitations and their driving factors in P. armandii ecosystems in terms of soil extracellular enzyme activity (EEA) and stoichiometry (EES). In this study, we established monoculture P. armandii (MPA) and Sanqi–P. armandii agroforestry (SPA) systems to investigate microbial resource limitations in P. armandii soils over 12 months (semi-monthly sampling). Sanqi cultivation decreased EEAs in P. armandii soils in the SPA system. Moreover, the vector length in both the bulk and rhizosphere soils of P. armandii decreased significantly from 1.31 to 1.12 and 1.29 to 1.21, respectively, indicating a decrease in the microbial C limitation of P. armandii soils. A vector angle < 45° in both systems revealed that N, rather than P, predominantly restricted microbial metabolism. The most influential factors affecting vector length and angle were Sanqi cultivation and seasonal dynamics. Structural equation modelling (SEM) revealed that fungi-to-bacteria ratios and soil chemical properties were direct factors positively affecting vector length. Overall, our findings suggest that Sanqi cultivation benefited soil microorganisms in P. armandii soils and should be encouraged to supply N to promote the sustainable development of P. armandii.