The effect of organophosphate bacteria on heavy metal elements in the rhizomes, fibrous roots, and rhizosphere soil of Paris polyphylla var. yunnanensis. Paris polyphylla var. yunnanensis is a perennial herbaceous medicinal plant in the Liliaceae family. The rhizomes, as its main medicinal parts, have detoxifying, hemostatic, antibacterial, anti-inflammatory, immune regulating, and anti-tumor effects. In recent years, in order to alleviate market supply and demand, the artificial planting area of Dianzhong Lou has been continuously expanded. However, due to immature planting technology and extensive use of fertilizers, the growth environment of Dianzhong Lou has been severely damaged, resulting in poor quality of the planted Dianzhong Lou and gradually decreasing yield. Therefore, finding reasonable fertilization methods to increase the usable value of Dianzhong lou is a hot topic in the standardized planting system of Dianzhong lou.

In recent years, with the rapid development of the traditional Chinese medicine industry, issues such as excessive heavy metal content in Chinese medicinal materials caused by environmental pollution have also received widespread attention. Heavy metal elements have the characteristics of high concealment, difficult degradation, and easy accumulation. They are easy to accumulate in the human body and difficult to excrete. When the content of heavy metals in the human body reaches a certain level, organ functional disorders will occur, which can trigger a series of diseases. When the mercury content in the body exceeds the standard, it can damage brain tissue and cause neurological disorders; Excessive cadmium and chromium elements can induce carcinogenesis in organs such as the kidneys; Excessive lead element can affect human digestion and memory function; Excessive arsenic can have adverse effects on various tissues and organs in the human body, such as muscles, blood, and immune system. Organic phosphorus solubilizing bacteria, as a new type of microbial fertilizer, can mineralize organic phosphorus compounds, convert insoluble organic phosphorus compounds in the soil into phosphorus elements that can be directly absorbed and utilized by plants, increase soil available phosphorus content, and alleviate the phenomenon of phosphorus deficiency in plant growth. In addition, a large number of studies have shown that organophosphate solubilizing bacteria can also passivate heavy metals such as lead, cadmium, copper, etc., by combining phosphate ions released from soil organic phosphorus with heavy metals to form phosphate precipitates, reducing the mobility and bioavailability of heavy metals in the soil environment, and playing a role in fixation or passivation, thereby reducing the harm caused by heavy metals. Organic phosphorus solubilizing bacteria mainly include bacteria, fungi, and actinomycetes, among which bacteria have the most types and quantities. Research has found that some strains of Bacillus, Pseudomonas, and Corynebacterium have strong phosphorus solubilizing ability and are often used as phosphorus bacterial fertilizers in agriculture. According to reports, the use of organophosphate solubilizing bacteria has a definite effect on the effective components and growth and development of economic crops such as corn and peanuts, and has been widely promoted and applied. The research conclusions on the heavy metal content of Dianzhong Lou medicinal materials and soil from different types or origins are already very mature. Yang et al. also explored the effects of arbuscular mycorrhizal fungi on the heavy metal elements of Dianzhong Lou rhizomes and soil. However, the effects of inoculating organophosphate solubilizing bacteria on the heavy metal content of Dianzhong Lou medicinal materials and soil have not been reported yet.

Therefore, in this study, a single factor indoor potted plant was used to select three dominant species of organic phosphorus solubilizing bacteria as strains, which were inoculated and combined in the cultivation medium. The rhizomes, fibrous roots, and rhizosphere soil of Dianzhong Lou were used as the research objects. Inductively coupled plasma mass spectrometry (ICP-MS) was used to detect five heavy metal elements, namely mercury (Hg), cadmium (Cd), lead (Pb), chromium (Cr), and arsenic (As), in order to explore the effects of organic phosphorus solubilizing bacteria and their combinations on the heavy metal element content of Dianzhong Lou rhizomes, fibrous roots, and rhizosphere soil, and to provide theoretical basis for the standardized artificial planting and development of Dianzhong Lou.

 

