The responses of soil microbial biomass carbon and nitrogen to the introduction of Polygonatum cyrtonema and Sarcandra glabra under a Cunninghamia lanceolata forest

Planting under forest is an important way to imitate wild cultivation of Chinese herbal medicine, which can not only improve land use efficiency and economic benefits, but also improve forest ecological function through plant-soil interaction.However, the effect of Polygonatum cyrtonema and Sarcandra glabra planted under Cunninghamia lanceolata forest on soil microbial biomass is not clear. Based on this, an underforest planting experiment was conducted in Luoguopu State-owned forest Farm, Guangan City, Sichuan Province. A randomized block design was used, setting up cultivation areas for Polygonatum cyrtonema and Sarcandra glabra, as well as a control area of pure Cunninghamia lanceolata forest. Soil samples were collected from 0~20 cm soil layer, and soil microbial biomass carbon (MBC) and microbial biomass nitrogen (MBN) were determined. The results showed that: Planting Polygonatum cyrtonema under Cunninghamia lanceolata forest significantly increased MBC and MBN contents by 3.94% and 109.71% respectively; planting Sarcandra glabra under Cunninghamia lanceolata forest increased MBC and MBN contents by 1.07% and 26.38% respectively, but the difference was not significant; planting Polygonatum cyrtonema and Sarcandra glabra under Cunninghamia lanceolata forest also significantly decreased MBC/MBN by 50.24% and 19.00% respectively. Redundancy analysis showed that 84.4% of the changes in MBC, MBN and MBC/MBN ratio could be explained by soil physical and chemical factors. Available potassium (AK), nitrate nitrogen (NO₃⁻) and soil organic carbon (SOC) were the key factors affecting MBC, MBN and MBC/MBN ratio. In conclusion, planting Polygonatum cyrtonema and Sarcandra glabra under Cunninghamia lanceolata forest increased soil microbial biomass and promoted soil nutrient cycling by increasing soil AK, NO₃⁻ and SOC contents.