Study on the Effects of Lime Treatment on the Content, Yield, and Quality of Five Heavy Metals in Chuanxiong
Chuanxiong is the dried rhizome of Ligusticum chuanxiong Hort., a plant in the Umbelliferae family. It is an important blood activating and pain relieving herb in traditional Chinese medicine, known as the “Qi medicine in the blood”. It has the effects of promoting blood circulation, dispelling wind, and relieving pain. It is commonly used for chest pain, menstrual disorders, headaches, and wind dampness pain. The active ingredients of Chuanxiong mainly include lactones, phenolic acids, and alkaloids, such as ferulic acid, ligustilide A, ligustilide I, Z-ligustilide, ferulic acid pine bark ester, and chlorogenic acid. Modern pharmacological studies have shown that Ligusticum chuanxiong has multiple effects, including anti-inflammatory, analgesic, anti-tumor, platelet aggregation inhibition, and vasodilation.
Ligusticum chuanxiong Hort is a famous genuine medicinal material produced in Sichuan, and is also a commonly used bulk medicinal material and an important export variety. With nearly a thousand years of planting history, it is mainly produced in Sichuan, with Dujiangyan Irrigation Project, Pengzhou, Pixian and other places as authentic places. Currently, Pengzhou and Meishan have the largest output. The commercially available Chuanxiong is all artificially cultivated, with over 90% of it produced in Sichuan. Chuanxiong is a plant rich in heavy metal cadmium (Cd). In recent years, the excessive heavy metal content (mainly Cd) in Chuanxiong has seriously affected its export earnings, medication safety, local economic development, and the income of medicinal farmers. According to incomplete statistics, the Cd content in Ligusticum chuanxiong exceeds the standard by up to 87.99%. The excessive Cd content in Ligusticum chuanxiong medicinal materials has brought adverse effects on product safety and the healthy development of the industry.
Soil remediation technology is a conventional soil improvement method to alleviate the excessive use of heavy metals in soil. This technology usually involves applying amendments to change the physical and chemical properties of the soil, reduce the content of available heavy metals in the soil, and decrease the absorption of heavy metals by plants. Previous studies have confirmed that the main factors affecting the Cd content in Chuanxiong medicinal herbs are their Cd enrichment characteristics and the concentration of ionized Cd in the soil. Lime can reduce the Cd content in Chuanxiong medicinal herbs by increasing soil pH and lowering the concentration of ionized Cd in the soil. Dong et al. found that cross-linked amino starch can bind with Cd2+through coordination and adsorb Cd2+in water. Lu et al. reported that the application of a composite amendment composed of lime, calcium magnesium phosphate fertilizer, peat, activated carbon, and sodium polyacrylate can not only significantly increase the aboveground biomass of water spinach, but also reduce the content of Pb and Cd in water spinach. At present, research on the methods of reducing heavy metal content in Chuanxiong mainly focuses on the application of soil amendments such as quicklime, silicon fertilizer, and KH2PO4 NaOH buffer solution alone, while the use of composite forms has only been explored by the research group in the early stage. Based on the research of the research group, this article uses field experiments to explore the effects of single application of quicklime or combined application with starch, activated carbon, potassium dihydrogen phosphate, and sodium polyacrylate on the heavy metal content, medicinal yield, and quality of Chuanxiong. Multivariate statistical analysis methods are used to compare the differences in various indicators, in order to screen methods that can effectively reduce the heavy metal content of Chuanxiong while ensuring the yield and quality of medicinal materials, and provide reference for high-quality green production of Chuanxiong.

The effects of quicklime treatment on the content of five heavy metals, yield, and quality of Chuanxiong vary, and both the place of origin and treatment method have an impact on each indicator. Taking into account the effect of reducing heavy metals (mainly Cd), medicinal yield, and quality, T-1 (quicklime 6.67kg/hm2) and T-2 (quicklime 6.67kg/hm2+potassium dihydrogen phosphate 1.33kg/hm2) can be used as methods to reduce heavy metals in the production of Chuanxiong in Pengzhou and Meishan. T-3 (quicklime 6.67kg/hm2+starch 6.67kg/hm2) and T-4 (quicklime 6.67kg/hm2+sodium polyacrylate 10kg/hm2) can be used as methods to reduce heavy metals in the production of Chuanxiong in Meishan and Pengzhou, respectively. T-2 treatment can significantly reduce the content of Cd and As in Ligusticum chuanxiong, significantly increase the content of alcohol soluble extract and ligustilide I in Ligusticum chuanxiong, and to some extent improve the yield of medicinal materials; T-1, T-3, and T-4 can significantly reduce the Cd content in Chuanxiong and increase the yield of medicinal materials. The above methods not only achieve the effect of reducing heavy metals, but also ensure the yield and quality of medicinal materials. In addition, the Cd reduction effect of the above method is basically consistent with the results reported by Ren, a member of the research team, which further indicates that the stability of the above method in reducing Cd in Chuanxiong is good, and can provide reference for the production of low Cd Chuanxiong medicinal materials in the main production areas of Chuanxiong.
