Tea, as an ancient non-alcoholic beverage, is widely beloved, originating from China1. It is currently one of the world's three major beverages and is universally recognized as one of the best health drinks2. Approximately 3 billion people worldwide have the habit of drinking tea, consuming around 18 to 20 billion cups of tea daily3. Human exposure to environmental pollutants occurs through various pathways, with dietary intake being a significant route4. By consuming tea, people intake beneficial components such as tea polyphenols, amino acids, polysaccharides, caffeine, as well as trace elements like Cu, Zn, Se5. Nevertheless, concurrent exposure to detrimental heavy metal elements such as Pb, Cd, and As takes place, leading to an elevation in health risks associated with the accumulation of these heavy metal residues6.
Soil, as a crucial medium for tea production, determines the yield and quality of tea leaves. The contamination of soil by heavy metals not only directly affects crop yield and quality, given its attributes of concealment, irreversibility, lag, and long-term consequences, but it also presents a potential hazard to human health through the process of bioaccumulation in the food chain. Several researchers have conducted analyses and assessments of heavy metals in agricultural soil. As an illustration, Shammi et al7 gathered soil samples from agricultural land in northwestern Bangladesh, examining the levels of heavy metals such as Zn, Cu, Pb, Cd, Cr, and others. The study disclosed diverse degrees of heavy metal pollution in the agricultural soils of the region. Shi et al8 applied both the single-factor index method and the comprehensive pollution index method to appraise the extent of heavy metal pollution in tea plantation soils situated on river terraces within the mountainous regions of Guizhou Province. They found that the soil in this region has experienced severe pollution, particularly from Cd contamination. Zhang et al9 focused on agricultural suburban soils in the representative county of Huzhu in Qinghai Province. A total of 120 soil samples were gathered, and the analysis encompassed the examination of soil pH along with the levels of heavy metals such as Cr, Cd, Pb, As, and Hg. The findings revealed that the ecological risk of farmland soils in this particular region falls within a medium to low range, with notable pollution observed specifically in Cr, As, and Pb.
Studies conducted by certain scholars on the soil environmental quality in China's key tea-producing regions suggest that, while the heavy metal content in soils within these primary tea-producing areas typically remains within national standards, the substantial capacity of soil to accumulate heavy metals introduces diverse levels of ecological risks in each tea plantation. These risks can significantly impact the yield and quality of tea leaves. Zhang et al10, focusing on the soil of a tea plantation in Pingshan County, Yibin City, found that Cd plays a controlling role in potential toxic element pollution across the entire study area. In their investigation, He et al11 identified that Cd and Hg are the primary contributors to potential ecological risks in tea plantations in southern Shaanxi. Similarly, Yi et al12, in their research, noted a significant correlation between the heavy metal content in tea plantation soils and the corresponding heavy metal content in tea leaves. Therefore, conducting investigations on the heavy metal content in tea plantation soils and pollution assessments is of paramount importance for ensuring the safety and quality of tea.
Shangluo City is rich in mineral resources, with 35 large deposits and 108 registered types of minerals accounting for 46 species in Shaanxi Province, potentially leading to elevated background values of heavy metal elements in local tea plantation soils13. Currently, there is limited research on heavy metals in the tea-growing areas of Shangluo City. Some studies have focused on point-to-area sample collection, while others have employed single-factor determinations. Nevertheless, there has not been a comprehensive investigation into the distribution characteristics of heavy metal pollution and risk assessment in the soils of Shangluo tea plantations14. This study collects soil samples from representative, widespread, and applicable tea plantation sites in the main tea production counties of Shangluo, aiming to comprehensively analyze the situation of heavy metal pollution in tea plantation soils throughout the city.Through the collection of soil samples from tea plantations in the primary tea production counties of Shangluo, an analysis of heavy metal content, including Cd, Hg, As, Pb, and Cr, is conducted. This aims to attain a thorough comprehension of the current state of heavy metal pollution in the soils of tea plantations in Shangluo City, with a specific focus on Shangnan County. Through a comparative analysis of soil samples both horizontally and vertically within tea plantations, this study will conduct an in-depth analysis of the characteristics of soil heavy metal pollution in the research area, including the distribution patterns of heavy metal elements and influencing factors. Concurrently, a risk assessment will be carried out, taking into account the moderate to low level of soil ecological risk and the severity of specific element pollution. This research aims to provide a more detailed and accurate scientific basis for the safety and quality of tea, offering reference solutions for future soil protection and management. Therefore, we hypothesize that (1) there is significant spatial heterogeneity in the heavy metal content of soils in Shangluo tea-growing areas, potentially influenced by factors such as geographic location and land-use history, and (2) there is a strong correlation among some heavy metals, leading to their co-enrichment in soils in the main tea production areas.