Response of soil microbial biomass carbon and nitrogen to the planting of Polygonatum cyrtonema and Sarcandra glabra under Cunninghamia lanceolata forest

Understory planting is an important way of imitating 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 effects of planting Polygonatum cyrtonema and Sarcandra glabra under Cunninghamia lanceolata forest on soil microbial biomass are still unclear. Based on this, an understory planting experiment was conducted in Luoguopu state-owned forest farm, Guangan City, Sichuan Province. A randomized block design was used to set up the planting areas of P. cyrtonema and S. glabra, as well as the control area of pure C. 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 P. cyrtonema under C. lanceolata forest significantly increased MBC and MBN content by 3.94% and 109.71% respectively; planting S. glabra under C. lanceolata forest increased MBC and MBN content by 1.07% and 26.38% respectively, but the difference was not significant; planting P. cyrtonema and S. glabra under C. lanceolata forest also significantly decreased MBC/MBN by 50.24% and 19.00% respectively. Redundancy analysis showed that soil physical and chemical factors could explain 84.4% of the changes of MBC, MBN conten and MBC/MBN ratio. Soil available potassium (AK), nitrate nitrogen (NO₃⁻) and soil organic carbon (SOC) were the key factors affecting MBC, MBN content and MBC/MBN ratio. In conclusion, planting P. cyrtonema and S. glabra under C. lanceolata forest increased soil microbial biomass by increasing soil AK, NO₃⁻ and SOC content, thereby promoting soil nutrient cycling.