In-Forest Planting of High-Value Herb Sarcandra glabra Enhances Soil Carbon Storage without Affecting the Diversity of the Arbuscular Mycorrhiza Fungal Community and Composition of Cunninghamia lanceolata

Sarcandra glabra in-forest planting, an anthropogenic activity that may introduce a variety of disturbances into the forest, is being popularly promoted in southern China, while its consequential influences on soil nutrients, as well as the arbuscular mycorrhiza fungal (AMF) community of key forest keystone plants, are still unelucidated, which hampers the assessment of ecological safety and the improvement of agronomic measurements. In this research, topsoil from a 3-year-old Sarcandra glabra planted forest and a nearby control forest were sampled, and the annual variation in the soil nutrients and AMF community of the keystone tree Cunninghamia lanceolata were investigated. Our result showed that the total amount of soil organic carbon of the Sarcandra glabra cultivation group was significantly higher than that of the control group (p < 0.05), which indicated that Sarcandra glabra cultivation significantly enhanced the topsoil carbon storage. Yet, there were only insignificant differences in the Shannon index and Chao index of the AMF community between the two groups (p > 0.05). PCoA analysis found that the compositional differences between two groups were also insignificant. This indicated that Sarcandra glabra cultivation had no significant influence on the diversity and composition of the Cunninghamia lanceolata AMF community. However, we found that the differences in the total amounts of nitrogen and total phosphorus between the two groups were relatively lower in April and September, which indicated the higher nutrient demands and consumption of Sarcandra glabra in these two periods and suggested that a sufficient fertilizer application in these two stages would reduce the potential competition for nutrients between Sarcandra glabra and Cunninghamia lanceolata in order to ensure Sarcandra glabra production and forest health. Lastly, our results reported a total extra income ranging from of CNY 127,700 hm−2 (7 years of cultivation) to CNY 215,300 hm−2 (10 years cultivation) provided by Sarcandra glabra in-forest planting, which indicated its powerful potential for mitigating poverty. Our research systematically investigated the annual variation in the soil nutrient content and keystone tree AMF community caused by Sarcandra glabra cultivation and offers constructive guidance for Sarcandra glabra cultivation and fertilization management and ecological safety assessment.