Organic phosphorus solubilizing bacteria can convert insoluble organic phosphorus compounds in soil into effective phosphorus that can be directly absorbed and utilized by plants through mineralization of organic phosphorus compounds, promoting the growth and development of Dianzhong Lou and obtaining high-yield and high-quality Chinese medicinal materials. At the same time, organic phosphorus solubilizing bacteria can also fix and passivate heavy metal elements in soil, making them difficult for medicinal plants to directly absorb, thereby reducing the residual heavy metal elements in Chinese medicinal materials. In this study, compared with the non inoculated CK group, inoculation with organophosphate solubilizing bacteria could overall reduce the heavy metal content in the roots, rhizomes, and fibrous roots of Dianzhong Lou, with the most significant reduction in Pb element content. The reason for this is that organophosphate solubilizing bacteria can passivate Pb elements in the soil, transforming heavy metal Pb from a highly active form to a less active form, reducing its bioavailability, and thus reducing the absorption and utilization of Pb elements by Dianzhong Lou. There was also a significant improvement in the content of Hg and Cr elements, and the mixed inoculation treatment was better than the single inoculation treatment. The inoculation treatment of S6 and S7 groups had the best effect on Dianzhong Lou. Referring to the “Chinese Pharmacopoeia” (2020 edition) and the “National Food Safety Standard – Limits of Contaminants in Food” (GB2762-2017) as the limit standards for Dianzhong Lou medicinal materials, only some treatment groups in the rhizomes have slightly exceeded the standards for Pb and Cr, while most heavy metals in the roots have exceeded the limit standards. The reason may be due to the presence of a large number of root hairs on the surface of the roots, which can not only directly absorb various harmful substances in the soil, but also effectively increase the contact area with the soil, thereby increasing the absorption capacity of soil elements. After inoculation with organophosphate solubilizing bacteria in the rhizosphere soil of Dianzhong Tower, the heavy metal content in the soil did not change. Referring to the “Risk Control Standards for Agricultural Soil Pollution in Soil Environmental Quality” (GB15618-2018), all treatment groups’ soils did not exceed the national limit standards; According to the “Technical Specification for Soil Environmental Testing” (HJ/T166-2004) as the evaluation standard, the single pollution index of all treatment groups is less than 1. Except for the S1 group, whose Nemerow index is greater than 0.7, the soil belongs to the relatively clean level and there is a risk of pollution. The soil of the other treatment groups has reached the clean level. This indicates that the heavy metal content in the cultivation substrate used in this experiment fully meets the planting standards, and there is no significant risk of heavy metal pollution to the soil after inoculation with organophosphate degrading bacteria. Therefore, the experiment of inoculating Dianzhong Lou plants with organophosphate degrading bacteria is safe and effective.

The heavy metal enrichment coefficient is an important indicator that describes the accumulation trend of heavy metals in organisms, and indirectly reflects the ability of plants to enrich heavy metals in their rhizosphere soil. Generally speaking, the smaller the enrichment coefficient of heavy metals in crops, the poorer their ability to absorb heavy metals and the stronger their resistance to soil heavy metals. Rice has a strong enrichment effect on Cd, so it is necessary to pay attention to the residual amount of Cd in the soil and environment during rice cultivation to avoid excessive enrichment of Cd, which can lead to excessive Cd content in rice and endanger human health. In this study, inoculation with different organophosphate solubilizing bacteria resulted in varying enrichment abilities of the roots, rhizomes, and fibrous roots of Dianzhong Lou for five heavy metal elements: Hg, Cd, Pb, Cr, and As. Among them, the enrichment ability of Cr element was strongly depleted, while its bioaccumulation effect on Cd element was greater, but all were significantly lower than the critical content standard for hyperaccumulating plants (1000mg/kg). The enrichment coefficient of fibrous roots was greater than that of rhizomes, and the enrichment ability of the mixed inoculation treatment group was weaker than that of the single inoculation treatment group. Therefore, instead of considering the use of Dianzhong Lou’s fibrous roots as medicinal parts, the two most effective inoculation methods can be considered as references for bio fertilizers and applied in the artificial cultivation process of Dianzhong Lou to reduce the risk of heavy metal pollution in Dianzhong Lou medicinal materials. In future experiments on organophosphate solubilizing bacteria and Dianzhong Lou field promotion and cultivation, priority should be given to regulating Cd elements.

Correlation analysis shows that after inoculation with organophosphate solubilizing bacteria, there is a significant correlation between Hg and Pb elements, while the correlation between other elements is relatively small. This indicates that after inoculation with organophosphate solubilizing bacteria, Dianzhonglou has a significant synergistic effect on the absorption of Hg and Pb elements in the rhizosphere soil; In addition, the correlation between rhizomes and fibrous roots is stronger than that between rhizosphere soil, indicating a stronger mutual influence of heavy metal element content between rhizomes, fibrous roots, and between rhizomes and fibrous roots. Therefore, in future field cultivation, the synergistic effects between rhizomes and fibrous roots, as well as between Hg and Pb elements, should be considered simultaneously.

In summary, inoculation with different organophosphate solubilizing bacteria or composite bacteria can effectively reduce the content of Hg, Pb, and Cr elements in the roots, rhizomes, and fibrous roots of Dianthus polyphylla, thereby reducing the risk of excessive heavy metal content in Dianthus polyphylla medicinal materials. It can also effectively control the risk of heavy metal pollution in the rhizosphere soil of Dianthus polyphylla. Meanwhile, the results of this study also indicate that mixed inoculation of the three bacteria and mixed inoculation of strain B Wiedmannii and B The two methods of proteolyticus have the best impact on Dianzhong Lou. Therefore, these two inoculation methods of organophosphate solubilizing bacteria can be promoted and applied to the artificial planting process of Dianzhong Lou, providing reference for the remediation of heavy metal pollution in the artificial planting process of Dianzhong Lou.