## 2 Materials and Methods
### Data Sources  
Research data were primarily gathered from published literature indexed in major academic databases, including CNKI ([[https://www.cnki.net/](https://www.cnki.net/)]([https://www.cnki.net/](https://www.cnki.net/))) and Web of Science ([[https://www.webofknowledge.com/](https://www.webofknowledge.com/)]([https://www.webofknowledge.com/](https://www.webofknowledge.com/))). These sources provide extensive research on Cd contamination in agricultural soils across Hainan Island.  
Search terms included \"Hainan\", \"cadmium\", \"heavy metal pollution\", \"farmland\", and \"agricultural land\", resulting in an initial collection of 1278 articles.  
Figure 1: Location map of the research area.  
To compile a comprehensive dataset, a structured literature search was performed across CNKI and Web of Science, targeting studies published between 2000 and 2024 on Cd contamination in Hainan's agricultural soils. Search terms included \"Hainan\", \"cadmium\", \"heavy metal\", \"farmland\", \"cultivated land\", and \"agricultural land\", resulting in an initial collection of 1278 articles.  
The selection process was guided by two main exclusion stages: exclusion based on abstracts and titles, and exclusion based on full-text content (Figure 2).  
Step 1: Exclusion Based on Abstracts and Titles  
A preliminary review of abstracts and titles narrowed the dataset to 143 articles, excluding studies in which the following were true:  
1. Titles or abstracts did not mention \"cadmium\";
2. Studies were not conducted in Hainan;
3. Research focused on non-soil subjects, such as crops or water;
4. Non-agricultural land was studied, such as mining areas;
5. The research area was not precisely described, and vague terms like \"a certain place\" were used.  
Step 2: Exclusion Based on Full-Text Content  
To establish a high-quality dataset, a rigorous selection framework was applied to assess full-text articles, ensuring reliability and methodological consistency. The criteria were as follows:  
1. Studies must be based on field sampling of agricultural soils rather than controlled laboratory experiments.
2. Surface soil samples (0-15 cm or 0-20 cm) must be collected from agricultural fields.
3. The research area or scope (e.g., city/latitude and longitude) must be explicitly defined, along with detailed information on sampling points (e.g., quantity and location).
4. The Cd content statistics must be explicitly reported, including mean, standard deviation, range, and coefficient of variation.
5. Sampling time must be recorded; if it was missing, it was estimated as 'publication year--2' for consistency.
6. All sampling and analytical procedures must be strictly followed to meet certified industry standards to ensure data comparability, reproducibility, and compliance with  
Figure 2: Data filtering process.  
international environmental monitoring frameworks. Adherence to these protocols minimizes methodological biases and enhances the reliability of risk assessments.
7. Rigorous quality assurance and quality control procedures must be followed during soil preparation and analysis.  
Following a detailed full-text assessment, 60 articles were deemed suitable based on predefined selection criteria. After eliminating duplicate records, 38 unique articles (see Table S1) were included in the final dataset to ensure data reliability and representativeness, covering 4110 sampling points. These sampling points were distributed across Hainan Island to ensure geographical diversity and representativeness.