Single application of quicklime (T-1 quicklime 6.67kg/m2) achieved good Cd reduction effects in Meishan (26.85%) and Pengzhou (35.18%). Adding quicklime during the production of Chuanxiong can increase soil pH, reduce soil available Cd content, and lower Cd content in Chuanxiong medicinal materials. Therefore, in production, appropriately increasing soil pH through the application of soil amendments such as quicklime is one of the effective measures to reduce the Cd content in Ligusticum chuanxiong. However, the addition of quicklime also increased the concentration of Ca in the soil. Studies have found that applying Ca to the soil can reduce the absorption and enrichment of Cd by plants such as corn and rapeseed, while He et al.’s research has shown that Ca has a certain activating effect on Cd, which can improve the absorption, accumulation, and transfer ability of heavy metal Cd to the aboveground parts of Spartina alterniflora. Therefore, further research is needed to clarify whether the increase of Ca in soil during the production process of Ligusticum chuanxiong affects its Cd absorption and enrichment.
There is a close relationship between P and Cd in soil, and phosphate can affect the effectiveness of Cd by inducing adsorption and precipitation. Applying potassium dihydrogen phosphate to soil can increase soil pH, promote the formation of insoluble phosphate precipitation and hydroxylated phosphate lead cadmium minerals, and reduce the effectiveness of Cd. There are research reports that potassium dihydrogen phosphate can significantly reduce cadmium accumulation in rice. Among the six treatments, Meishan T-2 (quicklime 6.67kg/hm2+potassium dihydrogen phosphate 1.33kg/hm2) and Pengzhou T-3 (quicklime 6.67kg/hm2+starch 6.67kg/hm2) showed better Cd reduction effect than T-1, indicating that the combination of potassium dihydrogen phosphate, starch, and quicklime can enhance the Cd reduction effect. However, there are differences in the impact of the same measures on the Cd content of Chuanxiong in the two places. It is speculated that the effect of quicklime treatment on plant cadmium reduction is influenced by soil physicochemical properties. Relevant analysis and cluster analysis show that both the origin and treatment methods have an impact on various indicators, further confirming this hypothesis. Therefore, the selection of the best method for reducing heavy metals should be tailored to local conditions. It also indicates that the combination of suitable soil amendments can achieve better results in reducing heavy metals.
The research results showed that 7 out of 14 samples of Chuanxiong exceeded the standard for Cd content, with a exceedance rate of 50%. The 7 samples that exceeded the standard were all from the Pengzhou experimental site, which is consistent with the background value of Cd in Pengzhou soil (0 54mg/kg) is higher than the background value of Cd in Meishan soil (0.22mg/kg), and Meishan soil pH (7.53) is higher than Pengzhou soil pH (5.47). The main influencing factors of Cd content in Chuanxiong medicinal herbs are not only the high background value of soil Cd, but also their own high Cd enrichment characteristics. This study found that the Cd content in six treatment groups of Chuanxiong in Pengzhou decreased by 15.42% to 39.05% compared to CK, but their Cd content remained between 0.42 and 0.59mg/kg. Based on the current situation of C exceeding the standard in traditional high-quality production areas of Chuanxiong, the research team is conducting screening for new varieties of Chuanxiong enriched with low Cd, aiming to provide more possibilities for solving the problem of Cd exceeding the standard in heavy metals of Chuanxiong.
Through correlation analysis of various indicators, we found a significant negative correlation between Pb and Cd, suggesting that Chuanxiong has competitive inhibition in absorbing Pb and Cd. Xu Zhongzhong et al. found the same pattern in rapeseed, and Cd has a significant inhibitory effect on Pb absorption and accumulation; Pb is significantly positively correlated with ligustilide A and As in Ligusticum chuanxiong. It is speculated that the Pb content in plants during the growth and development of Ligusticum chuanxiong has a certain influence on the synthesis of ligustilide A. There is a mutual promotion effect when Ligusticum chuanxiong absorbs As and Pb, and Hu found that there is a mutual promotion effect when rice roots absorb As and Pb in rice.