Panax notoginseng–Polygonatum kingianum Rotation Alters Soil Ecoenzymatic Stoichiometry and Reduces Enzyme-Inferred Microbial Carbon Limitation in the Pinus armandii Understory

Long-term continuous cropping of Panax notoginseng (Sanqi) in the understory agroforestry systems of southwest China disrupts soil biogeochemical cycles and triggers microbial nutrient limitation. Nonetheless, it remains unclear how rotation with compatible medicinal plants regulates soil ecoenzymatic stoichiometry and enzyme-inferred microbial nutrient limitation in Pinus armandii understories. Here, pure P. armandii stands (Pa), Sanqi continuous cropping (SS), and Sanqi–Polygonatum kingianum rotation (SP) systems were established as three treatments in a subtropical plateau mountain region. We analyzed edaphic properties, extracellular enzyme activities, and bacterial and fungal gene abundance. The results showed that both SS and SP markedly reduced soil extracellular enzyme activities relative to the Pa control. Vector analysis indicated universal phosphorus limitation across all treatments (vector angle > 45°), with no significant difference in phosphorus limitation intensity between SS and SP. Notably, SS increased vector length and thus aggravated enzyme-inferred microbial carbon limitation, while SP effectively alleviated such carbon constraint. Seasonal variation exerted a stronger influence on soil ecoenzymatic traits and microbial nutrient limitation than planting systems. Redundancy analysis (RDA) revealed that total potassium soil organic carbon and the fungi-to-bacteria ratio were the key factors governing soil enzyme profiles in the SP system. Besides bacterial and fungal abundance, the F:B ratio, soil physical properties, soil chemical properties in the SP system also directly drove vector length. Collectively, based on one-year monitoring, SP rotation modified soil ecoenzymatic stoichiometry and reduced enzyme-inferred microbial carbon limitation in the P. armandii understory. These findings provide empirical evidence for optimizing medicinal plant agroforestry management in P. armandii forests.